no-for-in-array

Disallow iterating over an array with a for-in loop.

✅

Extending "plugin:@typescript-eslint/recommended-type-checked" in an ESLint configuration enables this rule.

💭

This rule requires type information to run.

A for-in loop (for (var i in o)) iterates over the properties of an Object. While it is legal to use for-in loops with array types, it is not common. for-in will iterate over the indices of the array as strings, omitting any "holes" in the array.

.eslintrc.cjs

module.exports = {
  "rules": {
    "@typescript-eslint/no-for-in-array": "error"
  }
};

Try this rule in the playground ↗

Examples

❌ Incorrect

declare const array: string[];

for (const i in array) {
  console.log(array[i]);
}

for (const i in array) {
  console.log(i, array[i]);
}

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✅ Correct

declare const array: string[];

for (const value of array) {
  console.log(value);
}

for (let i = 0; i < array.length; i += 1) {
  console.log(i, array[i]);
}

array.forEach((value, i) => {
  console.log(i, value);
})

for (const [i, value] of array.entries()) {
  console.log(i, value);
}

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Options

This rule is not configurable.

When Not To Use It

If your project is a rare one that intentionally loops over string indices of arrays, you can turn off this rule. You might consider using ESLint disable comments for those specific situations instead of completely disabling this rule.

Resources

• Rule source
• Test source