More examples of `infer`

[John Smart]

Documentation is lacking in examples of how to use infer in the Conditional Types section.

For example, one can use infer to unshift an array literal:

type Unshift<T extends any[]> = T extends [infer Head, ...infer Tail] ? Head : never;
type A = Unshift<['a','b','c']>

type A is literal 'a'.

Another useful example would be to demonstrate infer within a template to parse a string type:

type TSVersion = "4.1.2"
type ExtractSemver<Semver extends string> =
    Semver extends `${infer Major}.${infer Minor}.${infer Patch}` ?
    { major: Major, minor: Minor, patch: Patch } : never
type TS = ExtractSemver<TSVersion>

type TS is literal { major: "4", minor: "1", patch : "2"}

Also useful would be to demonstrate the default of infer will be a single char, and to demonstrate recursion:

type NotEmptyString<S extends string> = S extends "" ? never : S;
type Reverse<S extends string> = 
	S extends "" ? S : 
	S extends `${infer B}${infer E}` ? `${Reverse<E>}${B}` : never;
type WorkItAndReverseIt = Reverse<"Foobar">;

type WorkItAndReverseIt is literal rabooF.