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Comment by arsome | Hacker News Reader

Is there a proper way to use custom objects as keys in hashmaps in JS (well, ideally TypeScript compliant)? Coming out of the C++ world where this is trivial and something I use quite often this seems pretty infuriating. Maybe a general data structure package that's worth pulling in?

EDIT: After doing a bit of research I found "tstl", looks interesting, their hashmap appears to support custom hashers and equality comparisons:

https://tstl.dev/api/classes/std.hashmap.html

I'm curious about experiences with it if anyone's had the chance?

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Create a wrapper class that takes a stringifying function that converts the key objects into a primitive and use a Map<string, [K, V]> as the backing store. fast-json-stable-stringify can be used as the default stringifier, and if the objects are large or have a natural key a custom stringifier can also be supplied. Since we are using TS we can have our custom class implement Map<K, V>.

That's very similar to the solution I'm using and that I hate - it involves unnecessary generation and storage of strings and is far less ideal than something like the implementation in tstl I linked above where you can simply provide a custom hash function and equality comparison.

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.

A quick look at the source implies that that iteration order is not predictable.

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