head

Head<Type> constructs a type which equals to first element in type Type

type FirstParameter<Type extends AnyFunction> = Head<Parameters<Type>>;

type StringValue = FirstParameter<typeof decodeURIComponent>;
//   ^? string

⚠️ Limitations:

• Returns never when type Type is [] or has 0 length

TS Playground – https://tsplay.dev/Wo8gMw