TypeScript Academy · Lesson

A Mini Type-Level Route Parser

Parse route paths into typed parameter objects.

The Goal

We build a route parser that reads a path like "users/:id/posts/:postId" and produces a typed params object { id: string; postId: string }, entirely at compile time. This is a real pattern used by typed routers.

type Params = ParseRoute<"users/:id/posts/:postId">;
// Goal: { id: string; postId: string }

Recognizing a Parameter

A path segment is a parameter when it starts with ":". We match that prefix with a template literal and infer the parameter name after the colon.

Notation: real TypeScript writes template literal types with backtick-delimited strings containing dollar-brace holes. In these snippets we show that pattern as Tpl<...>, listing each part in order; e.g. a backtick template matching the literal prefix then Rest appears as Tpl<'prefix', infer Rest>.

type ParamName<S> = S extends Tpl<':', infer Name> ? Name : never;
// Tpl<...> denotes a backtick template literal type: a colon then Name

type X = ParamName<':id'>;   // 'id'
type Y = ParamName<'users'>; // never

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