guard-for-in

Require for-in loops to include an if statement.

Looping over objects with a for-in loop will include properties that are inherited through the prototype chain. This behavior can lead to unexpected items in your for loop.

Invalid:

for (const key in obj) {
  foo(obj, key);
}

Valid:

for (const key in obj) {
  if (Object.hasOwn(obj, key)) {
    foo(obj, key);
  }
}

for (const key in obj) {
  if (!Object.hasOwn(obj, key)) {
    continue;
  }
  foo(obj, key);
}