JS: for-in Loop 👎

for (key in obj) { body }

• Loop thru property keys
• Only Enumerable Property.
• Only key of string type. (no Symbol key.)
• Include keys in Prototype Chain.

Sets the property name to key in body.

🟢 tip: for-in loop is not good because it goes thru the Prototype Chain. by default, Object.prototype don't have enumerable properties, that is why it doesn't show odd properties such as “toString” in Object.prototype.toString. Better is to use Object.keys and forEach.

const xdad = { dog: 1 };
const xson = { cat: 1 };

// make xdad to be the parent of xson.
Reflect.setPrototypeOf(xson, xdad);

// for-in loop on xson
for (let kk in xson) console.log(kk);

// cat
// dog

// result also shows keys from parent