I was looking for something like a Nullable type in TypeScript when I noticed that while there is no Nullable type (or is there?), there is a NonNullable type defined in:
C:\Program Files\Microsoft VS Code\resources\app\extensions\node_modules\typescript\lib\lib.es6.d.ts
The definition of NonNullable is:
/**
* Exclude null and undefined from T
*/
type NonNullable <T> = T extends null | undefined ? never : T;
Can someone please explain (or point me to the relevant documentation about) what this definition means? I could not locate the documentation around this. Specifically, I could not find what the ? operator and the never keyword means in the context of generic constraints.
I found other similar definitions in the same file:
/**
* Exclude from T those types that are assignable to U
*/
type Exclude<T, U> = T extends U ? never : T;
/**
* Extract from T those types that are assignable to U
*/
type Extract<T, U> = T extends U ? T : never;