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no-regexp-lastindex-mutation

Disallow mutation of RegExp#lastIndex.

Targeted pattern scope​

This rule targets writes to .lastIndex on known regular-expression instances (regex literals, RegExp(...), new RegExp(...), and aliases).

What this rule reports​

  • Assignments to regex.lastIndex
  • Update expressions on regex.lastIndex (for example regex.lastIndex++)
  • Deletions of regex.lastIndex
  • Computed-property equivalents like regex["lastIndex"] = ...

Why this rule exists​

RegExp#lastIndex is mutable hidden state that changes matching behavior across calls. Manual writes to it can introduce nondeterministic behavior and coupling between call sites.

This rule enforces immutable regex flow by disallowing explicit lastIndex mutation.

❌ Incorrect​

const matcher = /foo/;
matcher.lastIndex = 0;

✅ Correct​

const matcher = /foo/;
matcher.source;

Additional examples​

// ❌ Mutates regex cursor state
const matcher = new RegExp("foo", "g");
matcher.lastIndex++;

// ✅ Uses regex without explicit cursor mutation
const matcher = new RegExp("foo", "g");
matcher.test("foo");

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-regexp-lastindex-mutation": "error",
        },
    },
];

When not to use it​

If your code intentionally manipulates lastIndex for specialized parsing pipelines, this rule may be too strict for that subsystem.

Rule catalog ID: R931

Further reading​