[Typescript] 27. Medium - Capitalize

Implement Capitalize<T> which converts the first letter of a string to uppercase and leave the rest as-is.

For example

type capitalized = Capitalize<'hello world'> // expected to be 'Hello world'

 

// Non built-in approach
/* _____________ Your Code Here _____________ */
interface Map {
    a: 'A'
    b: 'B'
    c: 'C'
    d: 'D'
    e: 'E'
    f: 'F'
    g: 'G'
    h: 'H'
    i: 'I'
    j: 'J'
    k: 'K'
    l: 'L'
    m: 'M'
    n: 'N'
    o: 'O'
    p: 'P'
    q: 'Q'
    r: 'R'
    s: 'S'
    t: 'T'
    u: 'U'
    v: 'V'
    w: 'W'
    x: 'X'
    y: 'Y'
    z: 'Z'
}
type MyCapitalize<S extends string> = S extends `${infer First}${infer REST}` ? `${First extends keyof Map ? Map[First] : First}${REST}`: ''

type x = Capitalize<'a'>
/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<MyCapitalize<'foobar'>, 'Foobar'>>,
  Expect<Equal<MyCapitalize<'FOOBAR'>, 'FOOBAR'>>,
  Expect<Equal<MyCapitalize<'foo bar'>, 'Foo bar'>>,
  Expect<Equal<MyCapitalize<''>, ''>>,
  Expect<Equal<MyCapitalize<'a'>, 'A'>>,
  Expect<Equal<MyCapitalize<'b'>, 'B'>>,
  Expect<Equal<MyCapitalize<'c'>, 'C'>>,
  Expect<Equal<MyCapitalize<'d'>, 'D'>>,
  Expect<Equal<MyCapitalize<'e'>, 'E'>>,
  Expect<Equal<MyCapitalize<'f'>, 'F'>>,
  Expect<Equal<MyCapitalize<'g'>, 'G'>>,
  Expect<Equal<MyCapitalize<'h'>, 'H'>>,
  Expect<Equal<MyCapitalize<'i'>, 'I'>>,
  Expect<Equal<MyCapitalize<'j'>, 'J'>>,
  Expect<Equal<MyCapitalize<'k'>, 'K'>>,
  Expect<Equal<MyCapitalize<'l'>, 'L'>>,
  Expect<Equal<MyCapitalize<'m'>, 'M'>>,
  Expect<Equal<MyCapitalize<'n'>, 'N'>>,
  Expect<Equal<MyCapitalize<'o'>, 'O'>>,
  Expect<Equal<MyCapitalize<'p'>, 'P'>>,
  Expect<Equal<MyCapitalize<'q'>, 'Q'>>,
  Expect<Equal<MyCapitalize<'r'>, 'R'>>,
  Expect<Equal<MyCapitalize<'s'>, 'S'>>,
  Expect<Equal<MyCapitalize<'t'>, 'T'>>,
  Expect<Equal<MyCapitalize<'u'>, 'U'>>,
  Expect<Equal<MyCapitalize<'v'>, 'V'>>,
  Expect<Equal<MyCapitalize<'w'>, 'W'>>,
  Expect<Equal<MyCapitalize<'x'>, 'X'>>,
  Expect<Equal<MyCapitalize<'y'>, 'Y'>>,
  Expect<Equal<MyCapitalize<'z'>, 'Z'>>,
]

 

// Built-in 
type MyCapitalize<S extends string> = Capitalize<S>;