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no-cache-api-mutation

Disallow in-place mutation of Service Worker Cache API state.

Targeted pattern scope​

This rule targets mutating methods on CacheStorage (caches) and Cache instances derived from caches.open(...).

What this rule reports​

  • caches.delete(...)
  • cache.add(...)
  • cache.addAll(...)
  • cache.put(...)
  • cache.delete(...)

Why this rule exists​

Cache API writes mutate shared offline/network state that may be observed across requests, tabs, and worker lifecycles. In-place mutations can make behavior nondeterministic and hard to reason about.

This rule encourages immutable cache derivation flows instead of direct mutable writes.

❌ Incorrect​

caches.delete("v1");

✅ Correct​

caches.keys();

Additional examples​

// ❌ Mutates cache entries in place
async function hydrate(req: Request, res: Response): Promise<void> {
    const cache = await caches.open("v1");
    await cache.put(req, res);
}

// ✅ Read-only cache access
async function read(req: Request): Promise<Response | undefined> {
    const cache = await caches.open("v1");
    return await cache.match(req) ?? undefined;
}

ESLint flat config example​

import immutable from "eslint-plugin-immutable-2";

export default [
    {
        files: ["**/*.{js,mjs,cjs,ts,mts,cts,tsx}"],
        plugins: { immutable },
        rules: {
            "immutable/no-cache-api-mutation": "error",
        },
    },
];

When not to use it​

If your service worker intentionally performs imperative cache writes in a dedicated caching subsystem, this rule may be too strict for those modules.

Rule catalog ID: R933

Further reading​