Understanding Object Keys in JavaScript (Interview Trap Question)

I was recently asked an interesting JavaScript question in an interview: let obj = {}; let a = { a : 10 }; let b = { b : 20 }; obj [ a ] = 10 ; obj [ b ] = 20 ; console . log ( obj [ a ]); 🤔 What will be the output? 👉 Output: 20 🔍 Why does this happen? In JavaScript, object keys can only be of type string or symbol . When you use an object as a key: obj [ a ] = 10 ; JavaScript internally converts the key to a string using .toString() : a . toString () // "[object Object]" b . toString () // "[object Object]" So effectively, your code becomes: obj [ " [object Object] " ] = 10 ; obj [ " [object Object] " ] = 20 ; 👉 The second assignment overwrites the first one. So: console . log ( obj [ a ]); // 20 📌 Follow-up Question 1: How to fix this issue? ✅ Option 1: Use Map (Recommended) If you want to use objects as keys, use Map : const map = new Map (); map . set ( a , 10 ); map . set ( b , 20 ); console . log ( map . get ( a )); // 10 console . log ( map . get ( b )); // 20 👉 Map preserves re