If a site keeps loading old JavaScript after deployment, first find out whether the page is referencing an old bundle URL or whether a cache is returning old bytes for the current URL. The durable fix is to give changed JavaScript a new, content-hashed URL and make stable HTML revalidate; use a CDN purge to remove an already-stale edge response, not as a substitute for sound cache rules.
Identify which layer is serving the old code
Several separate stores can affect what a visitor sees: the browser’s HTTP cache, a CDN, cached HTML or an application shell, and a service worker’s Cache Storage. These layers can overlap, so clearing a visitor’s browser cache may hide the symptom without fixing the update path.
- Reproduce the problem and inspect requests. Open browser developer tools, select the Network panel, reload the affected page, and inspect the document and JavaScript requests. Record the script URL referenced by the loaded HTML, the JavaScript request URL and status, the response body and Content-Type, and the Cache-Control, ETag, and any CDN cache-status headers. This helps distinguish old HTML that points to an old bundle from a stale response for the current bundle URL. See MDN’s HTTP caching guide and Cloudflare’s cache response documentation.
- Compare the served bundle URL with the current release. Check whether the build produced a new content hash and whether the current HTML or manifest references that URL. If the JavaScript changed but its URL stayed the same, fix the build or deployment process so changed content gets a different URL. If the URL changed but the page still references the old one, investigate HTML or app-shell caching.
- Check the actual response headers. Compare browser-facing headers with your intended cache policy. HTML at a stable URL should revalidate; long-lived immutable caching belongs on assets whose URLs change when their bytes change.
- Inspect CDN rules and cache keys. Look for custom rules that cache HTML, override origin headers, or vary the cache key. Purging can remove a bad CDN object, but it does not correct a stable JavaScript URL that was overwritten in place or old HTML held elsewhere.
- Check for a service worker. A service worker can intercept requests and use Cache Storage independently of the browser’s HTTP cache and the CDN. Test the page while it is controlled by the worker, and review its cache versioning and activation cleanup. A replacement worker may wait while open pages remain controlled by the old one, so do not remove cached resources that those pages may still need.
- Verify the result with a fresh page load. Confirm that the document references the current bundle URL, the response contains the expected JavaScript and headers, and the relevant layer shows a current response, cache miss, or revalidation as appropriate. A CDN purge does not necessarily change a browser cache, service-worker store, or already-open document.
Use different cache policies for HTML and JavaScript
A cache may reuse a response while it is fresh. Once it is stale, validators such as ETag let a browser ask whether the representation changed; if not, the server can respond with 304 Not Modified. That saves retransmitting an unchanged body, but it only works as intended when the page and asset URLs reflect the release strategy.
Content-hashed JavaScript assets
When a file’s contents change, embed a content hash or version in its filename or URL. A changed file then has a new URL, while the old URL continues to identify the old bytes. For a public, content-hashed JavaScript asset, a common baseline is:
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Cache-Control: public, max-age=31536000, immutable
MDN describes versioned URLs as the cache-busting pattern and gives a one-year freshness example for immutable assets. This is a configuration example, not a requirement for every host; make sure your build actually changes the URL when the file changes. See MDN’s Cache-Control reference.
HTML and other stable entry points
The main HTML document often stays at the same URL while its script references change. A common baseline is:
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Cache-Control: no-cache
no-cache allows storage but requires revalidation before reuse; it does not mean “do not store.” ETag or Last-Modified validators can let the server confirm that an unchanged document is still current without sending its full body. For personalized responses, MDN’s example adds private. Hosting platforms, application frameworks, CDN rules, and personalization can require adjustments.
Choose a release strategy that keeps references coherent
Versioned assets plus revalidated HTML are generally the more reliable static-asset strategy. Changed code receives a new URL, while updated HTML directs clients to it. Fixed-name assets can work with frequent revalidation, but every cache layer must correctly check for changes before reusing them.
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| Approach | Update discovery | Cache behavior | Deployment considerations |
|---|---|---|---|
| Versioned assets plus revalidated HTML | Changed JavaScript gets a new URL; refreshed HTML points to it. | Versioned assets can be cached for a long time; stable HTML revalidates. | Requires content fingerprinting and a deployment that publishes HTML and referenced assets coherently. Keep old hashed files available long enough for already-open pages that may still request them. |
| Mutable fixed-name assets with frequent revalidation | The same URL must be checked for updates before reuse. | Frequent validation can reduce the benefit of long-lived caching. | Every relevant browser and CDN layer must honor the revalidation policy; a stale HTML or asset response can still disrupt the update path. |
The first approach follows MDN’s guidance to change the URL rather than modify a resource in place. A managed CDN purge can help with a known stale object, but invalidation steps depend on the provider.
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Cloudflare Pages
Cloudflare Pages documents that custom caching on a custom domain may cause stale assets after deployment. Its documented action for stale assets is Caching > Configuration > Purge Everything. Pages also documents browser revalidation with ETag/If-None-Match and a one-week asset TTL at a data center, while noting that assets may disappear at any time. Those behaviors apply to Pages and should not be assumed for other CDN products. See Cloudflare Pages’ cache purge guidance.
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Cloudflare Workers static assets
Cloudflare documents defaults for static assets that include Cache-Control: public, max-age=0, must-revalidate, ETag, and CF-Cache-Status. A _headers file can override headers on static assets, but it does not apply to responses generated by Worker code. If the response is server-rendered or Worker-first, set the relevant headers in the Worker rather than assuming a static-asset header rule will affect it. See Cloudflare Workers static asset headers.
Next.js
Next.js documents public, max-age=31536000, immutable for hashed immutable assets. Its guidance also notes that a CDN or reverse proxy needs to respect cache directives and cache-key variation. Confirm that advice against your deployment architecture and the assets in question; it is specific to Next.js-generated assets and its documented hosting context. See Next.js headers configuration.
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Common mistakes that keep the problem coming back
- Giving a stable filename a long immutable lifetime. If bytes change without the URL changing, caches have no URL-based signal that they should fetch a different object.
- Purging the CDN but leaving HTML stale. The page can continue to reference an old bundle URL even if the current bundle is available at a new one.
- Changing headers in the wrong layer. A hosting platform’s static-asset rules may not apply to HTML or to responses generated dynamically by application or Worker code.
- Assuming every cache is the CDN. Browser HTTP caches, service-worker Cache Storage, open tabs, and edge caches have different lifecycles and controls.
- Removing old hashed files immediately. A page opened before deployment may still refer to an older bundle. Retaining previous versioned assets through the relevant transition reduces the chance that such a page requests a missing file.
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