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no-typed-array-mutation

Disallow in-place mutation of JavaScript TypedArray instances.

Targeted pattern scope​

This rule targets mutating calls on known TypedArray instances created via constructors such as Uint8Array, Int32Array, Float64Array, and related TypedArray variants.

What this rule reports​

  • TypedArray mutations via .copyWithin(...)
  • TypedArray mutations via .fill(...)
  • TypedArray mutations via .reverse(...)
  • TypedArray mutations via .set(...)
  • TypedArray mutations via .sort(...)

Why this rule exists​

TypedArrays are mutable by design. Once shared across code paths, in-place mutation methods silently alter state and can cause hard-to-debug data corruption in binary protocols, parsing pipelines, and cache layers.

This rule keeps byte-oriented data flow predictable by discouraging mutation after initialization.

❌ Incorrect​

const bytes = new Uint8Array([1, 2, 3]);
bytes.fill(0);

✅ Correct​

const bytes = new Uint8Array([1, 2, 3]);
const copy = bytes.slice();
copy[0];

Additional examples​

// ❌ Mutates typed array in place
const packet = new Uint8Array([3, 2, 1]);
packet.reverse();

// ✅ Derives a new view instead of mutating source
const packet = new Uint8Array([3, 2, 1]);
const sortedPacket = packet.slice().sort();

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-typed-array-mutation": "error",
        },
    },
];

When not to use it​

If your project intentionally mutates TypedArrays in tightly scoped performance-critical internals, this rule may be too strict for those files.

Rule catalog ID: R917

Further reading​