forInGuards

Reports for-in loops without an if statement to filter inherited properties.

✅ This rule is included in the ts preset.

Looping over objects with a for-in loop will include properties inherited through the prototype chain. This can lead to unexpected items being iterated over when the object inherits from a non-standard prototype.

Examples

❌ Incorrect

for (const key in object) {
  doSomething(key);
}

for (const key in object) {
  doSomething(key);
}

for (const key in object) {
  if (condition) {
    doSomething(key);
  }
  doSomethingElse(key);
}

✅ Correct

for (const key in object) {
  if (Object.hasOwn(object, key)) {
Error ts(2550)  ― Property 'hasOwn' does not exist on type 'ObjectConstructor'. Do you need to change your target library? Try changing the 'lib' compiler option to 'es2022' or later.
    doSomething(key);
  }
}

for (const key in object) {
  if (!Object.hasOwn(object, key)) {
Error ts(2550)  ― Property 'hasOwn' does not exist on type 'ObjectConstructor'. Do you need to change your target library? Try changing the 'lib' compiler option to 'es2022' or later.
    continue;
  }
  doSomething(key);
}

for (const key in object) {
  if (Object.prototype.hasOwnProperty.call(object, key)) {
    doSomething(key);
  }
}

for (const key in object);

for (const key in object) {
}

Options

This rule is not configurable.

When Not To Use It

If you are certain that the objects you iterate over do not have inherited properties from custom prototypes, you might choose to disable this rule. This is common when iterating over plain objects created with object literals or Object.create(null).

Further Reading

• MDN: for…in
• MDN: Object.hasOwn()
• MDN: Object.prototype.hasOwnProperty()

Equivalents in Other Linters

• Biome: useGuardForIn
• Deno: guard-for-in
• ESLint: guard-for-in
• Oxlint: eslint/guard-for-in