No. 261 — Title case — Annotated solution

Title case

Implement TitleCase<S> so every space-separated word in S gets a capital first letter.

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Par 3

Puzzle

title-case.ts

type TitleCase<S extends string> = ???

type TitleCase<S extends string> = ???

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Replace ??? — your solution is checked against the cases below. Tab indents; press Escape then Tab to move focus out.

Checks

TitleCase<'hello world'>—
→ 'Hello World'
TitleCase<'foo'>—
→ 'Foo'
TitleCase<'a b c'>—
→ 'A B C'

How a check is judged Exact type equality, not assignability — an intersection is not the same as the flattened object.

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Annotated solution

Published September 14, 2026

The solution

type TitleCase<S extends string> =
  S extends `${infer W} ${infer Rest}` ? `${Capitalize<W>} ${TitleCase<Rest>}` : Capitalize<S>

The common wrong answer

type TitleCase<S extends string> = Capitalize<S>

Capitalize capitalizes a string, singular — the first character of the whole thing and nothing else. "hello world" becomes "Hello world". The built-in has no notion of a word, so anything word-shaped has to be found by you first.

Line by line

`${infer W} ${infer Rest}`
The literal space between the placeholders is what makes this a word splitter. Inference stops at the first one it finds, so W is always a single word.
: Capitalize<S>
The base case is the last word, and it is the only place Capitalize is applied to the whole remaining string — by then, the whole remaining string *is* one word.

Takeaway

The intrinsic string types work on whole strings. Words, segments and paths are things you carve out with template patterns first, and only then hand over.