Typescript, restrict keys to array elements

Question

Edited: Change ids type

I have an array with the following values

const ids: number[] = [45, 56];

const obj: any = {
  45: "HELLO",
  56: "WORLD",
};

I would like to type the current any type of my object to restrict it to my ids array values.

I tried with lookup type but I didn't succeed…

Any idea ?

Regards

As an aside, the line const ids: string[] = [45, 56]; is in error; 45 and 56 are not strings. — jcalz
– jcalz, Commented Jul 10, 2019 at 10:49

Titian Cernicova-Dragomir · Accepted Answer · 2019-07-10 09:42:04Z

14

You can use the Record mapped type. You also need to use a const assertion to capture the literal types of the array elements:

const ids = [45, 56] as const;

const obj: Record<typeof ids[number], string> = {
    45: "HELLO",
    56: "WORLD",
};
const obj2: Record<typeof ids[number], string> = {
    45: "HELLO",
    56: "WORLD",
    57: "WORLD",  // error
};

answered Jul 10, 2019 at 9:42

Titian Cernicova-Dragomir

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2 Comments

ScreamZ Over a year ago

Wow! Wonderful, I didn't known that as const existed but it worked, it's used to say that this array is immutable ?

Titian Cernicova-Dragomir Over a year ago

@ScreamZ yes it makes the array readonly, and as a side effect of this tuples and literal types are preserved (you get ids types as [45, 56] instead of number[])

Igor Sukharev · Accepted Answer · 2021-06-26 10:34:06Z

2

If you need to create a function that returns a type-checkable object with keys that correspond to array:

function indexKeys<K extends string>(keys: readonly K[]) {
  type Result = Record<K, number>;

  const result: Result = {} as Result;

  const {length} = keys;

  for (let i = 0; i < length; i++) {
    const k = keys[i];

    result[k] = i;
  }

  return result;
};

Here the type-checker will complain:

// Property 'zz' does not exist on type 'Result'.
const {aa, zz} = indexKeys(['aa', 'bb']);

edited Jun 26, 2021 at 10:34

answered Jun 26, 2021 at 10:05

Igor Sukharev

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