no-for-in-array

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

🧠

Extending "plugin:@typescript-eslint/recommended-requiring-type-checking" 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]);
}

✅ 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);
}

This rule is not configurable.

Options

When Not To Use It

If you want to iterate through a loop using the indices in an array as strings, you can turn off this rule.

Resources

• Rule source
• Test source