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An import map lets JavaScript modules keep a stable import name while the browser loads the current file URL behind it. If dozens of files refer to the same module, you can often change its mapped URL in one place instead of updating each importer. The “40 files” in the headline is a scenario, not a measured or typical result.
What an import map changes
A module specifier is the text used in a static import statement or a dynamic import() call. Browsers need a URL to load the module. An import map supplies the browser with a lookup from a specifier to a URL. MDN describes the feature this way: “Import maps allow developers to control how the browser resolves module specifiers when importing JavaScript modules.”
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For example, code can use the stable specifier app-library while the map points it to a versioned, hashed file such as /assets/app-library-a1b2c3.js. If a build produces a new filename, update the map target; importing code can continue using app-library.
How one URL change can replace many edits
Without indirection, importers may contain the physical filename directly. When that filename changes, every file that refers to it may need an edit. With an import map, those files refer to the stable specifier instead, and the map associates that name with the current asset URL.
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Choose a stable specifier for the dependency, such as
app-library. -
Add a mapping from that specifier to the dependency’s current URL in the HTML document.
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Use the stable specifier in the JavaScript modules that import the dependency.
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When the asset URL changes, update the mapping to the intended new URL and deploy the matching asset.
This reduces URL maintenance; it does not publish the new asset or guarantee that a new library version is compatible with existing code. If the new version changes APIs or behavior, source changes may still be necessary.
Choose global or scoped mappings
Use a global mapping for one target
The top-level imports object maps a specifier for the document. It is the straightforward choice when all importers should resolve a name to the same module URL.
Use scopes when different parts need different versions
The scopes object lets resolution vary according to the URL path of the importing module. This can allow separate parts of an application to use different versions of the same dependency. A scope is conditional on the importer’s path; a single global mapping cannot point the same specifier to two targets in the same context. The WHATWG HTML Standard describes scoped resolution, and MDN’s JavaScript modules guide covers its use.
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The browser must process the import map before loading module scripts that depend on its entries. A map added after a module specifier has already been resolved cannot reliably redirect that import. Keep the map early in the document, ahead of the dependent module scripts. See MDN’s import-map reference for the element and its behavior.
Know what import maps do not cover
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An import map resolves module specifiers in static and dynamic JavaScript imports; it does not rewrite the
srcvalue of a<script type="module">element.Rank #4
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Document import maps do not apply to modules loaded into workers or worklets.
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A mapping changes URL resolution, not the dependency’s code, API, or runtime behavior.
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MDN documents HTMLScriptElement.supports("importmap") as a feature-detection method. If your audience includes browsers that do not support import maps, use a compatibility or fallback strategy appropriate to the application.
Check browser support for the exact feature
Ordinary import maps have broad browser support. MDN labels them Baseline Widely Available and says they have been available across browsers since March 2023, while noting that some parts vary. Historical minimum versions listed by Web Platform DX are Chrome 89, Edge 89, Firefox 108, Safari 16.4, and Safari on iOS 16.4.
Multiple import-map elements are a separate, more limited capability, not something to infer from ordinary import-map support. Web Platform DX lists support for multiple maps in Chrome 133 and Safari 18.4, but not Firefox; it labels the feature Limited availability. Check the browser matrix for your users before relying on multiple map elements. A single map avoids depending on that newer capability.
When this approach is useful
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Use a global
importsmapping when one version or asset target should serve the whole document.Free tools Windows power users keep installed
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Use scoped mappings when distinct importer paths need separate versions and you can maintain the URL scope rules.
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Keep direct URLs where they are needed outside document module resolution, such as worker or worklet loading paths.
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