guard-for-in

for...in visits an object's own enumerable properties and the ones it inherits from its prototype chain. If anything was added to a prototype, the loop body runs for keys the object itself doesn't have.

The rule only looks at the shape of the loop. The body must be a single if, or start with an if that does nothing but continue. The rule doesn't check what the condition tests.

Configure

Off by default. To turn it on:

[lint.rules]
"guard-for-in" = "error" # or "warn"

Examples

Reported:

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

for (const key in settings) {
  if (Object.hasOwn(settings, key)) {
    apply(key, settings[key]);
  }
  count++;
}

Not reported:

for (const key in settings) {
  if (Object.hasOwn(settings, key)) {
    apply(key, settings[key]);
  }
}

for (const key in settings) {
  if (!Object.hasOwn(settings, key)) continue;
  apply(key, settings[key]);
  count++;
}

Object.keys() returns only the object's own keys, so a for...of loop over it doesn't need a guard:

for (const key of Object.keys(settings)) {
  apply(key, settings[key]);
}