No. 233 — StartsWith — Annotated solution

StartsWith

Implement `StartsWith<S, P>` so it reports whether the string `S` begins with the prefix `P`. Beginning with, not merely containing.

Try the puzzle yourself

Par 3

Puzzle

starts-with.ts

type StartsWith<S extends string, P extends string> = ???

type StartsWith<S extends string, P 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

StartsWith<'hello world', 'hello'>—
→ true
StartsWith<'hello world', 'world'>—
→ false
StartsWith<'abc', ''>—
→ true
StartsWith<'', 'a'>—
→ false

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 August 17, 2026

The solution

type StartsWith<S extends string, P extends string> =
  S extends `${P}${string}` ? true : false

The common wrong answer

type StartsWith<S extends string, P extends string> =
  S extends `${string}${P}${string}` ? true : false

This asks whether `P` appears *anywhere* — it is `includes`, not `startsWith`. The leading `${string}` lets the match slide past the beginning, so `"hello world"` reports `true` for the prefix `"world"`.

Line by line

`${P}${string}`
A template literal pattern is anchored at both ends. Putting `P` first means the very first characters of `S` must be `P`; the trailing `${string}` then accepts any remainder, including none at all.
StartsWith<'abc', ''>
The empty prefix collapses the pattern to `${string}`, which every string satisfies — so the answer is `true`, matching how `"abc".startsWith("")` behaves at runtime.

Takeaway

Where you put `${string}` in a pattern is where you allow slack. Leading slack means "contains", trailing slack means "starts with", and slack on both sides means you have written a substring test by accident.