打字稿泛型语法：有什么作用<Type>达到？

Question

Reading the typescript handbook on generics: https://www.typescriptlang.org/docs/handbook/2/generics.html

I got confused on the syntax for below example:

function loggingIdentity<Type>(arg: Type[]): Type[] {
  console.log(arg.length);
  return arg;
}

I understand that:

1. arg: Type[] specifies the input argument.
2. Type[] that comes before { ... } specifies the return type

However,

3. What is the purpose of <Type> in loggingIdentity<Type> ?

What does it achieve?

Answer 1

The <Type> part is what makes it a generic: it defines a variable of sorts (which you can name anything you want, though T or Type are common). This is the placeholder for any type.

So you can call loggingIdentity(arg) where arg is of type number or string or any complex type you want. The declaration with <Type> is what allows it; and the usage of Type in the argument and the return type is your declaration that the method will return the same type of value as was passed in.