What does the typeof keyword return when used with a variable in Typescript?

Question

What does the typeof keyword return when used with a variable in Typescript?

In TypeScript, a class can be defined as shown below:

class Sup {

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

If you write the following code:

let x = Sup;

Why does the type of x show up as typeof Sup (highlighted in vscode) and what does typeof Sup mean? Is it related to the typeof operator?

Moreover, how would you type something like let y = Object.create(Sup)? Is it correctly typed as

let y: typeof Sup = Object.create(Sup)

?

javascript typescript visual-studio-code

Answer 1

Answer №1

typeof in TypeScript has a unique definition compared to regular JavaScript. It serves as an operator for determining the type of an entity within the value space.

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

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

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

// additional 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

When assigning SomeClass to a variable, the variable's type becomes typeof SomeClass. The format remains unchanged, unlike other examples, due to the ambiguity involved in simplifying class types; it stays as typeof SomeClass for clarity.

Answer 2

typeof in TypeScript has a unique definition compared to regular JavaScript. It serves as an operator for determining the type of an entity within the value space.

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

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

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

// additional 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

When assigning SomeClass to a variable, the variable's type becomes typeof SomeClass. The format remains unchanged, unlike other examples, due to the ambiguity involved in simplifying class types; it stays as typeof SomeClass for clarity.

Answer 3

Answer №2

When encountering the statement let x = Sup; in your scenario, it essentially means that variable x is capable of storing the constructor function Sup, rather than an actual instance of it:

class Sup { }

let x: typeof Sup;

x = Sup;       // acceptable
x = new Sup(); // not valid

Answer 4

When encountering the statement let x = Sup; in your scenario, it essentially means that variable x is capable of storing the constructor function Sup, rather than an actual instance of it:

class Sup { }

let x: typeof Sup;

x = Sup;       // acceptable
x = new Sup(); // not valid

What does the typeof keyword return when used with a variable in Typescript?

Answer №1

Answer №2

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