What is the method for retrieving built-in string manipulation types?

To achieve a return using template literal type, you must return a template literal:

type Greetings = `Hello ${string}`

const name = 'World'
function greet(name: string): Greetings {
  return `Hello ${name}`; // no errors
  // return `Hola ${name}`; // error
  // return `Hello${name}`; // error
}

Similarly, how can one return intrinsic string manipulation types?

type UppercaseFirstLetter = Capitalize<string>
function capitalizeFirstLetter(word: string): UppercaseFirstLetter { 
    return `${word.charAt(0).toUpperCase() + word.slice(1)}` 
}

This approach does not produce the desired result.

Answer №1

An issue arises with functions like toUpperCase() and slice() as they do not support string literal types, instead always returning an arbitrary string.

To address this, one suggestion is to transform capitalWord() into a generic function and utilize assertions on the return value.

type Capital<T extends string> = Capitalize<T>;

function capitalWorld<T extends string>(str: T): Capital<T> {
    return `${str.charAt(0).toUpperCase() + str.slice(1)}` as Capital<T>;
}

const lowercase = "hello world";
const capitalized = capitalWorld(lowercase);
//    ^? const capitalized: "Hello world"

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