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I'm using a for loop to get strings from an array. I need to use these strings as object keys.

This code works when run but TypeScript complains:

const arr = ['one', 'two'];

const map = {
    one: 'Vaue is 1',
    two: 'Value is 2'
}

for (let i=0; i < arr.length; i++) {
    const item = arr[i];
    const res = map[item]
    console.log(res)
}

Element implicitly has an 'any' type because expression of type 'string' can't be used to index type '{ one: string; two: string; }'.
  No index signature with a parameter of type 'string' was found on type '{ one: string; two: string; }'.(7053)
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    How is the compiler supposed to know that item is a valid key of map? As the error tells you, the type is inferred as string, and there are plenty of strings that are neither 'one' nor 'two'. Can you give arr a const assertion? Commented May 15, 2023 at 10:31

4 Answers 4

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One possible solution is to declare arr as const. For example:

const arr = ['one', 'two'] as const;

const map = {
    one: 'Vaue is 1',
    two: 'Value is 2'
}

for (let i=0; i < arr.length; i++) {
    const item = arr[i]; // "one" | "two"
    const res = map[item]
    console.log(res)
}

Another solution, if the array needs to be based on the map, is to type arr as a collection of keys of map:

const map = {
    one: 'Vaue is 1',
    two: 'Value is 2'
}

const arr: (keyof typeof map)[] = ['one', 'two'];

for (let i=0; i < arr.length; i++) {
    const item = arr[i]; // "one" | "two"
    const res = map[item]
    console.log(res)
}
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Comments

0

You have to tell the compiler at least type for on of your variables map or arr.

Option 1. Type Array:

const map = {
  one: 'Vaue is 1',
  two: 'Value is 2',
};

const arr: (keyof typeof map)[] = ['one', 'two'];

This way we make sure that arr can only contain map's keys

Option 2. Type map:

const map2: Record<string, unknown> = {
  one: 'Vaue is 1',
  two: 'Value is 2',
};

const arr2 = ['one', 'two'];

We just tell that map has any string key and that does the trick.

playground

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0

you can declare this keys as a type, check this out:

type KeysMap = 'one' | 'two';

const arr: KeysMap[] = ['one', 'two'];

const map = {
 one: 'Vaue is 1',
 two: 'Value is 2'
}

for (let i=0; i < arr.length; i++) {
  const item = arr[i];
  const res = map[item]
  console.log(res)
}

this way typescript knows that the type options are equal to the map keys

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Comments

0

This happens because, as the message says, typescript doesn't know that the items in arr will match a key of map. Therefore, you need to define in your code that arr is an array with keys of the type map:

const arr: (keyof typeof map)[] = ['one', 'two'];
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