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