What does the following TypeScript 4.1+ template literal type produce? type PropEventSource = { on(event: `${K}Changed`, callback: (value: T[K]) => void): void; };

Question

TypeScript Professional — Hard

What does the following TypeScript 4.1+ template literal type produce? type PropEventSource = { on(event: `${K}Changed`, callback: (value: T[K]) => void): void; };

Accepted Answer

The template literal type `${K}Changed` in the 'on' method restricts event strings to the form 'propertyNameChanged'. This ensures type safety by only allowing valid event names. The callback parameter is typed to receive the correct property value, maintaining type integrity.