How to get object from custom key type from map in TypeScript?

Question

I have a custom class called Foo:

class Foo {
    constructor(public v: any) {
    }
}

And I have a map where Foo is my Key:

const map = new Map<Foo, string>();

AFAIK TypeScript doesn't have comparision overloading. How can I make sure that getting the key works properly?

const foo = new Foo(1234);
map.get(foo);

Here is the entire code:

class Foo {
    constructor(public v: any) {

    }
}

const map = new Map<Foo, string>();


const foo = new Foo(1234);
map.set(foo, "HELLO WORLD");

const foo2 = new Foo(1234);
console.log(map.get(foo2)); // DOESN'T work

console.log(map.get(foo)); // DOES work of course

You can find my problem here: https://www.typescriptlang.org/play?#code/MYGwhgzhAEBiD29oG8BQ0PWPAdhALgE4Cuw+8hAFAA7EBGIAlsNAG4Bc0YOAngJQpU6TAF9UYodjz5oAWzDVoAXmg4ApgHdoAWQUAeBPAA00AoUY4A5gD5KfANxDUUgtABmiZas1xElAIwATADMACwOqPLUAHQQaviUHsbQAEQAEgCiADJZAPLQAOq5AEpZACIpEZK4rkmBXupahgEh4Y4u8CBq0SDwlpRR0ZbxiYiBfA7QAPRT0AAm8GoQOADkMhoUANZOHV09fQMKQyNJE-bTs2W5GQDK0BuE26hAA

Thats the point. Both objects are same in value and therefore equal for my usecase — Daniel Stephens
– Daniel Stephens, Commented Jan 15, 2021 at 5:49
Correct but thats not the end. If there would be no workarounds any Array.find() function would loose its purpose. Thats why such function has a custom "find" callback function — Daniel Stephens
– Daniel Stephens, Commented Jan 15, 2021 at 5:54

Yuri7os · Accepted Answer · 2021-01-15 16:13:40Z

1

The problem you have isn't related to TypeScript, but rather JavaScript not having referential transparency. Therefore comparison like this new Foo(123) === new Foo(123) is always falsy.

Solution

The JavaScript's Map has a rather limited api. If you want to find specific entry using custom comparison function (in this case comparing by value), I'd suggest using .entries() method to get all the entries from the Map in form of an array of tuples [key, value][]. You can then find there whatever entry you want using Array.find(). The Map itself doesn't have any such method.

However, even better approach would be to rethink your data structure. Considering the limitations of the Map it would be better to have a primitive data type as a key in the Map. Since I don't know the specifics of your case I can only offer this general suggestion.

edited Jan 15, 2021 at 16:13

answered Jan 15, 2021 at 7:55

Yuri7os

1361 silver badge4 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

How to get object from custom key type from map in TypeScript?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related