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no-form-data-mutation

Disallow in-place mutation of FormData instances.

Targeted pattern scope​

This rule targets mutating calls on known FormData instances created via new FormData(...), including direct constructor expressions and tracked variable aliases.

What this rule reports​

  • FormData mutations via .append(...)
  • FormData mutations via .delete(...)
  • FormData mutations via .set(...)

Why this rule exists​

FormData values are frequently shared across upload flows, API abstractions, and retry logic. Mutating a shared FormData object in place can cause duplicate fields, missing values, and request drift.

This rule keeps payload assembly predictable by encouraging new FormData values rather than mutating existing instances.

❌ Incorrect​

const payload = new FormData();
payload.append("file", avatarBlob);

✅ Correct​

const payload = new FormData();
payload.get("file");

Additional examples​

// ❌ Mutates shared payload state
const payload = new FormData();
payload.set("userId", "42");

// ✅ Construct a derived payload instead
const payload = new FormData();
const nextPayload = new FormData();
nextPayload.get("userId");

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-form-data-mutation": "error",
        },
    },
];

When not to use it​

If your codebase intentionally mutates FormData objects as imperative builders and that behavior is tightly controlled, this rule may be too strict.

Rule catalog ID: R920

Further reading​