angular - What does ... interface Type<T> extends Function { new (...args: any[]): T; } ... mean?

Issue

Interface Type<T> is an interface which is extending global type Function. I found this in typescript documentation Function about global function.

What does the above code mean? I got this code from angular source code.

export const Type = Function;

export function isType(v: any): v is Type<any> {return typeof v === 'function';}

export interface Type<T> extends Function { new (...args: any[]): T; }

I went through Typescript documentation but not able to figure it out.

Solution

The definition

interface Type<T> extends Function { new (...args: any[]): T; }

means that a value of type Type<T> is a function which is also known to be a constructor for instances of type T. Since it extends the Function interface it is a Function, and the syntax { new (⋯): ⋯ } is a construct signature, so it's also a constructor.

That means if you have

declare const f: Type<Date>;

then f is a Function and has all Function's properties like length and bind:

f.apply; // okay
f.call; // okay
f.bind; // okay
f.length; // okay

but it is also a constructor for Date objects:

const d = new f();
//    ^? const d: Date;

---

That being said, I don't know why it's defined this way. All constructor types automatically inherit from Function; you don't need to declare it. You can only use the new operator on functions anyway... classes are just special functions:

class Foo {}; 
console.log(typeof Foo) // "function"

That means the definition

interface Type<T> { new(...args: any[]): T; }

behaves the same as your version. Perhaps it's defined this way to make the relationship with Function explicit? Not sure. But that's out of scope for the question as asked.

Playground link to code

Answered By - jcalz