How can I get the length of a string using TypeScript types?

Question

I'm trying to get the length of a string as a number:

type Length = LengthOfString<"hello">
//   ^? should be 5

I'm not sure where to begin on this. How can I accomplish this?

(I'm trying to learn more about typescript's type system -- I am not applying this to JS code)

Tristan F.-R. · Accepted Answer · 2022-11-15 12:42:54Z

5

One way is to use a template literal string to split of each char of the string. We append some element to the tuple L for each char we can retrieve.

type LengthOfString<T extends string, L extends any[] = []> = 
  T extends `${string}${infer R}` 
    ? LengthOfString<R, [...L, string]>
    : L["length"]

When there are no more chars left, we return the length of L

Result:

type Length = LengthOfString<"hello">
//   ^? 5

Playground

edited Nov 15, 2022 at 12:42

Tristan F.-R.

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answered Nov 15, 2022 at 12:39

Tobias S.

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2 Comments

Tristan F.-R. Over a year ago

Great! By the way, I put the ^? there because it's used as a label for types. See this playground

Tobias S. Over a year ago

@LeoDog896 - Ah of course. I always put the computed type there for the answer but forget to remove it in the playground :)

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