Inferring Types with infer

Inside a conditional type, infer declares a type variable that TypeScript fills in from the match. T extends Promise<infer V> ? V : T extracts what a promise resolves to, and F extends (...args: any[]) => infer R ? R : never extracts a function's return type. The built-in helpers ReturnType and Awaited are written exactly this way.

How it reads

• infer V captures whatever sits inside Promise<...>
• Result<typeof makeStar> derives the Star type from the function itself
• When the pattern does not match, the false branch is used

Cloud tip: Reach for infer when you want a type that is buried inside another one; it keeps derived types in sync with their source.