JS: for-in Loop 👎

for (key in obj) { body }

• Loop thru property keys
• Only Enumerable Property.
• Only key of string type.
• Include keys in Prototype Chain.

Sets the property name to key in body.

💡 TIP: for-in loop is almost never useful, because you don't want to go thru parent chain. Use for-of Loop, or Object.keys and forEach .

const aa = { aa1: 1, aa2: 2 };
const bb = { bb1: 1, bb2: 2 };

// make aa to be the parent of bb.
Reflect.setPrototypeOf(bb, aa);

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

// bb1
// bb2
// aa1
// aa2

// result also show keys from parent