StartsWith

In this medium-level challenge, you'll implement StartsWith<T, U> which checks whether a string type T begins with a prefix string type U, returning true or false.

type a = StartsWith<'abc', 'ac'> // expected to be false
type b = StartsWith<'abc', 'ab'> // expected to be true
type c = StartsWith<'abc', 'abcd'> // expected to be false

type cases = [
  Expect<Equal<StartsWith<'abc', 'ac'>, false>>,
  Expect<Equal<StartsWith<'abc', 'ab'>, true>>,
  Expect<Equal<StartsWith<'abc', 'abc'>, true>>,
  Expect<Equal<StartsWith<'abc', 'abcd'>, false>>,
  Expect<Equal<StartsWith<'abc', ''>, true>>,
  Expect<Equal<StartsWith<'abc', ' '>, false>>,
  Expect<Equal<StartsWith<'', ''>, true>>,
]

Detailed Explanation

type StartsWith<T extends string, U extends string> =
  T extends `${U}${infer _Rest}` ? true : false

How it works:

• We use a template literal type pattern ${U}${infer _Rest} to check if T can be decomposed into the prefix U followed by any remaining string _Rest.
• If T matches this pattern, the string starts with U and we return true.
• If T does not match, we return false. This handles cases where U is longer than T or where the characters simply do not match.
• When U is an empty string "", the pattern ${""}{infer _Rest} always matches any string, correctly returning true.

This challenge helps you understand template literal type pattern matching and how to apply this concept in real-world scenarios.