Limit the keys in TypeScript to only the elements of an array

Question

Limit the keys in TypeScript to only the elements of an array

Modified: Updating ID Types

I have an array that contains the following values:

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

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

I want to specify a type for my object to only allow values that are in my ids array.

I attempted to use a lookup type but didn't achieve the desired result...

Any suggestions?

Thank you

typescript types typescript-typings

Answer 1

Answer №1

To store the literal types of array elements, you can utilize the Record mapped type along with a const assertion:

const numbers = [12, 24] as const;

const data: Record<typeof numbers[number], string> = {
    12: "GOOD",
    24: "DAY",
};
const data2: Record<typeof numbers[number], string> = {
    12: "GOOD",
    24: "DAY",
    25: "MORNING",  // error
};

Answer 2

To store the literal types of array elements, you can utilize the Record mapped type along with a const assertion:

const numbers = [12, 24] as const;

const data: Record<typeof numbers[number], string> = {
    12: "GOOD",
    24: "DAY",
};
const data2: Record<typeof numbers[number], string> = {
    12: "GOOD",
    24: "DAY",
    25: "MORNING",  // error
};

Answer 3

Answer №2

If you're looking to create a function that generates an object with type-checkable keys corresponding to an array, you can use the following code:

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

  const result: Output = {} as Output;

  const {length} = keys;

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

    result[k] = i;
  }

  return result;
};

If you try to access a key that doesn't exist in the generated object, the type-checker will raise an error:

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

Answer 4

If you're looking to create a function that generates an object with type-checkable keys corresponding to an array, you can use the following code:

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

  const result: Output = {} as Output;

  const {length} = keys;

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

    result[k] = i;
  }

  return result;
};

If you try to access a key that doesn't exist in the generated object, the type-checker will raise an error:

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

Limit the keys in TypeScript to only the elements of an array

Answer №1

Answer №2

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