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Kush Bhandari
Kush Bhandari

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Understanding Object Keys in JavaScript (Interview Trap Question)

#javascript #interview

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]);
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🤔 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;
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JavaScript internally converts the key to a string using .toString():

a.toString() // "[object Object]"
b.toString() // "[object Object]"
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So effectively, your code becomes:

obj["[object Object]"] = 10;
obj["[object Object]"] = 20;
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👉 The second assignment overwrites the first one.

So:

console.log(obj[a]); // 20
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📌 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
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👉 Map preserves reference identity, so a and b are treated as different keys.

⚠️ Option 2: Convert keys to unique strings (Not ideal) 

let obj = {};

let a = JSON.stringify({ a: 10 });
let b = JSON.stringify({ b: 20 });

obj[a] = 10;
obj[b] = 20;

console.log(obj[a]); // 10
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👉 This works, but:

  • It’s fragile
  • Depends on consistent serialization
  • Not suitable for complex objects

⚠️ Option 3: Use Symbol 

let obj = {};

let a = Symbol("a");
let b = Symbol("b");

obj[a] = 10;
obj[b] = 20;

console.log(obj[a]); // 10
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👉 Symbols are always unique:

Symbol("a") === Symbol("a") // false
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📌 Follow-up Question 2: What is the data type of object keys?

In JavaScript:

  • Object keys are always:

    • string
    • symbol

Even if you use:

obj[1] = "value";
obj[true] = "value";
obj[null] = "value";
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They are converted to strings:

obj["1"]
obj["true"]
obj["null"]
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🚀 When should you use Map instead of Object?

Use Map when:

  • Keys are objects or functions
  • You need preserved insertion order
  • You want better performance for frequent add/remove
  • You need strict key equality (no coercion)

⚡ Bonus: Key Differences (Object vs Map)

Feature Object Map
Key Types String, Symbol Any type
Key Conversion Yes (to string) No
Order Guarantee Not reliable Preserved
Performance Good Better for dynamic use
Iteration Limited Built-in methods

🧠 Final Takeaway

  • Objects coerce keys to strings, which can cause collisions.
  • This is why both a and b became "[object Object]".
  • To safely use non-string keys, always prefer Map.

This is a classic interview question designed to test:

  • JavaScript coercion rules
  • Object internals
  • Practical knowledge of Map

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