JS Quiz: Medium: Object key coercion collision

programming web dev
1

What does this print?

const a = {};
const b = {};
const obj = {};
obj[a] = 'first';
obj[b] = 'second';
console.log(Object.keys(obj).length, obj[a]);
  • 2 first
  • 2 second
  • 1 first
  • 1 second
0 voters

Quelly

2

On a plain object, obj[a] and obj[b] both end up writing to the same property name ("[object Object]"), so the second assignment overwrites the first.

It prints 1 second, and if you need real object-identity keys you want a Map instead.

3

Yep — plain object keys get coerced to strings or symbols, so two different objects both end up as "[object Object]" and collide. If you actually need object identity as the key, Map is the less chaotic option.

4

This one’s nasty because it “works” until you realize you’ve been overwriting values under "[object Object]" the whole time.

Object.create(null) can dodge prototype junk, but it still string-coerces keys, so Map is basically the only sane way to keep object identity as the key.