for...in loops include inherited properties

for...in is useful, but it does not only read an object's own keys.
It also iterates enumerable properties inherited from the prototype chain.

const foo = { one: 1 };
const bar = { two: 2, __proto__: foo };

console.log(Object.keys(bar)); // ["two"]

for (const key in bar) {
  console.log(key); // "two", "one"
}

If you only need own properties:

• Use Object.keys(obj), Object.values(obj), or Object.entries(obj).
• Or guard inside for...in with Object.hasOwn(obj, key).

for (const key in bar) {
  if (Object.hasOwn(bar, key)) {
    console.log(key);
  }
}

Knowing this behavior helps avoid hard-to-spot bugs when working with object maps.