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no-param-reassign

Disallow function parameter reassignment outside the first expression statement.

Targeted pattern scope​

This rule checks parameter mutations through:

  • assignment expressions (param = ...), and
  • update expressions (param++, param--).

It applies only when the target resolves to a parameter variable.

What this rule reports​

This rule reports parameter reassignment except when it occurs inside the first ExpressionStatement of a function body.

Why this rule exists​

Parameter mutation can obscure function contracts and complicate reasoning about input values.

❌ Incorrect​

function f(value: number) {
 sideEffect();
 value += 1;
}

✅ Correct​

function f(value: number) {
 value += 1;
 sideEffect();
}
function f(value: number): number {
 const nextValue = value + 1;
 return nextValue;
}
function initialize(value: number) {
 value = normalize(value);
 // ✅ allowed: reassignment occurs in first expression statement
 return value;
}

Behavior and migration notes​

This rule reports only and does not provide an autofix.

To migrate, prefer introducing local variables for transformed parameter values.

Options​

This rule has no options.

Additional examples​

function increment(count: number): number {
 doWork();
 count++;
 // ❌ reported: update occurs after the first expression statement
 return count;
}

ESLint flat config example​

import etcMisc from "eslint-plugin-etc-misc";

export default [
 {
  plugins: { "etc-misc": etcMisc },
  rules: {
   "etc-misc/no-param-reassign": "error",
  },
 },
];

When not to use it​

Disable this rule if your codebase allows unrestricted parameter mutation.

Package documentation​

Rule catalog ID: R051

Further reading​

Adoption resources​

  • Start at warning level in CI, then move to error after cleanup.
  • Use focused codemods/autofix batches per package or directory.