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Merge pull request #1529 from meirzamoodle/MDL-87922_import_map
MDL-87922 [docs] Add guide for JavaScript import maps in Moodle
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‎docs/guides/javascript/react/importmap.md‎

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---
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title: Aliasing and Import Maps
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tags:
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- Javascript
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- ESM
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- React
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- Import Maps
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description: How Moodle uses native browser import maps to resolve bare module specifiers to real URLs at runtime, including built-in specifiers, custom entries, and the ESM serving endpoint.
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---
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Moodle uses native browser import maps as the mechanism for resolving bare module specifiers
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(for example `react` or `@moodle/lms/`) to real URLs at runtime. This replaces the need for
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bundler-specific alias configuration and allows Moodle components to write standard ESM
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`import` statements that work directly in the browser.
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## What is an Import Map?
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An import map is a JSON object, embedded in the page as a `<script type="importmap">` tag,
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that tells the browser how to resolve bare specifiers used in `import` statements.
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```html
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<script type="importmap">{
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"imports": {
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"@moodle/lms/": "http://localhost/MDL-87922/r.php/core/esm/-1/@moodle/lms/",
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"@moodlehq/design-system": "http://localhost/MDL-87922/r.php/core/esm/-1/@moodlehq/design-system",
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"react": "http://localhost/MDL-87922/r.php/core/esm/-1/react",
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"react/": "http://localhost/MDL-87922/r.php/core/esm/-1/react/",
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"react-dom": "http://localhost/MDL-87922/r.php/core/esm/-1/react-dom",
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"react-dom/": "http://localhost/MDL-87922/r.php/core/esm/-1/react-dom/"
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}
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}</script>
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```
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With this map in place, any ES module on the page can write:
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```js
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import React from 'react';
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import { jsx } from 'react/jsx-runtime';
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import { someUtil } from '@moodle/lms/core/utils';
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```
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…and the browser resolves the specifier to the correct URL without any bundler step at runtime.
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## How Moodle generates the Import Map
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The import map is built and injected into the page automatically by
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`page_requirements_manager::get_import_map()`, which is called during the page `<head>`
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render phase.
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### The `import_map` class
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**`core\output\requirements\import_map`** is the single source of truth for all
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specifier → URL mappings and specifier → filesystem path mappings. It implements
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`JsonSerializable` so it can be written directly into the page as JSON, and it is also
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consulted by the ESM controller when serving files.
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Key responsibilities:
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- Holds a list of specifier → entry mappings.
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- Accepts a **default loader URL** (the ESM serving endpoint) which is used to derive
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concrete URLs for entries that use the default loader.
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- Provides `add_import()` to register additional specifiers, or to override the built-in
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ones, from a `pre_render` hook.
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#### Built-in specifiers
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The following specifiers are registered by default in `add_standard_imports()`:
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| Specifier | URL in import map | Filesystem path |
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|---|---|---|
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| `@moodle/lms/` | `{loaderBase}@moodle/lms/` | Component's `js/esm/build/` directory |
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| `@moodlehq/design-system` | `{loaderBase}@moodlehq/design-system` | `lib/js/bundles/design-system.js` |
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| `react` | `{loaderBase}react` | `lib/js/bundles/react/react.js` |
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| `react/` | `{loaderBase}react/` | `lib/js/bundles/react/` (prefix) |
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| `react-dom` | `{loaderBase}react-dom` | `lib/js/bundles/react-dom/react-dom.js` |
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| `react-dom/` | `{loaderBase}react-dom/` | `lib/js/bundles/react-dom/` (prefix) |
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The `react/` prefix entry covers all sub-specifiers such as `react/jsx-runtime`.
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:::note
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`{loaderBase}` is the base URL of the ESM serving endpoint — the URL that
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`page_requirements_manager::get_import_map()` sets as the default loader. In practice it looks like
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`https://example.com/esm/12345/`, where `12345` is the JS revision number returned by
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`page_requirements_manager::get_jsrev()`. All specifier URLs in the import map are built by
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appending the bare specifier to this base URL.
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The revision value `/esm/-1/` means the revision is invalid or in development mode. When the
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revision is `-1`, the ESM controller applies short-lived cache headers so the browser re-fetches
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the file on every page load instead of serving a stale cached copy.
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:::
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#### Adding a custom specifier
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You can extend the import map from a `pre_render` hook before the page is rendered:
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```php
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use core\output\requirements\import_map;
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// Fetch the shared singleton from the DI container.
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$importmap = \core\di::get(import_map::class);
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// Map 'my-lib' to an absolute URL (e.g. a CDN). Used verbatim in the import map.
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$importmap->add_import('my-lib', loader: new \core\url('https://cdn.example.com/my-lib.js'));
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// Map '@myplugin/' using a filesystem path relative to $CFG->root.
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// The URL in the import map will be '{loaderBase}@myplugin/'.
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// The ESM controller uses $path to locate the file on disk.
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$importmap->add_import('@myplugin/', path: 'local_myplugin/js/esm/build');
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```
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The `add_import()` signature is:
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```php
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public function add_import(
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string $specifier,
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?\core\url $loader = null,
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?string $path = null,
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bool $loadfromcomponent = false,
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): void
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```
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- **`$specifier`** — The bare specifier string used in `import` statements
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(e.g. `react`, `@moodle/lms/`).
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- **`$loader`** — An absolute `\core\url`. When provided, this URL is written directly
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into the import map and `$path` is ignored.
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- **`$path`** — A filesystem path relative to `$CFG->root`, used by the ESM controller
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to locate the file on disk. Has no effect on the URL written into the import map
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(the URL always uses `{loaderBase}{specifier}`).
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- **`$loadfromcomponent`** — When `true`, the specifier is treated as a
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`<component>/<module>` prefix and resolved to the component's `js/esm/build/`
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directory on disk. Used internally for `@moodle/lms/`.
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## The ESM serving endpoint
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All ESM files are served by `core\route\controller\esm_controller::serve`, registered
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under the route:
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```
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/esm/{revision:[0-9-]+}/{scriptpath:.*}
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```
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The controller is intentionally thin: it delegates all resolution to the `import_map`
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registry, which is the single source of truth. For a given `scriptpath` it calls
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`import_map::get_path_for_script()`, which:
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1. Sorts entries longest-key-first so a more-specific prefix wins
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(e.g. `react/` is matched before `react`).
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2. Finds the first entry whose specifier is a prefix of the requested path.
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3. Returns the absolute filesystem path to the file, or `null` if no entry matches.
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If the resolved path exists on disk the file is served; otherwise a 404 is returned.
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### Resolving component modules
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Entries registered with `$loadfromcomponent = true` (i.e. `@moodle/lms/`) are resolved
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differently: the path after the prefix is split into `<component>/<module>`, the component
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directory is looked up via `core\component`, and the file is resolved to:
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```
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<component_directory>/js/esm/build/<module>.js
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```
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For example, `@moodle/lms/mod_book/viewer` resolves to
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`<dirroot>/mod/book/js/esm/build/viewer.js`.
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## HTTP caching
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The controller applies the following caching strategy:
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| Revision | Behaviour |
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|---|---|
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| Valid (positive integer) | Long-lived immutable cache headers + ETag. Returns `304 Not Modified` when the client already has the file cached. |
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| `-1` (development / invalid) | Short-lived cache headers. Forces the browser to re-fetch on every page load. |
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The revision value comes from `page_requirements_manager::get_jsrev()` and changes
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whenever JavaScript files are updated, automatically busting caches.
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## Writing a component React module
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To expose a React module through the import map:
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1. Build your TypeScript/React source (`.ts` or `.tsx`) into a compiled JavaScript file
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at `<component_directory>/js/esm/build/<module>.js` using the Moodle build tools.
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See the React build tooling guide for details.
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2. Import it in browser code using the `@moodle/lms/` scope:
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```js
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import { BookViewer } from '@moodle/lms/mod_book/viewer';
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```
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The import map translates `@moodle/lms/mod_book/viewer` → the ESM endpoint URL for
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`@moodle/lms/mod_book/viewer`, which the controller resolves to
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`<dirroot>/mod/book/js/esm/build/viewer.js`.
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:::note
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The `@moodle/lms/` specifier ends with a trailing slash. This is the standard import map
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convention for **package scopes**: any import that begins with `@moodle/lms/` is prefixed
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with the loader base URL, allowing the entire namespace to be served from one endpoint
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without registering each module individually.
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:::
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## Overriding a built-in specifier
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You can replace any of the default specifiers in a `pre_render` hook. For example, to swap
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in a local React build during development:
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```php
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$importmap = \core\di::get(\core\output\requirements\import_map::class);
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$importmap->add_import(
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'react',
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loader: new \core\url('/local/devtools/react-debug.js'),
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);
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```
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Calling `add_import()` with the same specifier twice overwrites the previous entry, so
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ordering matters when multiple hooks are involved.
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## See also
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- [JavaScript Modules](../modules.md) — AMD and ESM module authoring in Moodle.
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- [MDN: Import maps](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script/type/importmap)
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- [HTML spec: import maps](https://html.spec.whatwg.org/multipage/webappapis.html#import-maps)

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