如何检查资源是否被浏览器从 Javascript 缓存？

Question

Is there a way to check if a resource is cached by the browser without downloading the resource? Most of the questions and answers are old and I believe things have changed now and there are better ways.

Answer 1

You can leverage the fetch API and it's correspondent AbortController to achieve this functionality with a margin of error (expected false negatives).

It goes like this, fetch the required resource with a signal attached. Abort in a small amount of time eg. 4ms. If the fetch is returned in the short amount of time it's absolutely cached. If the fetch was aborted, it's probably not cached. Here's some code:

      async checkImageCached(url, waitTimeMs = 4) {
        const ac = new AbortController()
        const cachePromise = fetch(url, {signal: ac.signal})
          .then(() => true)
          .catch(() => false)
        setTimeout(() => ac.abort(), waitTimeMs)
        return cachePromise
      }

Answer 2

Currently only works on Firefox, unfortunately, but another option is to use only-if-cached , though it only works for requests on the same origin (because it requires mode: 'same-origin' as well):

fetch(urlOnSameOrigin, { mode: 'same-origin', cache: 'only-if-cached'})
  .then((response) => {
    if (response.ok) {
      console.log('Cached');
    } else if (response.status === 504) {
      console.log('Not cached');
    }
  })
  .catch(() => {
    console.log('Network error');
  });

only-if-cached — The browser looks for a matching request in its HTTP cache.

• If there is a match, fresh or stale, it will be returned from the cache.

• If there is no match, the browser will respond with a 504 Gateway timeout status.