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.