Hacker News Reader

Comment by upzylon | Hacker News Reader

If the problem is just that you want to use non-primitive keys, that's what Maps are for.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

I'm aware of them, but the problem with that is you can only compare an object by instance if I recall correctly. There's no way to override the hashing and equality check that I could find, meaning even for simple objects you're basically forced to serialize your keys in some way still, which is disgustingly inefficient and slow for no good reason.

So much this. I remember being so excited that Map and Set were added to the language, only to find that they were worse than useless for objects. At least Set can be handy in a pinch, but I have never seen an actual usecase for Map that isn’t solved better with plain old JS objects. Curious if anyone else has?

There definitely are valid use cases for a Map, and even more now that we have WeakMaps (e.g. associating data DOM nodes). Or mapping functions to values, e.g. something like this:

  const returnCache = new Map<() => any, any>()

  const memoized = <T extends () => any>(f: T): ReturnType<T> => {
    if (returnCache.has(f)) return returnCache.get(f)
    const v = f()
    returnCache.set(f, v)
    return v
  }

> I have never seen an actual usecase for Map that isn’t solved better with plain old JS objects. Curious if anyone else has?

With a Map, you can easily and efficiently associate data with DOM nodes without changing them.

Reply on news.ycombinator.com