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no-map-set-mutation

Disallow in-place mutation of Map, Set, WeakMap, and WeakSet instances.

Targeted pattern scope​

This rule targets collection mutator calls on known collection instances created via new Map(...), new Set(...), new WeakMap(...), and new WeakSet(...).

What this rule reports​

  • Map and WeakMap mutations via .set(...) and .delete(...)
  • Set and WeakSet mutations via .add(...) and .delete(...)
  • Map and Set mutations via .clear(...)

Why this rule exists​

Mutable collections are a common hidden state channel. Mutating an existing Map/Set makes data flow harder to reason about, especially when references are shared across modules or async paths.

❌ Incorrect​

const indexById = new Map<string, number>();
indexById.set("u1", 1);

const activeTags = new Set<string>();
activeTags.add("new");

✅ Correct​

const indexById = new Map<string, number>([["u1", 1]]);

const activeTags = new Set<string>(["new"]);

Additional examples​

// ❌ Mutates an existing shared cache
const cache = new WeakMap<object, number>();
cache.set(target, 1);

// ✅ Creates a new cache value from previous entries
const nextCache = new Map(existingCache);
nextCache.set(target, 1);

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-map-set-mutation": "error",
        },
    },
];

When not to use it​

If your codebase intentionally uses long-lived mutable caches and mutation-heavy data structures for performance-critical internals, this rule may be too strict. In that case, keep it enabled for application/business logic and override it in narrowly scoped infrastructure files.

Rule catalog ID: R915

Further reading​