Question

What's the output?

const a = {};
const b = { key: 'b' };
const c = { key: 'c' };

a[b] = 123;
a[c] = 456;

console.log(a[b]);

• A: 123
• B: 456
• C: undefined
• D: ReferenceError

Answer 1

Answer: B

Object keys are automatically converted into strings. We are trying to set an object as a key to object a, with the value of 123.

However, when we stringify an object, it becomes "[object Object]". So what we are saying here, is that a["[object Object]"] = 123. Then, we can try to do the same again. c is another object that we are implicitly stringifying. So then, a["[object Object]"] = 456.

Then, we log a[b], which is actually a["[object Object]"]. We just set that to 456, so it returns 456.