Typescript index signatures

Question

I wish to restrict the assignable types of an object to a specific type. Example, an object where all keys must have number values, such as below:

interface NumberMap {
    [key: string]: number;
}

This works for enforcing the value restriction however I then won't be able to determine what keys actually exist inside the map variable.

const map: NumberMap = {
  one: 1,
  two: 2,
  three: 3,
};

// no error, but incorrect, this key does *not* exist
const lol = map.weoiroweiroew;

// Also cannot do this
type MyKeys = keyof map;

Is there a way to enforce an index signature without losing the information of what keys the object implementing that signature has?

Your comment does not answer my question. There is no value restriction if I do that. — ed'
– ed', Commented Jun 13, 2018 at 11:41
@JonasLochmann he wants to ensure that the type of the object literal extends the interface but get the actual type of the object literal too — Titian Cernicova-Dragomir
– Titian Cernicova-Dragomir, Commented Jun 13, 2018 at 11:43

Titian Cernicova-Dragomir · Accepted Answer · 2018-06-13 11:41:08Z

7

The only way to do both restrict values, and preserve type information about what keys are actually in the implementing type is to use a generic helper function. The function will enforce that the object literal extends the interface, but it will infer the actual type of the object literal:

interface NumberMap {
    [key: string]: number;
}

function createNumberMap<T extends NumberMap>(v: T) {
    return v;
}
const map = createNumberMap({
    one: 1,
    two: 2,
    three: 3,
    // no: '' // error
});
map.one //ok
map.no //error

answered Jun 13, 2018 at 11:41

Titian Cernicova-Dragomir

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2 Comments

ed' Over a year ago

Ah yes, forgot about that trick :) No way around using a helper function?

Titian Cernicova-Dragomir Over a year ago

@EdwardSammutAlessi not as far as I know, at least no easy way. There is a way that requires you declare an extra type that will perform the validation, but I would not really recommend it

Priyal · Accepted Answer · 2020-08-05 19:06:57Z

2

How about this ? I think this will restrict your objects keys to the given 3 indexes

type Index = 'one' | 'two' | 'three'
type NumberMap = { [k in Index]?: number }

answered Aug 5, 2020 at 19:06

Priyal

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