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Import maps can keep a JavaScript module’s import name stable while its file URL changes. Instead of editing every file that refers to a hashed asset, you can update the mapping in the HTML document. That can turn a 40-file edit scenario into one map change—but it does not update application code or make incompatible library versions work together.
How an import map can replace repeated filename edits
A browser needs a URL to load a JavaScript module. The text between from and the quotes in an import statement—or passed to import()—is its module specifier. An import map tells the browser which URL that specifier should resolve to. MDN describes the feature this way: “Import maps allow developers to control how the browser resolves module specifiers when importing JavaScript modules.”
Without a map, code may refer directly to a file whose name includes a build hash. If that hash changes, each importer containing the old filename may need editing. With a map, importers can use one stable specifier while the map points it at the current hashed file. The “40 files” in this article’s title is a scenario, not a measured average or a published statistic about projects.
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import { formatDate } from "./format-date.a81c9f.js";If a build replaces that asset with a differently hashed filename, importers that contain the old URL must be updated or otherwise made to refer to the new file.
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After: importers use a stable name
<script type="importmap">
{
"imports": {
"format-date": "/assets/format-date.a81c9f.js"
}
}
</script>
<script type="module" src="/app.js"></script>import { formatDate } from "format-date";When the asset’s hash changes, keep the specifier in JavaScript and update the URL in the map. MDN’s import-map guide demonstrates this hashed-filename pattern.
Change the mapping without changing every import
For a single target shared across the document, add or edit the specifier under the map’s top-level imports object. The key is the stable name used by modules; the value is the URL the browser should load. Use a URL that actually exists at deployment time.
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- Choose a stable specifier. Replace direct references to a changeable asset URL with the same module name in the relevant static or dynamic imports.
- Add the map to the HTML document. Put a
<script type="importmap">element containing valid JSON before module scripts that rely on its entries. - Point the entry at the current asset. When a build changes the hashed filename, update the map’s value to the deployed URL.
- Deploy the map and target asset together. The map controls resolution; it does not publish the file or guarantee that the URL is reachable.
- Check the library’s API compatibility. If a new dependency version changes the API or behavior your code uses, application changes may still be necessary. An import map redirects names to URLs; it does not adapt code to a new version.
The map must be processed before dependent module scripts load. A later map cannot reliably redirect a specifier that the browser has already resolved. See MDN’s documentation on import-map timing and behavior.
Choose global imports or scoped mappings
A top-level imports mapping is the simplest option when one target should serve all importers. Use scopes when different parts of an application need different targets for the same specifier. Scope matching depends on the importing module’s URL path, so the directory structure and scope prefixes matter.
| Decision | Global imports |
scopes |
|---|---|---|
| Best fit | One target should serve the document | Different application areas need different versions |
| Resolution | The specifier resolves to the global target unless a scope applies | The importing module’s URL path can select a different mapping |
| Main consideration | Simple central update | More configuration; scope paths must match as intended |
For example, a map can route a shared package name to one version globally and use another target for imports originating under a particular application path. This allows different parts of an application to use different versions; it does not make one global mapping point to two targets in the same context. The WHATWG HTML Standard describes scoped resolution, and MDN’s JavaScript modules guide covers scopes and module behavior.
Know where import maps apply—and where they do not
- They apply to module specifiers in static imports and dynamic
import()calls in the document. - They do not rewrite a module script’s
srcattribute. That attribute is a URL, not an import specifier resolved through the map. - They do not apply to modules loaded in workers or worklets. Those contexts need their own loading approach.
- They do not guarantee compatibility. A resolved URL can load successfully while the application still fails because the new module’s interface differs.
MDN documents the feature’s scope and the HTMLScriptElement.supports("importmap") feature-detection method in its modules guide. If your target includes browsers without the needed support, plan a compatible fallback rather than assuming the mapping will be honored.
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Check browser support for the feature you use
Ordinary import maps are broadly supported, but support for multiple import-map elements is a separate, narrower capability. MDN labels import maps Baseline Widely Available since March 2023 while cautioning that some parts have varying support. The feature label is not a promise for every import-map behavior or legacy browser.
Web Platform DX lists these historical first-support versions for import maps: Chrome 89 (March 2, 2021), Edge 89 (March 4, 2021), Firefox 108 (December 13, 2022), Safari 16.4 (March 27, 2023), and Safari on iOS 16.4 (March 27, 2023). These are minimum versions reported by that reference, not current latest releases.
Multiple import maps have more limited support: Web Platform DX lists Chrome 133 (February 4, 2025) and Safari 18.4 (March 31, 2025), and reports no Firefox support. Its multiple import maps compatibility entry classifies the feature as Limited availability. Check your actual browser matrix before relying on multiple map elements. A single map avoids depending on that newer capability. The general import-map version data is in the Web Platform DX import maps entry.
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