@studnicky/v8/for-in-loops

Reports every for...in statement. The rule is a syntax selector and does not attempt to determine whether an object has inherited keys, accessors, or a particular runtime shape.

For repeated traversal of the same object, compute Object.values(object) or Object.keys(object) once outside the repeated loop and iterate that array. At 5,000,000 property visits, hoisted Object.keys took 25.132 ms and hoisted Object.values 2.172 ms, versus 95.987 ms for for...in. Do not recompute Object.entries(object) inside the replacement loop: that form measured 367.317 ms (3.83× slower than for...in) because it creates a key-value array for every property on every repeat.

Fixable: No · Options: No · Suggested severity: error

✗ Incorrect

for (const key in settings) {
  useSetting(key, settings[key]);
}

✓ Correct

const keys = Object.keys(settings);

for (let index = 0; index < keys.length; index += 1) {
  const key = keys[index];
  if (key !== undefined) {
    useSetting(key, settings[key]);
  }
}

const values = Object.values(settings);

for (let index = 0; index < values.length; index += 1) {
  const value = values[index];
  if (value !== undefined) {
    useValue(value);
  }
}