Trim Right

In this medium-level challenge, you'll implement a TrimRight type that removes trailing whitespace characters from a string type, similar to JavaScript's trimEnd() but operating entirely at the type level.

[object Object]

type cases = [
  Expect<Equal<TrimRight<'str'>, 'str'>>,
  Expect<Equal<TrimRight<'str '>, 'str'>>,
  Expect<Equal<TrimRight<'str     '>, 'str'>>,
  Expect<Equal<TrimRight<'     str     '>, '     str'>>,
  Expect<Equal<TrimRight<'   foo bar  \n\t '>, '   foo bar'>>,
  Expect<Equal<TrimRight<''>, ''>>,
  Expect<Equal<TrimRight<'\n\t '>, ''>>,
]

Detailed Explanation

type Whitespace = ' ' | '\n' | '\t';
 
type TrimRight<S extends string> =
  S extends `${infer Rest}${Whitespace}`
    ? TrimRight<Rest>
    : S;

How it works:

• The Whitespace union type defines the three characters we consider whitespace: space, newline, and tab
• TrimRight uses a conditional type with template literal inference to check if the string S ends with any whitespace character
• If it does, the pattern ${infer Rest}${Whitespace} captures everything before the trailing whitespace character as Rest, and the type recurses on Rest to strip the next trailing character
• When the string no longer ends with a whitespace character, the recursion stops and returns the trimmed string as-is
• This peels off one trailing whitespace character per recursive step, handling strings with multiple trailing whitespace characters of mixed types (spaces, newlines, tabs)

This challenge helps you understand template literal types and recursive string manipulation, and how to apply these concepts in real-world scenarios.