forInArrays

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

✅ This rule is included in the ts logical preset.

A for-in loop (for (const i in o)) iterates over all enumerable properties of an object, including those that are not array indices. This can lead to unexpected behavior when used with arrays, as it may include properties that are not part of the array’s numeric indices. It also returns the index key (i) as a string, which is not the expected numeric type for array indices.

Consider using a construct more suited for arrays, such as a for-of loop (for (const i of o)).

Examples

❌ Incorrect

const values = ["a", "b", "c"];

for (const key in values) {
  console.log(values[key]);
}

✅ Correct

const values = ["a", "b", "c"];

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

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 Flint disable comments and/or configuration file disables for those specific situations instead of completely disabling this rule.

Equivalents in Other Linters

• ESLint: @typescript-eslint/no-for-in-array
• Oxlint: typescript/no-for-in-array