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<li><a href="#modules-nodemodule-api">Modules: <code>node:module</code> API</a>
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<li><a href="#the-module-object">The <code>Module</code> object</a>
<ul>
<li><a href="#modulebuiltinmodules"><code>module.builtinModules</code></a></li>
<li><a href="#modulecreaterequirefilename"><code>module.createRequire(filename)</code></a></li>
<li><a href="#moduleisbuiltinmodulename"><code>module.isBuiltin(moduleName)</code></a></li>
<li><span class="stability_1"><a href="#moduleregisterspecifier-parenturl-options"><code>module.register(specifier[, parentURL][, options])</code></a></span></li>
<li><a href="#modulesyncbuiltinesmexports"><code>module.syncBuiltinESMExports()</code></a></li>
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<li><span class="stability_1"><a href="#customization-hooks">Customization Hooks</a></span>
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<li><a href="#enabling">Enabling</a></li>
<li><a href="#chaining">Chaining</a></li>
<li><a href="#communication-with-module-customization-hooks">Communication with module customization hooks</a></li>
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<li><span class="stability_1"><a href="#initialize"><code>initialize()</code></a></span></li>
<li><span class="stability_1"><a href="#resolvespecifier-context-nextresolve"><code>resolve(specifier, context, nextResolve)</code></a></span></li>
<li><span class="stability_1"><a href="#loadurl-context-nextload"><code>load(url, context, nextLoad)</code></a></span></li>
<li><span class="stability_1"><a href="#globalpreload"><code>globalPreload()</code></a></span></li>
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<li><a href="#transpilation">Transpilation</a></li>
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<li><a href="#modulefindsourcemappath"><code>module.findSourceMap(path)</code></a></li>
<li><a href="#class-modulesourcemap">Class: <code>module.SourceMap</code></a>
<ul>
<li><a href="#new-sourcemappayload--linelengths-"><code>new SourceMap(payload[, { lineLengths }])</code></a></li>
<li><a href="#sourcemappayload"><code>sourceMap.payload</code></a></li>
<li><a href="#sourcemapfindentrylineoffset-columnoffset"><code>sourceMap.findEntry(lineOffset, columnOffset)</code></a></li>
<li><a href="#sourcemapfindoriginlinenumber-columnnumber"><code>sourceMap.findOrigin(lineNumber, columnNumber)</code></a></li>
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<details role="navigation" id="toc" open><summary>Table of contents</summary><ul>
<li><a href="#modules-nodemodule-api">Modules: <code>node:module</code> API</a>
<ul>
<li><a href="#the-module-object">The <code>Module</code> object</a>
<ul>
<li><a href="#modulebuiltinmodules"><code>module.builtinModules</code></a></li>
<li><a href="#modulecreaterequirefilename"><code>module.createRequire(filename)</code></a></li>
<li><a href="#moduleisbuiltinmodulename"><code>module.isBuiltin(moduleName)</code></a></li>
<li><span class="stability_1"><a href="#moduleregisterspecifier-parenturl-options"><code>module.register(specifier[, parentURL][, options])</code></a></span></li>
<li><a href="#modulesyncbuiltinesmexports"><code>module.syncBuiltinESMExports()</code></a></li>
</ul>
</li>
<li><span class="stability_1"><a href="#customization-hooks">Customization Hooks</a></span>
<ul>
<li><a href="#enabling">Enabling</a></li>
<li><a href="#chaining">Chaining</a></li>
<li><a href="#communication-with-module-customization-hooks">Communication with module customization hooks</a></li>
<li><a href="#hooks">Hooks</a>
<ul>
<li><span class="stability_1"><a href="#initialize"><code>initialize()</code></a></span></li>
<li><span class="stability_1"><a href="#resolvespecifier-context-nextresolve"><code>resolve(specifier, context, nextResolve)</code></a></span></li>
<li><span class="stability_1"><a href="#loadurl-context-nextload"><code>load(url, context, nextLoad)</code></a></span></li>
<li><span class="stability_1"><a href="#globalpreload"><code>globalPreload()</code></a></span></li>
</ul>
</li>
<li><a href="#examples">Examples</a>
<ul>
<li><a href="#import-from-https">Import from HTTPS</a></li>
<li><a href="#transpilation">Transpilation</a></li>
<li><a href="#import-maps">Import maps</a></li>
</ul>
</li>
</ul>
</li>
<li><span class="stability_1"><a href="#source-map-v3-support">Source map v3 support</a></span>
<ul>
<li><a href="#modulefindsourcemappath"><code>module.findSourceMap(path)</code></a></li>
<li><a href="#class-modulesourcemap">Class: <code>module.SourceMap</code></a>
<ul>
<li><a href="#new-sourcemappayload--linelengths-"><code>new SourceMap(payload[, { lineLengths }])</code></a></li>
<li><a href="#sourcemappayload"><code>sourceMap.payload</code></a></li>
<li><a href="#sourcemapfindentrylineoffset-columnoffset"><code>sourceMap.findEntry(lineOffset, columnOffset)</code></a></li>
<li><a href="#sourcemapfindoriginlinenumber-columnnumber"><code>sourceMap.findOrigin(lineNumber, columnNumber)</code></a></li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul></details>
<div role="main" id="apicontent">
<h2>Modules: <code>node:module</code> API<span><a class="mark" href="#modules-nodemodule-api" id="modules-nodemodule-api">#</a></span><a aria-hidden="true" class="legacy" id="module_modules_node_module_api"></a></h2>
<div class="api_metadata">
<span>Added in: v0.3.7</span>
</div>
<section><h3>The <code>Module</code> object<span><a class="mark" href="#the-module-object" id="the-module-object">#</a></span><a aria-hidden="true" class="legacy" id="module_the_module_object"></a></h3>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li>
</ul>
<p>Provides general utility methods when interacting with instances of
<code>Module</code>, the <a href="modules.html#the-module-object"><code>module</code></a> variable often seen in <a href="modules.html">CommonJS</a> modules. Accessed
via <code>import 'node:module'</code> or <code>require('node:module')</code>.</p>
<h4><code>module.builtinModules</code><span><a class="mark" href="#modulebuiltinmodules" id="modulebuiltinmodules">#</a></span><a aria-hidden="true" class="legacy" id="module_module_builtinmodules"></a></h4>
<div class="api_metadata">
<span>Added in: v9.3.0, v8.10.0, v6.13.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string[]></a></li>
</ul>
<p>A list of the names of all modules provided by Node.js. Can be used to verify
if a module is maintained by a third party or not.</p>
<p>Note: the list doesn't contain <a href="modules.html#built-in-modules-with-mandatory-node-prefix">prefix-only modules</a> like <code>node:test</code>.</p>
<p><code>module</code> in this context isn't the same object that's provided
by the <a href="modules.html#the-module-wrapper">module wrapper</a>. To access it, require the <code>Module</code> module:</p>
<pre class="with-26-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-comment">// module.mjs</span>
<span class="hljs-comment">// In an ECMAScript module</span>
<span class="hljs-keyword">import</span> { builtinModules <span class="hljs-keyword">as</span> builtin } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:module'</span>;</code><code class="language-js cjs"><span class="hljs-comment">// module.cjs</span>
<span class="hljs-comment">// In a CommonJS module</span>
<span class="hljs-keyword">const</span> builtin = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:module'</span>).<span class="hljs-property">builtinModules</span>;</code><button class="copy-button">copy</button></pre>
<h4><code>module.createRequire(filename)</code><span><a class="mark" href="#modulecreaterequirefilename" id="modulecreaterequirefilename">#</a></span><a aria-hidden="true" class="legacy" id="module_module_createrequire_filename"></a></h4>
<div class="api_metadata">
<span>Added in: v12.2.0</span>
</div>
<ul>
<li><code>filename</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="url.html#the-whatwg-url-api" class="type"><URL></a> Filename to be used to construct the require
function. Must be a file URL object, file URL string, or absolute path
string.</li>
<li>Returns: <a href="modules.html#requireid" class="type"><require></a> Require function</li>
</ul>
<pre><code class="language-js mjs"><span class="hljs-keyword">import</span> { createRequire } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:module'</span>;
<span class="hljs-keyword">const</span> <span class="hljs-built_in">require</span> = <span class="hljs-title function_">createRequire</span>(<span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>);
<span class="hljs-comment">// sibling-module.js is a CommonJS module.</span>
<span class="hljs-keyword">const</span> siblingModule = <span class="hljs-built_in">require</span>(<span class="hljs-string">'./sibling-module'</span>);</code> <button class="copy-button">copy</button></pre>
<h4><code>module.isBuiltin(moduleName)</code><span><a class="mark" href="#moduleisbuiltinmodulename" id="moduleisbuiltinmodulename">#</a></span><a aria-hidden="true" class="legacy" id="module_module_isbuiltin_modulename"></a></h4>
<div class="api_metadata">
<span>Added in: v18.6.0, v16.17.0</span>
</div>
<ul>
<li><code>moduleName</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> name of the module</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> returns true if the module is builtin else returns false</li>
</ul>
<pre><code class="language-js mjs"><span class="hljs-keyword">import</span> { isBuiltin } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:module'</span>;
<span class="hljs-title function_">isBuiltin</span>(<span class="hljs-string">'node:fs'</span>); <span class="hljs-comment">// true</span>
<span class="hljs-title function_">isBuiltin</span>(<span class="hljs-string">'fs'</span>); <span class="hljs-comment">// true</span>
<span class="hljs-title function_">isBuiltin</span>(<span class="hljs-string">'wss'</span>); <span class="hljs-comment">// false</span></code> <button class="copy-button">copy</button></pre>
<h4><code>module.register(specifier[, parentURL][, options])</code><span><a class="mark" href="#moduleregisterspecifier-parenturl-options" id="moduleregisterspecifier-parenturl-options">#</a></span><a aria-hidden="true" class="legacy" id="module_module_register_specifier_parenturl_options"></a></h4>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v20.8.0</td>
<td><p>Add support for WHATWG URL instances.</p></td></tr>
<tr><td>v20.6.0</td>
<td><p><span>Added in: v20.6.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a>.2 - Release candidate</div><p></p>
<ul>
<li><code>specifier</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="url.html#the-whatwg-url-api" class="type"><URL></a> Customization hooks to be registered; this should be
the same string that would be passed to <code>import()</code>, except that if it is
relative, it is resolved relative to <code>parentURL</code>.</li>
<li><code>parentURL</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="url.html#the-whatwg-url-api" class="type"><URL></a> If you want to resolve <code>specifier</code> relative to a base
URL, such as <code>import.meta.url</code>, you can pass that URL here. <strong>Default:</strong>
<code>'data:'</code></li>
<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a>
<ul>
<li><code>parentURL</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="url.html#the-whatwg-url-api" class="type"><URL></a> If you want to resolve <code>specifier</code> relative to a
base URL, such as <code>import.meta.url</code>, you can pass that URL here. This
property is ignored if the <code>parentURL</code> is supplied as the second argument.
<strong>Default:</strong> <code>'data:'</code></li>
<li><code>data</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> Any arbitrary, cloneable JavaScript value to pass into the
<a href="#initialize"><code>initialize</code></a> hook.</li>
<li><code>transferList</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object[]></a> <a href="worker_threads.html#portpostmessagevalue-transferlist">transferrable objects</a> to be passed into the
<code>initialize</code> hook.</li>
</ul>
</li>
</ul>
<p>Register a module that exports <a href="#customization-hooks">hooks</a> that customize Node.js module
resolution and loading behavior. See <a href="#customization-hooks">Customization hooks</a>.</p>
<h4><code>module.syncBuiltinESMExports()</code><span><a class="mark" href="#modulesyncbuiltinesmexports" id="modulesyncbuiltinesmexports">#</a></span><a aria-hidden="true" class="legacy" id="module_module_syncbuiltinesmexports"></a></h4>
<div class="api_metadata">
<span>Added in: v12.12.0</span>
</div>
<p>The <code>module.syncBuiltinESMExports()</code> method updates all the live bindings for
builtin <a href="esm.html">ES Modules</a> to match the properties of the <a href="modules.html">CommonJS</a> exports. It
does not add or remove exported names from the <a href="esm.html">ES Modules</a>.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>);
<span class="hljs-keyword">const</span> assert = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:assert'</span>);
<span class="hljs-keyword">const</span> { syncBuiltinESMExports } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:module'</span>);
fs.<span class="hljs-property">readFile</span> = newAPI;
<span class="hljs-keyword">delete</span> fs.<span class="hljs-property">readFileSync</span>;
<span class="hljs-keyword">function</span> <span class="hljs-title function_">newAPI</span>(<span class="hljs-params"></span>) {
<span class="hljs-comment">// ...</span>
}
fs.<span class="hljs-property">newAPI</span> = newAPI;
<span class="hljs-title function_">syncBuiltinESMExports</span>();
<span class="hljs-keyword">import</span>(<span class="hljs-string">'node:fs'</span>).<span class="hljs-title function_">then</span>(<span class="hljs-function">(<span class="hljs-params">esmFS</span>) =></span> {
<span class="hljs-comment">// It syncs the existing readFile property with the new value</span>
assert.<span class="hljs-title function_">strictEqual</span>(esmFS.<span class="hljs-property">readFile</span>, newAPI);
<span class="hljs-comment">// readFileSync has been deleted from the required fs</span>
assert.<span class="hljs-title function_">strictEqual</span>(<span class="hljs-string">'readFileSync'</span> <span class="hljs-keyword">in</span> fs, <span class="hljs-literal">false</span>);
<span class="hljs-comment">// syncBuiltinESMExports() does not remove readFileSync from esmFS</span>
assert.<span class="hljs-title function_">strictEqual</span>(<span class="hljs-string">'readFileSync'</span> <span class="hljs-keyword">in</span> esmFS, <span class="hljs-literal">true</span>);
<span class="hljs-comment">// syncBuiltinESMExports() does not add names</span>
assert.<span class="hljs-title function_">strictEqual</span>(esmFS.<span class="hljs-property">newAPI</span>, <span class="hljs-literal">undefined</span>);
});</code> <button class="copy-button">copy</button></pre>
<p><i id="module_customization_hooks"></i></p>
</section><section><h3>Customization Hooks<span><a class="mark" href="#customization-hooks" id="customization-hooks">#</a></span><a aria-hidden="true" class="legacy" id="module_customization_hooks"></a></h3>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v20.6.0</td>
<td><p>Added <code>initialize</code> hook to replace <code>globalPreload</code>.</p></td></tr>
<tr><td>v18.6.0, v16.17.0</td>
<td><p>Add support for chaining loaders.</p></td></tr>
<tr><td>v16.12.0</td>
<td><p>Removed <code>getFormat</code>, <code>getSource</code>, <code>transformSource</code>, and <code>globalPreload</code>; added <code>load</code> hook and <code>getGlobalPreload</code> hook.</p></td></tr>
<tr><td>v8.8.0</td>
<td><p><span>Added in: v8.8.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a>.2 - Release candidate</div><p></p>
<p><i id="enabling_module_customization_hooks"></i></p>
<h4>Enabling<span><a class="mark" href="#enabling" id="enabling">#</a></span><a aria-hidden="true" class="legacy" id="module_enabling"></a></h4>
<p>Module resolution and loading can be customized by registering a file which
exports a set of hooks. This can be done using the <a href="#moduleregisterspecifier-parenturl-options"><code>register</code></a> method
from <code>node:module</code>, which you can run before your application code by
using the <code>--import</code> flag:</p>
<pre><code class="language-bash">node --import ./register-hooks.js ./my-app.js</code> <button class="copy-button">copy</button></pre>
<pre class="with-44-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-comment">// register-hooks.js</span>
<span class="hljs-keyword">import</span> { register } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:module'</span>;
<span class="hljs-title function_">register</span>(<span class="hljs-string">'./hooks.mjs'</span>, <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>);</code><code class="language-js cjs"><span class="hljs-comment">// register-hooks.js</span>
<span class="hljs-keyword">const</span> { register } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:module'</span>);
<span class="hljs-keyword">const</span> { pathToFileURL } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:url'</span>);
<span class="hljs-title function_">register</span>(<span class="hljs-string">'./hooks.mjs'</span>, <span class="hljs-title function_">pathToFileURL</span>(__filename));</code><button class="copy-button">copy</button></pre>
<p>The file passed to <code>--import</code> can also be an export from a dependency:</p>
<pre><code class="language-bash">node --import some-package/register ./my-app.js</code> <button class="copy-button">copy</button></pre>
<p>Where <code>some-package</code> has an <a href="packages.html#exports"><code>"exports"</code></a> field defining the <code>/register</code>
export to map to a file that calls <code>register()</code>, like the following <code>register-hooks.js</code>
example.</p>
<p>Using <code>--import</code> ensures that the hooks are registered before any application
files are imported, including the entry point of the application. Alternatively,
<code>register</code> can be called from the entry point, but dynamic <code>import()</code> must be
used for any code that should be run after the hooks are registered:</p>
<pre class="with-46-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { register } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:module'</span>;
<span class="hljs-title function_">register</span>(<span class="hljs-string">'http-to-https'</span>, <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>);
<span class="hljs-comment">// Because this is a dynamic `import()`, the `http-to-https` hooks will run</span>
<span class="hljs-comment">// to handle `./my-app.js` and any other files it imports or requires.</span>
<span class="hljs-keyword">await</span> <span class="hljs-keyword">import</span>(<span class="hljs-string">'./my-app.js'</span>);</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { register } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:module'</span>);
<span class="hljs-keyword">const</span> { pathToFileURL } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:url'</span>);
<span class="hljs-title function_">register</span>(<span class="hljs-string">'http-to-https'</span>, <span class="hljs-title function_">pathToFileURL</span>(__filename));
<span class="hljs-comment">// Because this is a dynamic `import()`, the `http-to-https` hooks will run</span>
<span class="hljs-comment">// to handle `./my-app.js` and any other files it imports or requires.</span>
<span class="hljs-keyword">import</span>(<span class="hljs-string">'./my-app.js'</span>);</code><button class="copy-button">copy</button></pre>
<p>In this example, we are registering the <code>http-to-https</code> hooks, but they will
only be available for subsequently imported modules—in this case, <code>my-app.js</code>
and anything it references via <code>import</code> (and optionally <code>require</code>). If the
<code>import('./my-app.js')</code> had instead been a static <code>import './my-app.js'</code>, the
app would have <em>already</em> been loaded <strong>before</strong> the <code>http-to-https</code> hooks were
registered. This due to the ES modules specification, where static imports are
evaluated from the leaves of the tree first, then back to the trunk. There can
be static imports <em>within</em> <code>my-app.js</code>, which will not be evaluated until
<code>my-app.js</code> is dynamically imported.</p>
<p><code>my-app.js</code> can also be CommonJS. Customization hooks will run for any
modules that it references via <code>import</code> (and optionally <code>require</code>).</p>
<p>Finally, if all you want to do is register hooks before your app runs and you
don't want to create a separate file for that purpose, you can pass a <code>data:</code>
URL to <code>--import</code>:</p>
<pre><code class="language-bash">node --import <span class="hljs-string">'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register("http-to-https", pathToFileURL("./"));'</span> ./my-app.js</code> <button class="copy-button">copy</button></pre>
<h4>Chaining<span><a class="mark" href="#chaining" id="chaining">#</a></span><a aria-hidden="true" class="legacy" id="module_chaining"></a></h4>
<p>It's possible to call <code>register</code> more than once:</p>
<pre class="with-44-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-comment">// entrypoint.mjs</span>
<span class="hljs-keyword">import</span> { register } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:module'</span>;
<span class="hljs-title function_">register</span>(<span class="hljs-string">'./foo.mjs'</span>, <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>);
<span class="hljs-title function_">register</span>(<span class="hljs-string">'./bar.mjs'</span>, <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>);
<span class="hljs-keyword">await</span> <span class="hljs-keyword">import</span>(<span class="hljs-string">'./my-app.mjs'</span>);</code><code class="language-js cjs"><span class="hljs-comment">// entrypoint.cjs</span>
<span class="hljs-keyword">const</span> { register } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:module'</span>);
<span class="hljs-keyword">const</span> { pathToFileURL } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:url'</span>);
<span class="hljs-keyword">const</span> parentURL = <span class="hljs-title function_">pathToFileURL</span>(__filename);
<span class="hljs-title function_">register</span>(<span class="hljs-string">'./foo.mjs'</span>, parentURL);
<span class="hljs-title function_">register</span>(<span class="hljs-string">'./bar.mjs'</span>, parentURL);
<span class="hljs-keyword">import</span>(<span class="hljs-string">'./my-app.mjs'</span>);</code><button class="copy-button">copy</button></pre>
<p>In this example, the registered hooks will form chains. These chains run
last-in, first out (LIFO). If both <code>foo.mjs</code> and <code>bar.mjs</code> define a <code>resolve</code>
hook, they will be called like so (note the right-to-left):
node's default ← <code>./foo.mjs</code> ← <code>./bar.mjs</code>
(starting with <code>./bar.mjs</code>, then <code>./foo.mjs</code>, then the Node.js default).
The same applies to all the other hooks.</p>
<p>The registered hooks also affect <code>register</code> itself. In this example,
<code>bar.mjs</code> will be resolved and loaded via the hooks registered by <code>foo.mjs</code>
(because <code>foo</code>'s hooks will have already been added to the chain). This allows
for things like writing hooks in non-JavaScript languages, so long as
earlier registered hooks transpile into JavaScript.</p>
<p>The <code>register</code> method cannot be called from within the module that defines the
hooks.</p>
<h4>Communication with module customization hooks<span><a class="mark" href="#communication-with-module-customization-hooks" id="communication-with-module-customization-hooks">#</a></span><a aria-hidden="true" class="legacy" id="module_communication_with_module_customization_hooks"></a></h4>
<p>Module customization hooks run on a dedicated thread, separate from the main
thread that runs application code. This means mutating global variables won't
affect the other thread(s), and message channels must be used to communicate
between the threads.</p>
<p>The <code>register</code> method can be used to pass data to an <a href="#initialize"><code>initialize</code></a> hook. The
data passed to the hook may include transferrable objects like ports.</p>
<pre class="with-53-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { register } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:module'</span>;
<span class="hljs-keyword">import</span> { <span class="hljs-title class_">MessageChannel</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:worker_threads'</span>;
<span class="hljs-comment">// This example demonstrates how a message channel can be used to</span>
<span class="hljs-comment">// communicate with the hooks, by sending `port2` to the hooks.</span>
<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
port1.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">msg</span>) =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(msg);
});
port1.<span class="hljs-title function_">unref</span>();
<span class="hljs-title function_">register</span>(<span class="hljs-string">'./my-hooks.mjs'</span>, {
<span class="hljs-attr">parentURL</span>: <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>,
<span class="hljs-attr">data</span>: { <span class="hljs-attr">number</span>: <span class="hljs-number">1</span>, <span class="hljs-attr">port</span>: port2 },
<span class="hljs-attr">transferList</span>: [port2],
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { register } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:module'</span>);
<span class="hljs-keyword">const</span> { pathToFileURL } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:url'</span>);
<span class="hljs-keyword">const</span> { <span class="hljs-title class_">MessageChannel</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:worker_threads'</span>);
<span class="hljs-comment">// This example showcases how a message channel can be used to</span>
<span class="hljs-comment">// communicate with the hooks, by sending `port2` to the hooks.</span>
<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
port1.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">msg</span>) =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(msg);
});
port1.<span class="hljs-title function_">unref</span>();
<span class="hljs-title function_">register</span>(<span class="hljs-string">'./my-hooks.mjs'</span>, {
<span class="hljs-attr">parentURL</span>: <span class="hljs-title function_">pathToFileURL</span>(__filename),
<span class="hljs-attr">data</span>: { <span class="hljs-attr">number</span>: <span class="hljs-number">1</span>, <span class="hljs-attr">port</span>: port2 },
<span class="hljs-attr">transferList</span>: [port2],
});</code><button class="copy-button">copy</button></pre>
<h4>Hooks<span><a class="mark" href="#hooks" id="hooks">#</a></span><a aria-hidden="true" class="legacy" id="module_hooks"></a></h4>
<p>The <a href="#moduleregisterspecifier-parenturl-options"><code>register</code></a> method can be used to register a module that exports a set of
hooks. The hooks are functions that are called by Node.js to customize the
module resolution and loading process. The exported functions must have specific
names and signatures, and they must be exported as named exports.</p>
<pre><code class="language-js mjs"><span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">initialize</span>(<span class="hljs-params">{ number, port }</span>) {
<span class="hljs-comment">// Receives data from `register`.</span>
}
<span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">resolve</span>(<span class="hljs-params">specifier, context, nextResolve</span>) {
<span class="hljs-comment">// Take an `import` or `require` specifier and resolve it to a URL.</span>
}
<span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">load</span>(<span class="hljs-params">url, context, nextLoad</span>) {
<span class="hljs-comment">// Take a resolved URL and return the source code to be evaluated.</span>
}</code> <button class="copy-button">copy</button></pre>
<p>Hooks are part of a <a href="#chaining">chain</a>, even if that chain consists of only one
custom (user-provided) hook and the default hook, which is always present. Hook
functions nest: each one must always return a plain object, and chaining happens
as a result of each function calling <code>next<hookName>()</code>, which is a reference to
the subsequent loader's hook (in LIFO order).</p>
<p>A hook that returns a value lacking a required property triggers an exception. A
hook that returns without calling <code>next<hookName>()</code> <em>and</em> without returning
<code>shortCircuit: true</code> also triggers an exception. These errors are to help
prevent unintentional breaks in the chain. Return <code>shortCircuit: true</code> from a
hook to signal that the chain is intentionally ending at your hook.</p>
<p>Hooks are run in a separate thread, isolated from the main thread where
application code runs. That means it is a different <a href="https://tc39.es/ecma262/#realm">realm</a>. The hooks thread
may be terminated by the main thread at any time, so do not depend on
asynchronous operations (like <code>console.log</code>) to complete.</p>
<h5><code>initialize()</code><span><a class="mark" href="#initialize" id="initialize">#</a></span><a aria-hidden="true" class="legacy" id="module_initialize"></a></h5>
<div class="api_metadata">
<span>Added in: v20.6.0</span>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a>.2 - Release candidate</div><p></p>
<ul>
<li><code>data</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The data from <code>register(loader, import.meta.url, { data })</code>.</li>
</ul>
<p>The <code>initialize</code> hook provides a way to define a custom function that runs in
the hooks thread when the hooks module is initialized. Initialization happens
when the hooks module is registered via <a href="#moduleregisterspecifier-parenturl-options"><code>register</code></a>.</p>
<p>This hook can receive data from a <a href="#moduleregisterspecifier-parenturl-options"><code>register</code></a> invocation, including
ports and other transferrable objects. The return value of <code>initialize</code> can be a
<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise" class="type"><Promise></a>, in which case it will be awaited before the main application thread
execution resumes.</p>
<p>Module customization code:</p>
<pre><code class="language-js mjs"><span class="hljs-comment">// path-to-my-hooks.js</span>
<span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">initialize</span>(<span class="hljs-params">{ number, port }</span>) {
port.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">`increment: <span class="hljs-subst">${number + <span class="hljs-number">1</span>}</span>`</span>);
}</code> <button class="copy-button">copy</button></pre>
<p>Caller code:</p>
<pre class="with-44-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> assert <span class="hljs-keyword">from</span> <span class="hljs-string">'node:assert'</span>;
<span class="hljs-keyword">import</span> { register } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:module'</span>;
<span class="hljs-keyword">import</span> { <span class="hljs-title class_">MessageChannel</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:worker_threads'</span>;
<span class="hljs-comment">// This example showcases how a message channel can be used to communicate</span>
<span class="hljs-comment">// between the main (application) thread and the hooks running on the hooks</span>
<span class="hljs-comment">// thread, by sending `port2` to the `initialize` hook.</span>
<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
port1.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">msg</span>) =></span> {
assert.<span class="hljs-title function_">strictEqual</span>(msg, <span class="hljs-string">'increment: 2'</span>);
});
port1.<span class="hljs-title function_">unref</span>();
<span class="hljs-title function_">register</span>(<span class="hljs-string">'./path-to-my-hooks.js'</span>, {
<span class="hljs-attr">parentURL</span>: <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>,
<span class="hljs-attr">data</span>: { <span class="hljs-attr">number</span>: <span class="hljs-number">1</span>, <span class="hljs-attr">port</span>: port2 },
<span class="hljs-attr">transferList</span>: [port2],
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> assert = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:assert'</span>);
<span class="hljs-keyword">const</span> { register } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:module'</span>);
<span class="hljs-keyword">const</span> { pathToFileURL } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:url'</span>);
<span class="hljs-keyword">const</span> { <span class="hljs-title class_">MessageChannel</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:worker_threads'</span>);
<span class="hljs-comment">// This example showcases how a message channel can be used to communicate</span>
<span class="hljs-comment">// between the main (application) thread and the hooks running on the hooks</span>
<span class="hljs-comment">// thread, by sending `port2` to the `initialize` hook.</span>
<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
port1.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">msg</span>) =></span> {
assert.<span class="hljs-title function_">strictEqual</span>(msg, <span class="hljs-string">'increment: 2'</span>);
});
port1.<span class="hljs-title function_">unref</span>();
<span class="hljs-title function_">register</span>(<span class="hljs-string">'./path-to-my-hooks.js'</span>, {
<span class="hljs-attr">parentURL</span>: <span class="hljs-title function_">pathToFileURL</span>(__filename),
<span class="hljs-attr">data</span>: { <span class="hljs-attr">number</span>: <span class="hljs-number">1</span>, <span class="hljs-attr">port</span>: port2 },
<span class="hljs-attr">transferList</span>: [port2],
});</code><button class="copy-button">copy</button></pre>
<h5><code>resolve(specifier, context, nextResolve)</code><span><a class="mark" href="#resolvespecifier-context-nextresolve" id="resolvespecifier-context-nextresolve">#</a></span><a aria-hidden="true" class="legacy" id="module_resolve_specifier_context_nextresolve"></a></h5>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v20.10.0</td>
<td><p>The property <code>context.importAssertions</code> is replaced with <code>context.importAttributes</code>. Using the old name is still supported and will emit an experimental warning.</p></td></tr>
<tr><td>v18.6.0, v16.17.0</td>
<td><p>Add support for chaining resolve hooks. Each hook must either call <code>nextResolve()</code> or include a <code>shortCircuit</code> property set to <code>true</code> in its return.</p></td></tr>
<tr><td>v17.1.0, v16.14.0</td>
<td><p>Add support for import assertions.</p></td></tr>
</tbody></table>
</details>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a>.2 - Release candidate</div><p></p>
<ul>
<li><code>specifier</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a>
<ul>
<li><code>conditions</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string[]></a> Export conditions of the relevant <code>package.json</code></li>
<li><code>importAttributes</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> An object whose key-value pairs represent the
attributes for the module to import</li>
<li><code>parentURL</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> The module importing this one, or undefined
if this is the Node.js entry point</li>
</ul>
</li>
<li><code>nextResolve</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The subsequent <code>resolve</code> hook in the chain, or the
Node.js default <code>resolve</code> hook after the last user-supplied <code>resolve</code> hook
<ul>
<li><code>specifier</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li>
</ul>
</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise" class="type"><Promise></a>
<ul>
<li><code>format</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Null_type" class="type"><null></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> A hint to the load hook (it might be
ignored)
<code>'builtin' | 'commonjs' | 'json' | 'module' | 'wasm'</code></li>
<li><code>importAttributes</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> The import attributes to use when
caching the module (optional; if excluded the input will be used)</li>
<li><code>shortCircuit</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> A signal that this hook intends to
terminate the chain of <code>resolve</code> hooks. <strong>Default:</strong> <code>false</code></li>
<li><code>url</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The absolute URL to which this input resolves</li>
</ul>
</li>
</ul>
<blockquote>
<p><strong>Warning</strong> Despite support for returning promises and async functions, calls
to <code>resolve</code> may block the main thread which can impact performance.</p>
</blockquote>
<p>The <code>resolve</code> hook chain is responsible for telling Node.js where to find and
how to cache a given <code>import</code> statement or expression, or <code>require</code> call. It can
optionally return a format (such as <code>'module'</code>) as a hint to the <code>load</code> hook. If
a format is specified, the <code>load</code> hook is ultimately responsible for providing
the final <code>format</code> value (and it is free to ignore the hint provided by
<code>resolve</code>); if <code>resolve</code> provides a <code>format</code>, a custom <code>load</code> hook is required
even if only to pass the value to the Node.js default <code>load</code> hook.</p>
<p>Import type attributes are part of the cache key for saving loaded modules into
the internal module cache. The <code>resolve</code> hook is responsible for returning an
<code>importAttributes</code> object if the module should be cached with different
attributes than were present in the source code.</p>
<p>The <code>conditions</code> property in <code>context</code> is an array of conditions for
<a href="packages.html#conditional-exports">package exports conditions</a> that apply to this resolution
request. They can be used for looking up conditional mappings elsewhere or to
modify the list when calling the default resolution logic.</p>
<p>The current <a href="packages.html#conditional-exports">package exports conditions</a> are always in
the <code>context.conditions</code> array passed into the hook. To guarantee <em>default
Node.js module specifier resolution behavior</em> when calling <code>defaultResolve</code>, the
<code>context.conditions</code> array passed to it <em>must</em> include <em>all</em> elements of the
<code>context.conditions</code> array originally passed into the <code>resolve</code> hook.</p>
<pre><code class="language-js mjs"><span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">resolve</span>(<span class="hljs-params">specifier, context, nextResolve</span>) {
<span class="hljs-keyword">const</span> { parentURL = <span class="hljs-literal">null</span> } = context;
<span class="hljs-keyword">if</span> (<span class="hljs-title class_">Math</span>.<span class="hljs-title function_">random</span>() > <span class="hljs-number">0.5</span>) { <span class="hljs-comment">// Some condition.</span>
<span class="hljs-comment">// For some or all specifiers, do some custom logic for resolving.</span>
<span class="hljs-comment">// Always return an object of the form {url: <string>}.</span>
<span class="hljs-keyword">return</span> {
<span class="hljs-attr">shortCircuit</span>: <span class="hljs-literal">true</span>,
<span class="hljs-attr">url</span>: parentURL ?
<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(specifier, parentURL).<span class="hljs-property">href</span> :
<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(specifier).<span class="hljs-property">href</span>,
};
}
<span class="hljs-keyword">if</span> (<span class="hljs-title class_">Math</span>.<span class="hljs-title function_">random</span>() < <span class="hljs-number">0.5</span>) { <span class="hljs-comment">// Another condition.</span>
<span class="hljs-comment">// When calling `defaultResolve`, the arguments can be modified. In this</span>
<span class="hljs-comment">// case it's adding another value for matching conditional exports.</span>
<span class="hljs-keyword">return</span> <span class="hljs-title function_">nextResolve</span>(specifier, {
...context,
<span class="hljs-attr">conditions</span>: [...context.<span class="hljs-property">conditions</span>, <span class="hljs-string">'another-condition'</span>],
});
}
<span class="hljs-comment">// Defer to the next hook in the chain, which would be the</span>
<span class="hljs-comment">// Node.js default resolve if this is the last user-specified loader.</span>
<span class="hljs-keyword">return</span> <span class="hljs-title function_">nextResolve</span>(specifier);
}</code> <button class="copy-button">copy</button></pre>
<h5><code>load(url, context, nextLoad)</code><span><a class="mark" href="#loadurl-context-nextload" id="loadurl-context-nextload">#</a></span><a aria-hidden="true" class="legacy" id="module_load_url_context_nextload"></a></h5>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v20.6.0</td>
<td><p>Add support for <code>source</code> with format <code>commonjs</code>.</p></td></tr>
<tr><td>v18.6.0, v16.17.0</td>
<td><p>Add support for chaining load hooks. Each hook must either call <code>nextLoad()</code> or include a <code>shortCircuit</code> property set to <code>true</code> in its return.</p></td></tr>
</tbody></table>
</details>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a>.2 - Release candidate</div><p></p>
<ul>
<li><code>url</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The URL returned by the <code>resolve</code> chain</li>
<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a>
<ul>
<li><code>conditions</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string[]></a> Export conditions of the relevant <code>package.json</code></li>
<li><code>format</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Null_type" class="type"><null></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> The format optionally supplied by the
<code>resolve</code> hook chain</li>
<li><code>importAttributes</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li>
</ul>
</li>
<li><code>nextLoad</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The subsequent <code>load</code> hook in the chain, or the
Node.js default <code>load</code> hook after the last user-supplied <code>load</code> hook
<ul>
<li><code>url</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li>
</ul>
</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a>
<ul>
<li><code>format</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
<li><code>shortCircuit</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> A signal that this hook intends to
terminate the chain of <code>load</code> hooks. <strong>Default:</strong> <code>false</code></li>
<li><code>source</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer" class="type"><ArrayBuffer></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray" class="type"><TypedArray></a> The source for Node.js to evaluate</li>
</ul>
</li>
</ul>
<p>The <code>load</code> hook provides a way to define a custom method of determining how a
URL should be interpreted, retrieved, and parsed. It is also in charge of
validating the import assertion.</p>
<p>The final value of <code>format</code> must be one of the following:</p>
<table><thead><tr><th><code>format</code></th><th>Description</th><th>Acceptable types for <code>source</code> returned by <code>load</code></th></tr></thead><tbody><tr><td><code>'builtin'</code></td><td>Load a Node.js builtin module</td><td>Not applicable</td></tr><tr><td><code>'commonjs'</code></td><td>Load a Node.js CommonJS module</td><td>{ <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String"><code>string</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray"><code>TypedArray</code></a>, <code>null</code>, <code>undefined</code> }</td></tr><tr><td><code>'json'</code></td><td>Load a JSON file</td><td>{ <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String"><code>string</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray"><code>TypedArray</code></a> }</td></tr><tr><td><code>'module'</code></td><td>Load an ES module</td><td>{ <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String"><code>string</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray"><code>TypedArray</code></a> }</td></tr><tr><td><code>'wasm'</code></td><td>Load a WebAssembly module</td><td>{ <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a>, <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray"><code>TypedArray</code></a> }</td></tr></tbody></table>
<p>The value of <code>source</code> is ignored for type <code>'builtin'</code> because currently it is
not possible to replace the value of a Node.js builtin (core) module.</p>
<p>Omitting vs providing a <code>source</code> for <code>'commonjs'</code> has very different effects:</p>
<ul>
<li>When a <code>source</code> is provided, all <code>require</code> calls from this module will be
processed by the ESM loader with registered <code>resolve</code> and <code>load</code> hooks; all
<code>require.resolve</code> calls from this module will be processed by the ESM loader
with registered <code>resolve</code> hooks; only a subset of the CommonJS API will be
available (e.g. no <code>require.extensions</code>, no <code>require.cache</code>, no
<code>require.resolve.paths</code>) and monkey-patching on the CommonJS module loader
will not apply.</li>
<li>If <code>source</code> is undefined or <code>null</code>, it will be handled by the CommonJS module
loader and <code>require</code>/<code>require.resolve</code> calls will not go through the
registered hooks. This behavior for nullish <code>source</code> is temporary — in the
future, nullish <code>source</code> will not be supported.</li>
</ul>
<p>When <code>node</code> is run with <code>--experimental-default-type=commonjs</code>, the Node.js
internal <code>load</code> implementation, which is the value of <code>next</code> for the
last hook in the <code>load</code> chain, returns <code>null</code> for <code>source</code> when <code>format</code> is
<code>'commonjs'</code> for backward compatibility. Here is an example hook that would
opt-in to using the non-default behavior:</p>
<pre><code class="language-js mjs"><span class="hljs-keyword">import</span> { readFile } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs/promises'</span>;
<span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">load</span>(<span class="hljs-params">url, context, nextLoad</span>) {
<span class="hljs-keyword">const</span> result = <span class="hljs-keyword">await</span> <span class="hljs-title function_">nextLoad</span>(url, context);
<span class="hljs-keyword">if</span> (result.<span class="hljs-property">format</span> === <span class="hljs-string">'commonjs'</span>) {
result.<span class="hljs-property">source</span> ??= <span class="hljs-keyword">await</span> <span class="hljs-title function_">readFile</span>(<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(result.<span class="hljs-property">responseURL</span> ?? url));
}
<span class="hljs-keyword">return</span> result;
}</code> <button class="copy-button">copy</button></pre>
<blockquote>
<p><strong>Warning</strong>: The ESM <code>load</code> hook and namespaced exports from CommonJS modules
are incompatible. Attempting to use them together will result in an empty
object from the import. This may be addressed in the future.</p>
</blockquote>
<blockquote>
<p>These types all correspond to classes defined in ECMAScript.</p>
</blockquote>
<ul>
<li>The specific <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a> object is a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer"><code>SharedArrayBuffer</code></a>.</li>
<li>The specific <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray"><code>TypedArray</code></a> object is a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array"><code>Uint8Array</code></a>.</li>
</ul>
<p>If the source value of a text-based format (i.e., <code>'json'</code>, <code>'module'</code>)
is not a string, it is converted to a string using <a href="util.html#class-utiltextdecoder"><code>util.TextDecoder</code></a>.</p>
<p>The <code>load</code> hook provides a way to define a custom method for retrieving the
source code of a resolved URL. This would allow a loader to potentially avoid
reading files from disk. It could also be used to map an unrecognized format to
a supported one, for example <code>yaml</code> to <code>module</code>.</p>
<pre><code class="language-js mjs"><span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">load</span>(<span class="hljs-params">url, context, nextLoad</span>) {
<span class="hljs-keyword">const</span> { format } = context;
<span class="hljs-keyword">if</span> (<span class="hljs-title class_">Math</span>.<span class="hljs-title function_">random</span>() > <span class="hljs-number">0.5</span>) { <span class="hljs-comment">// Some condition</span>
<span class="hljs-comment">/*
For some or all URLs, do some custom logic for retrieving the source.
Always return an object of the form {
format: <string>,
source: <string|buffer>,
}.
*/</span>
<span class="hljs-keyword">return</span> {
format,
<span class="hljs-attr">shortCircuit</span>: <span class="hljs-literal">true</span>,
<span class="hljs-attr">source</span>: <span class="hljs-string">'...'</span>,
};
}
<span class="hljs-comment">// Defer to the next hook in the chain.</span>
<span class="hljs-keyword">return</span> <span class="hljs-title function_">nextLoad</span>(url);
}</code> <button class="copy-button">copy</button></pre>
<p>In a more advanced scenario, this can also be used to transform an unsupported
source to a supported one (see <a href="#examples">Examples</a> below).</p>
<h5><code>globalPreload()</code><span><a class="mark" href="#globalpreload" id="globalpreload">#</a></span><a aria-hidden="true" class="legacy" id="module_globalpreload"></a></h5>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v18.6.0, v16.17.0</td>
<td><p>Add support for chaining globalPreload hooks.</p></td></tr>
</tbody></table>
</details>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a>.0 - Early development</div><p></p>
<blockquote>
<p><strong>Warning:</strong> This hook will be removed in a future version. Use
<a href="#initialize"><code>initialize</code></a> instead. When a hooks module has an <code>initialize</code> export,
<code>globalPreload</code> will be ignored.</p>
</blockquote>
<ul>
<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> Information to assist the preload code
<ul>
<li><code>port</code> <a href="worker_threads.html#class-messageport" class="type"><MessagePort></a></li>
</ul>
</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> Code to run before application startup</li>
</ul>
<p>Sometimes it might be necessary to run some code inside of the same global
scope that the application runs in. This hook allows the return of a string
that is run as a sloppy-mode script on startup.</p>
<p>Similar to how CommonJS wrappers work, the code runs in an implicit function
scope. The only argument is a <code>require</code>-like function that can be used to load
builtins like "fs": <code>getBuiltin(request: string)</code>.</p>
<p>If the code needs more advanced <code>require</code> features, it has to construct
its own <code>require</code> using <code>module.createRequire()</code>.</p>
<pre><code class="language-js mjs"><span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">globalPreload</span>(<span class="hljs-params">context</span>) {
<span class="hljs-keyword">return</span> <span class="hljs-string">`\
globalThis.someInjectedProperty = 42;
console.log('I just set some globals!');
const { createRequire } = getBuiltin('module');
const { cwd } = getBuiltin('process');
const require = createRequire(cwd() + '/<preload>');
// [...]
`</span>;
}</code> <button class="copy-button">copy</button></pre>
<p>Another argument is provided to the preload code: <code>port</code>. This is available as a
parameter to the hook and inside of the source text returned by the hook. This
functionality has been moved to the <code>initialize</code> hook.</p>
<p>Care must be taken in order to properly call <a href="worker_threads.html#portref"><code>port.ref()</code></a> and
<a href="worker_threads.html#portunref"><code>port.unref()</code></a> to prevent a process from being in a state where it won't
close normally.</p>
<pre><code class="language-js mjs"><span class="hljs-comment">/**
* This example has the application context send a message to the hook
* and sends the message back to the application context
*/</span>
<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">globalPreload</span>(<span class="hljs-params">{ port }</span>) {
port.<span class="hljs-property">onmessage</span> = <span class="hljs-function">(<span class="hljs-params">evt</span>) =></span> {
port.<span class="hljs-title function_">postMessage</span>(evt.<span class="hljs-property">data</span>);
};
<span class="hljs-keyword">return</span> <span class="hljs-string">`\
port.postMessage('console.log("I went to the hook and back");');
port.onmessage = (evt) => {
eval(evt.data);
};
`</span>;
}</code> <button class="copy-button">copy</button></pre>
<h4>Examples<span><a class="mark" href="#examples" id="examples">#</a></span><a aria-hidden="true" class="legacy" id="module_examples"></a></h4>
<p>The various module customization hooks can be used together to accomplish
wide-ranging customizations of the Node.js code loading and evaluation
behaviors.</p>
<h5>Import from HTTPS<span><a class="mark" href="#import-from-https" id="import-from-https">#</a></span><a aria-hidden="true" class="legacy" id="module_import_from_https"></a></h5>
<p>In current Node.js, specifiers starting with <code>https://</code> are experimental (see
<a href="esm.html#https-and-http-imports">HTTPS and HTTP imports</a>).</p>
<p>The hook below registers hooks to enable rudimentary support for such
specifiers. While this may seem like a significant improvement to Node.js core
functionality, there are substantial downsides to actually using these hooks:
performance is much slower than loading files from disk, there is no caching,
and there is no security.</p>
<pre><code class="language-js mjs"><span class="hljs-comment">// https-hooks.mjs</span>
<span class="hljs-keyword">import</span> { get } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:https'</span>;
<span class="hljs-keyword">export</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">load</span>(<span class="hljs-params">url, context, nextLoad</span>) {
<span class="hljs-comment">// For JavaScript to be loaded over the network, we need to fetch and</span>
<span class="hljs-comment">// return it.</span>
<span class="hljs-keyword">if</span> (url.<span class="hljs-title function_">startsWith</span>(<span class="hljs-string">'https://'</span>)) {
<span class="hljs-keyword">return</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Promise</span>(<span class="hljs-function">(<span class="hljs-params">resolve, reject</span>) =></span> {
<span class="hljs-title function_">get</span>(url, <span class="hljs-function">(<span class="hljs-params">res</span>) =></span> {
<span class="hljs-keyword">let</span> data = <span class="hljs-string">''</span>;
res.<span class="hljs-title function_">setEncoding</span>(<span class="hljs-string">'utf8'</span>);
res.<span class="hljs-title function_">on</span>(<span class="hljs-string">'data'</span>, <span class="hljs-function">(<span class="hljs-params">chunk</span>) =></span> data += chunk);
res.<span class="hljs-title function_">on</span>(<span class="hljs-string">'end'</span>, <span class="hljs-function">() =></span> <span class="hljs-title function_">resolve</span>({
<span class="hljs-comment">// This example assumes all network-provided JavaScript is ES module</span>
<span class="hljs-comment">// code.</span>
<span class="hljs-attr">format</span>: <span class="hljs-string">'module'</span>,
<span class="hljs-attr">shortCircuit</span>: <span class="hljs-literal">true</span>,
<span class="hljs-attr">source</span>: data,
}));
}).<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">err</span>) =></span> <span class="hljs-title function_">reject</span>(err));
});
}
<span class="hljs-comment">// Let Node.js handle all other URLs.</span>
<span class="hljs-keyword">return</span> <span class="hljs-title function_">nextLoad</span>(url);
}</code> <button class="copy-button">copy</button></pre>
<pre><code class="language-js mjs"><span class="hljs-comment">// main.mjs</span>
<span class="hljs-keyword">import</span> { <span class="hljs-variable constant_">VERSION</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'https://coffeescript.org/browser-compiler-modern/coffeescript.js'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-variable constant_">VERSION</span>);</code> <button class="copy-button">copy</button></pre>
<p>With the preceding hooks module, running
<code>node --import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register(pathToFileURL("./https-hooks.mjs"));' ./main.mjs</code>
prints the current version of CoffeeScript per the module at the URL in
<code>main.mjs</code>.</p>
<h5>Transpilation<span><a class="mark" href="#transpilation" id="transpilation">#</a></span><a aria-hidden="true" class="legacy" id="module_transpilation"></a></h5>
<p>Sources that are in formats Node.js doesn't understand can be converted into
JavaScript using the <a href="#loadurl-context-nextload"><code>load</code> hook</a>.</p>
<p>This is less performant than transpiling source files before running Node.js;
transpiler hooks should only be used for development and testing purposes.</p>
<pre><code class="language-js mjs"><span class="hljs-comment">// coffeescript-hooks.mjs</span>
<span class="hljs-keyword">import</span> { readFile } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs/promises'</span>;
<span class="hljs-keyword">import</span> { dirname, extname, resolve <span class="hljs-keyword">as</span> resolvePath } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:path'</span>;
<span class="hljs-keyword">import</span> { cwd } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span>;
<span class="hljs-keyword">import</span> { fileURLToPath, pathToFileURL } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:url'</span>;
<span class="hljs-keyword">import</span> coffeescript <span class="hljs-keyword">from</span> <span class="hljs-string">'coffeescript'</span>;
<span class="hljs-keyword">const</span> extensionsRegex = <span class="hljs-regexp">/\.(coffee|litcoffee|coffee\.md)$/</span>;
<span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">load</span>(<span class="hljs-params">url, context, nextLoad</span>) {
<span class="hljs-keyword">if</span> (extensionsRegex.<span class="hljs-title function_">test</span>(url)) {
<span class="hljs-comment">// CoffeeScript files can be either CommonJS or ES modules, so we want any</span>
<span class="hljs-comment">// CoffeeScript file to be treated by Node.js the same as a .js file at the</span>
<span class="hljs-comment">// same location. To determine how Node.js would interpret an arbitrary .js</span>
<span class="hljs-comment">// file, search up the file system for the nearest parent package.json file</span>
<span class="hljs-comment">// and read its "type" field.</span>
<span class="hljs-keyword">const</span> format = <span class="hljs-keyword">await</span> <span class="hljs-title function_">getPackageType</span>(url);
<span class="hljs-keyword">const</span> { <span class="hljs-attr">source</span>: rawSource } = <span class="hljs-keyword">await</span> <span class="hljs-title function_">nextLoad</span>(url, { ...context, format });
<span class="hljs-comment">// This hook converts CoffeeScript source code into JavaScript source code</span>
<span class="hljs-comment">// for all imported CoffeeScript files.</span>
<span class="hljs-keyword">const</span> transformedSource = coffeescript.<span class="hljs-title function_">compile</span>(rawSource.<span class="hljs-title function_">toString</span>(), url);
<span class="hljs-keyword">return</span> {
format,
<span class="hljs-attr">shortCircuit</span>: <span class="hljs-literal">true</span>,
<span class="hljs-attr">source</span>: transformedSource,
};
}
<span class="hljs-comment">// Let Node.js handle all other URLs.</span>
<span class="hljs-keyword">return</span> <span class="hljs-title function_">nextLoad</span>(url);
}
<span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">getPackageType</span>(<span class="hljs-params">url</span>) {
<span class="hljs-comment">// `url` is only a file path during the first iteration when passed the</span>
<span class="hljs-comment">// resolved url from the load() hook</span>
<span class="hljs-comment">// an actual file path from load() will contain a file extension as it's</span>
<span class="hljs-comment">// required by the spec</span>
<span class="hljs-comment">// this simple truthy check for whether `url` contains a file extension will</span>
<span class="hljs-comment">// work for most projects but does not cover some edge-cases (such as</span>
<span class="hljs-comment">// extensionless files or a url ending in a trailing space)</span>
<span class="hljs-keyword">const</span> isFilePath = !!<span class="hljs-title function_">extname</span>(url);
<span class="hljs-comment">// If it is a file path, get the directory it's in</span>
<span class="hljs-keyword">const</span> dir = isFilePath ?
<span class="hljs-title function_">dirname</span>(<span class="hljs-title function_">fileURLToPath</span>(url)) :
url;
<span class="hljs-comment">// Compose a file path to a package.json in the same directory,</span>
<span class="hljs-comment">// which may or may not exist</span>
<span class="hljs-keyword">const</span> packagePath = <span class="hljs-title function_">resolvePath</span>(dir, <span class="hljs-string">'package.json'</span>);
<span class="hljs-comment">// Try to read the possibly nonexistent package.json</span>
<span class="hljs-keyword">const</span> type = <span class="hljs-keyword">await</span> <span class="hljs-title function_">readFile</span>(packagePath, { <span class="hljs-attr">encoding</span>: <span class="hljs-string">'utf8'</span> })
.<span class="hljs-title function_">then</span>(<span class="hljs-function">(<span class="hljs-params">filestring</span>) =></span> <span class="hljs-title class_">JSON</span>.<span class="hljs-title function_">parse</span>(filestring).<span class="hljs-property">type</span>)
.<span class="hljs-title function_">catch</span>(<span class="hljs-function">(<span class="hljs-params">err</span>) =></span> {
<span class="hljs-keyword">if</span> (err?.<span class="hljs-property">code</span> !== <span class="hljs-string">'ENOENT'</span>) <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">error</span>(err);
});
<span class="hljs-comment">// If package.json existed and contained a `type` field with a value, voilà</span>
<span class="hljs-keyword">if</span> (type) <span class="hljs-keyword">return</span> type;
<span class="hljs-comment">// Otherwise, (if not at the root) continue checking the next directory up</span>
<span class="hljs-comment">// If at the root, stop and return false</span>
<span class="hljs-keyword">return</span> dir.<span class="hljs-property">length</span> > <span class="hljs-number">1</span> && <span class="hljs-title function_">getPackageType</span>(<span class="hljs-title function_">resolvePath</span>(dir, <span class="hljs-string">'..'</span>));
}</code> <button class="copy-button">copy</button></pre>
<pre><code class="language-coffee"><span class="hljs-comment"># main.coffee</span>
<span class="hljs-keyword">import</span> { scream } <span class="hljs-keyword">from</span> <span class="hljs-string">'./scream.coffee'</span>
console.log scream <span class="hljs-string">'hello, world'</span>
<span class="hljs-keyword">import</span> { version } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span>
console.log <span class="hljs-string">"Brought to you by Node.js version <span class="hljs-subst">#{version}</span>"</span></code> <button class="copy-button">copy</button></pre>
<pre><code class="language-coffee"><span class="hljs-comment"># scream.coffee</span>
<span class="hljs-keyword">export</span> scream = <span class="hljs-function"><span class="hljs-params">(str)</span> -></span> str.toUpperCase()</code> <button class="copy-button">copy</button></pre>
<p>With the preceding hooks module, running
<code>node --import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register(pathToFileURL("./coffeescript-hooks.mjs"));' ./main.coffee</code>
causes <code>main.coffee</code> to be turned into JavaScript after its source code is
loaded from disk but before Node.js executes it; and so on for any <code>.coffee</code>,
<code>.litcoffee</code> or <code>.coffee.md</code> files referenced via <code>import</code> statements of any
loaded file.</p>
<h5>Import maps<span><a class="mark" href="#import-maps" id="import-maps">#</a></span><a aria-hidden="true" class="legacy" id="module_import_maps"></a></h5>
<p>The previous two examples defined <code>load</code> hooks. This is an example of a
<code>resolve</code> hook. This hooks module reads an <code>import-map.json</code> file that defines
which specifiers to override to other URLs (this is a very simplistic
implementation of a small subset of the "import maps" specification).</p>
<pre><code class="language-js mjs"><span class="hljs-comment">// import-map-hooks.js</span>
<span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs/promises'</span>;
<span class="hljs-keyword">const</span> { imports } = <span class="hljs-title class_">JSON</span>.<span class="hljs-title function_">parse</span>(<span class="hljs-keyword">await</span> fs.<span class="hljs-title function_">readFile</span>(<span class="hljs-string">'import-map.json'</span>));
<span class="hljs-keyword">export</span> <span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">resolve</span>(<span class="hljs-params">specifier, context, nextResolve</span>) {
<span class="hljs-keyword">if</span> (<span class="hljs-title class_">Object</span>.<span class="hljs-title function_">hasOwn</span>(imports, specifier)) {
<span class="hljs-keyword">return</span> <span class="hljs-title function_">nextResolve</span>(imports[specifier], context);
}
<span class="hljs-keyword">return</span> <span class="hljs-title function_">nextResolve</span>(specifier, context);
}</code> <button class="copy-button">copy</button></pre>
<p>With these files:</p>
<pre><code class="language-js mjs"><span class="hljs-comment">// main.js</span>
<span class="hljs-keyword">import</span> <span class="hljs-string">'a-module'</span>;</code> <button class="copy-button">copy</button></pre>
<pre><code class="language-json"><span class="hljs-comment">// import-map.json</span>
<span class="hljs-punctuation">{</span>
<span class="hljs-attr">"imports"</span><span class="hljs-punctuation">:</span> <span class="hljs-punctuation">{</span>
<span class="hljs-attr">"a-module"</span><span class="hljs-punctuation">:</span> <span class="hljs-string">"./some-module.js"</span>
<span class="hljs-punctuation">}</span>
<span class="hljs-punctuation">}</span></code> <button class="copy-button">copy</button></pre>
<pre><code class="language-js mjs"><span class="hljs-comment">// some-module.js</span>
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'some module!'</span>);</code> <button class="copy-button">copy</button></pre>
<p>Running <code>node --import 'data:text/javascript,import { register } from "node:module"; import { pathToFileURL } from "node:url"; register(pathToFileURL("./import-map-hooks.js"));' main.js</code>
should print <code>some module!</code>.</p>
</section><section><h3>Source map v3 support<span><a class="mark" href="#source-map-v3-support" id="source-map-v3-support">#</a></span><a aria-hidden="true" class="legacy" id="module_source_map_v3_support"></a></h3>
<div class="api_metadata">
<span>Added in: v13.7.0, v12.17.0</span>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<p>Helpers for interacting with the source map cache. This cache is
populated when source map parsing is enabled and
<a href="https://sourcemaps.info/spec.html#h.lmz475t4mvbx">source map include directives</a> are found in a modules' footer.</p>
<p>To enable source map parsing, Node.js must be run with the flag
<a href="cli.html#--enable-source-maps"><code>--enable-source-maps</code></a>, or with code coverage enabled by setting
<a href="cli.html#node_v8_coveragedir"><code>NODE_V8_COVERAGE=dir</code></a>.</p>
<pre class="with-26-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-comment">// module.mjs</span>
<span class="hljs-comment">// In an ECMAScript module</span>
<span class="hljs-keyword">import</span> { findSourceMap, <span class="hljs-title class_">SourceMap</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:module'</span>;</code><code class="language-js cjs"><span class="hljs-comment">// module.cjs</span>
<span class="hljs-comment">// In a CommonJS module</span>
<span class="hljs-keyword">const</span> { findSourceMap, <span class="hljs-title class_">SourceMap</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:module'</span>);</code><button class="copy-button">copy</button></pre>
<!-- Anchors to make sure old links find a target -->
<p><a id="module_module_findsourcemap_path_error"></a></p>
<h4><code>module.findSourceMap(path)</code><span><a class="mark" href="#modulefindsourcemappath" id="modulefindsourcemappath">#</a></span><a aria-hidden="true" class="legacy" id="module_module_findsourcemap_path"></a></h4>
<div class="api_metadata">
<span>Added in: v13.7.0, v12.17.0</span>
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<ul>
<li><code>path</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
<li>Returns: <a href="module.html#class-modulesourcemap" class="type"><module.SourceMap></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> Returns <code>module.SourceMap</code> if a source
map is found, <code>undefined</code> otherwise.</li>
</ul>
<p><code>path</code> is the resolved path for the file for which a corresponding source map
should be fetched.</p>
<h4>Class: <code>module.SourceMap</code><span><a class="mark" href="#class-modulesourcemap" id="class-modulesourcemap">#</a></span><a aria-hidden="true" class="legacy" id="module_class_module_sourcemap"></a></h4>
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<span>Added in: v13.7.0, v12.17.0</span>
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<h5><code>new SourceMap(payload[, { lineLengths }])</code><span><a class="mark" href="#new-sourcemappayload--linelengths-" id="new-sourcemappayload--linelengths-">#</a></span><a aria-hidden="true" class="legacy" id="module_new_sourcemap_payload_linelengths"></a></h5>
<ul>
<li><code>payload</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li>
<li><code>lineLengths</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number[]></a></li>
</ul>
<p>Creates a new <code>sourceMap</code> instance.</p>
<p><code>payload</code> is an object with keys matching the <a href="https://sourcemaps.info/spec.html#h.mofvlxcwqzej">Source map v3 format</a>:</p>
<ul>
<li><code>file</code>: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
<li><code>version</code>: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
<li><code>sources</code>: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string[]></a></li>
<li><code>sourcesContent</code>: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string[]></a></li>
<li><code>names</code>: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string[]></a></li>
<li><code>mappings</code>: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
<li><code>sourceRoot</code>: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
</ul>
<p><code>lineLengths</code> is an optional array of the length of each line in the
generated code.</p>
<h5><code>sourceMap.payload</code><span><a class="mark" href="#sourcemappayload" id="sourcemappayload">#</a></span><a aria-hidden="true" class="legacy" id="module_sourcemap_payload"></a></h5>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li>
</ul>
<p>Getter for the payload used to construct the <a href="#class-modulesourcemap"><code>SourceMap</code></a> instance.</p>
<h5><code>sourceMap.findEntry(lineOffset, columnOffset)</code><span><a class="mark" href="#sourcemapfindentrylineoffset-columnoffset" id="sourcemapfindentrylineoffset-columnoffset">#</a></span><a aria-hidden="true" class="legacy" id="module_sourcemap_findentry_lineoffset_columnoffset"></a></h5>
<ul>
<li><code>lineOffset</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The zero-indexed line number offset in
the generated source</li>
<li><code>columnOffset</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The zero-indexed column number offset
in the generated source</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li>
</ul>
<p>Given a line offset and column offset in the generated source
file, returns an object representing the SourceMap range in the
original file if found, or an empty object if not.</p>
<p>The object returned contains the following keys:</p>
<ul>
<li>generatedLine: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The line offset of the start of the
range in the generated source</li>
<li>generatedColumn: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The column offset of start of the
range in the generated source</li>
<li>originalSource: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The file name of the original source,
as reported in the SourceMap</li>
<li>originalLine: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The line offset of the start of the
range in the original source</li>
<li>originalColumn: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The column offset of start of the
range in the original source</li>
<li>name: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
</ul>
<p>The returned value represents the raw range as it appears in the
SourceMap, based on zero-indexed offsets, <em>not</em> 1-indexed line and
column numbers as they appear in Error messages and CallSite
objects.</p>
<p>To get the corresponding 1-indexed line and column numbers from a
lineNumber and columnNumber as they are reported by Error stacks
and CallSite objects, use <code>sourceMap.findOrigin(lineNumber, columnNumber)</code></p>
<h5><code>sourceMap.findOrigin(lineNumber, columnNumber)</code><span><a class="mark" href="#sourcemapfindoriginlinenumber-columnnumber" id="sourcemapfindoriginlinenumber-columnnumber">#</a></span><a aria-hidden="true" class="legacy" id="module_sourcemap_findorigin_linenumber_columnnumber"></a></h5>
<ul>
<li><code>lineNumber</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The 1-indexed line number of the call
site in the generated source</li>
<li><code>columnNumber</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The 1-indexed column number
of the call site in the generated source</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li>
</ul>
<p>Given a 1-indexed <code>lineNumber</code> and <code>columnNumber</code> from a call site in
the generated source, find the corresponding call site location
in the original source.</p>
<p>If the <code>lineNumber</code> and <code>columnNumber</code> provided are not found in any
source map, then an empty object is returned. Otherwise, the
returned object contains the following keys:</p>
<ul>
<li>name: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a> The name of the range in the
source map, if one was provided</li>
<li>fileName: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The file name of the original source, as
reported in the SourceMap</li>
<li>lineNumber: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The 1-indexed lineNumber of the
corresponding call site in the original source</li>
<li>columnNumber: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The 1-indexed columnNumber of the
corresponding call site in the original source</li>
</ul></section>
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