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If your chunks differ only by their #fragment, the Cache API treats them as one entry. The fragment is excluded when the cache compares URLs, so every put() to a URL such as /book.html#chunk-2 lands on the same key as /book.html#chunk-1, and the later write replaces the earlier response. The fix is to put the chunk identity into a part of the URL that matching uses (the path or the query string), or to store chunks in a key/value store your code manages.
Why the fragment never separates cache entries
A fragment is the part of a URL after #. Browsers do not send it to the server, and the Cache API does not use it to tell resources apart. The W3C Service Workers specification defines the matching step this way: “If queryURL does not equal cachedURL with the exclude fragment flag set, then return false.” In other words, the comparison deliberately drops the fragment before deciding whether two URLs match. W3C Service Workers specification
That rule is intentional. A fragment usually names a position inside one document, so the cache is not meant to store each in-page anchor as a separate resource. It becomes a problem when your application uses fragments as data identifiers, because the cache has no way to see the difference.
How the collision happens
Consider a worker that stores each chunk of a long document under its own hash URL:
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const cache = await caches.open('book-v1');
for (const n of [1, 2, 3]) {
const url = `/book.html#chunk-${n}`;
const res = await fetch(`/book.html`); // the fragment is never sent to the server
await cache.put(url, res.clone()); // all three writes share one cache key
}
const hit = await cache.match('/book.html#chunk-1');
// hit is the response stored last: the chunk-3 content, not chunk-1The fetches themselves may succeed, and the cache reports no error. Each put() adds a request/response pair, and when the effective key is the same, the newer pair takes the place of the older one. MDN Cache The result looks like a lost-write bug, but the cache is behaving as specified.
The symptoms usually show up as one of these:
- Every chunk request returns identical content, matching whichever chunk was stored last.
- Cache Storage shows fewer entries than you expected, often just one for the whole set.
- The bug appears only offline or after the service worker serves from cache, because online requests go to the network.
Query strings follow different rules
Query strings are part of matching, unlike fragments. That makes them a workable place to encode chunk identity, but one option can undo the distinction. The ignoreSearch option of Cache.match() defaults to false. When you set it to true, query variants match as though their query strings were absent, so /book.html?chunk=1 and /book.html?chunk=2 would collapse into one lookup. MDN Cache.match()
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Check that option first if you already moved identity into the query string and the collision persists.
Fix options
There are three practical approaches. Choose by whether each chunk must be a fetchable response that the browser can return for a normal request.
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Option 1: put the chunk identity in the path
This is the most natural choice when chunks are real resources. Each chunk gets its own path, and matching treats those paths as distinct keys.
const chunkUrl = (n) => `/book/chunk/${n}.html`;
for (const n of [1, 2, 3]) {
const res = await fetch(chunkUrl(n));
await cache.put(chunkUrl(n), res.clone());
}
const hit = await cache.match(chunkUrl(1)); // returns chunk 1Your server or build step must generate those paths, and each one must exist as a response you can fetch.
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Option 2: put the chunk identity in the query string
Use this when the chunk is served from one route with a parameter. Leave ignoreSearch at its default so the query remains part of matching.
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await cache.put(chunkUrl(2), (await fetch(chunkUrl(2))).clone());
const hit = await cache.match(chunkUrl(2)); // ignoreSearch is false by defaultDo not pass { ignoreSearch: true } to match() for these keys. That option would reduce every chunk to /book.html.
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Option 3: use an application-managed key/value store
If the chunks are data rather than fetchable responses, store them in a key/value store that your code controls, such as IndexedDB. You define the key, so no URL rule applies. You also become responsible for versioning, cleanup and storage limits, which the Cache API partly handles for you through its request/response model.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Debugging checklist
- Log the full URL you pass to
put()and tomatch(), including the fragment. - Remove the fragment from each URL in your head or in code. If two URLs become identical, the fragment is the cause.
- Open DevTools, go to the Application tab, and expand Cache Storage to list the entries actually stored for your origin.
- Confirm that
match()is not called withignoreSearch: truewhen chunks differ only by query. - Move the identity into the path or query, clear the old cache by name, and repeat the test.
Lifecycle cautions
Fixing the key does not finish the job. The Cache API stores what you tell it to store and nothing more:
- Entries do not expire or refresh automatically, and the API does not honor HTTP caching headers. Your code must delete outdated chunks.
- Version cache names, such as
book-v1, when the worker’s chunking logic changes, and delete old caches during activation. - Browsers may evict an origin’s cache data under storage pressure, so chunks should be treated as recoverable and fetched again when missing.
These points come from the MDN Cache reference. MDN Cache The behavior described here comes from the specification and API reference, not from a browser test of this exact project, so confirm the result in your own target browsers before you rely on it.
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