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no-url-search-params-mutation

Disallow in-place mutation of URLSearchParams instances.

Targeted pattern scope​

This rule targets mutating calls on values created from new URLSearchParams(...), including direct constructor expressions and tracked variable aliases.

What this rule reports​

  • URLSearchParams mutations via .append(...)
  • URLSearchParams mutations via .delete(...)
  • URLSearchParams mutations via .set(...)
  • URLSearchParams mutations via .sort(...)

Why this rule exists​

URLSearchParams values are frequently passed between routers, HTTP clients, and link builders. In-place updates silently mutate shared references and can leak unintended query changes.

This rule encourages immutable query composition by preventing mutation after construction.

❌ Incorrect​

const params = new URLSearchParams("page=1");
params.set("page", "2");

✅ Correct​

const params = new URLSearchParams("page=1");
params.get("page");

Additional examples​

// ❌ Mutates shared query instance
const base = new URLSearchParams("debug=false");
base.append("user", "42");

// ✅ Creates a derived query object
const base = new URLSearchParams("debug=false");
const next = new URLSearchParams(base.toString());
next.get("debug");

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-url-search-params-mutation": "error",
        },
    },
];

When not to use it​

If your code intentionally uses mutable query-builder objects and mutability is a deliberate design choice, this rule may be too strict.

Rule catalog ID: R918

Further reading​