JS: in (operator) 👎

key in obj

Return true if property key key is obj's own property or in Prototype Chain. (Both string and Symbol keys) Else, false.

const xx = { "dog": 1 };

console.log("dog" in xx);
// true
console.log("cat" in xx);
// false

// in-operator on parent property

const xdad = { dog: 1 };

// create a object xson, with parent xdad
const xson = Object.create(xdad);

console.log("dog" in xson);
// true

// the in operator works with symbol key
const xx = Symbol();
const yy = { [xx]: 4 };
console.assert(xx in yy);

🛑 warning: the syntax key in obj is not a part of the syntax for (key in obj) {body}. They look the same but don't have the same meaning. [see for-in Loop]