Why a missing JavaScript file can stay missing after you deploy it
I built cachewhy around a small but costly HTTP cache mistake. Imagine a request for /missing.js that returns 404 with Cache-Control: public, max-age=2678400 . That lifetime is 31 days. A browser may reuse the missing response even after the file is deployed. Changing the server header helps new responses, but it does not automatically erase a copy the browser already holds. The command fetches a…
When deploying a new version of a website, developers often overlook the impact a missing JavaScript file can have. A small but significant HTTP cache mistake can leave that file missing even after deployment. To illustrate this, consider a request for a non-existent file like /missing.js. The server responds with a 404 status code and specifies the Cache-Control: public, max-age=2678400, which means the response will be valid for 31 days.
Unfortunately, this long-lived cache entry can persist even after the file has been replaced on the server.
When a browser encounters a 404 response, it may store this empty response in both its private cache (local storage) and any shared caches it may interact with. So, even after the new version of the file is deployed, the browser could still serve the outdated 404 response to users. Changing the server header to indicate a fresh response doesn't automatically clear the browser's cached copy.
To help diagnose this issue, a tool called cachewhy was built. When run against a specific URL, cachewhy fetches the response and details the headers returned. It provides a breakdown of the response stored in both the private and shared cache, along with relevant headers such as Age, Date, and any delay encountered during the request.
The tool also differentiates between no-store and no-cache directives – the former prevents any caching of new responses, while the latter allows storage but requires validation before reuse.
Cachewhy takes into account various factors that can affect cache behavior. It follows redirects and ignores the response body after reading the headers, focusing solely on what the browser currently holds in its cache. The tool uses http-cache-semantics for calculating freshness and age, ensuring accurate results. It distinguishes between no-store and no-cache, explaining how each impacts the caching behavior.
However, cachewhy does have its limitations. It cannot determine what is already in a particular browser's cache, whether a service worker is intercepting the request, which cache key a CDN has chosen, or whether a provider rule is overriding the visible headers. Additionally, a CDN might serve the probe response from its own cache, adding another layer of complexity.
Despite these limitations, cachewhy is a valuable tool for debugging specific URLs and attaching a concise explanation to an issue. It comes with tests for various scenarios, including cached 404 responses, no-store directives, private responses, s-maxage, Age, stale revalidation, redirects, and timeouts. The repository is licensed under MIT, making it freely available for use at https://github.com/Arthur031221/cachewhy.
If you encounter cases where cachewhy's explanation seems too terse or goes beyond the information provided by the headers, feel free to reach out with examples, as the developers would appreciate hearing about real-world scenarios where the tool falls short.
Written by urgent.news from Dev.to's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
