no-for-in-array

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

This rule prohibits iterating over an array with a for-in loop.

Attributes

• Included in configs
  • ✅ Recommended
  • 🔒 Strict
• Fixable
  • 🔧 Automated Fixer
  • 🛠 Suggestion Fixer
• 💭 Requires type information

Rule Details

A for-in loop (for (var k 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. More common is to use for-of, which iterates over the values of an array. If you want to iterate over the indices, alternatives include:

array.forEach((value, index) => { ... });
for (const [index, value] of array.entries()) { ... }
for (let i = 0; i < array.length; i++) { ... }

Examples of code for this rule:

❌ Incorrect

for (const x in [3, 4, 5]) {
  console.log(x);
}

✅ Correct

for (const x in { a: 3, b: 4, c: 5 }) {
  console.log(x);
}

Options

// .eslintrc.json
{
  "rules": {
    "@typescript-eslint/no-for-in-array": "error"
  }
}

This rule is not configurable.

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.