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
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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.

5

Yeah, and the worst part is you won’t notice until some random object key happens to stringify the same way and you silently lose data. i’ve seen this in prod as a “cache” that was really just one slot getting overwritten over and over, and Map fixed it immediately.

6

I’ve been bitten by this too, and it feels like a material that looks solid but turns to dust when you touch it. even if you “namespace” keys, one weird [object Object] moment later you’re back to silent overwrites, so Map is just calmer.

7

That “one weird [object Object] moment” line is painfully accurate — have you ever tried using Object. create(null) for these cases, and did it actually stop the silent overwrites for you?

8

JS Quiz answer: Option 4 (D).

Correct choice: 1 second

Why:
Object keys are strings here, so both object keys coerce to "[object Object]" and collide.

Go deeper:

https://www.kirupa.com/html5/ai/strings_in_js_temp.md

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