打字稿中的“ typeof x”是什么类型？

Question

In typescript you can define a class like so:

class Sup {

    static member: any;
    static log() {
         console.log('sup');
    }
}

If you do the following:

let x = Sup; 

Why is the type of x equal to typeof Sup (when I highlighted the type in vscode) and what does typeof Sup mean? Is this linked to the typeof operator?

Furthermore, how do you type something like let y = Object.create(Sup) ? Is this typed as let y: typeof Sup = Object.create(Sup) ?

Answer 1

typeof has a different meaning in TypeScript's type space than in normal JS. It's an operator to get the type of something that exists in the value space.

let person = { name: 'bob', age: 26 }

type Person = typeof person // Person is { name: string, age: number }

// These are all equivalent
let personRef1: { name: string, age: number } = person
let personRef2: typeof person = person
let personRef3: Person = person
let personRef4 = person

// more examples
let secret = 'helloworld'
type Secret = typeof secret // string

let key = 123
type Key = typeof key // number

let add = (a: number, b: number) => a + b
type Add = typeof add // (a: number, b: number) => number

So, when you assign SomeClass to a variable, the variable's type will be typeof SomeClass . The only reason it's not simplified like the above examples is because there's no way to non-ambiguously simplify the type of a class; it stays as typeof SomeClass for simplicity's sake.

Answer 2

In your case, let x = Sup; (or more precisely, inferred typeof Sup ) means that variable x can hold Sup constructor function but not the instance itself:

class Sup { }

let x: typeof Sup;

x = Sup;       // ok
x = new Sup(); // invalid.