Why is the "typeof" function important in TypeScript member typings?

The definitions in the Electron typescript include an interface named MainInterface. This interface includes familiar members like app and autoUpdater, as well as less familiar ones such as BrowserView, BrowserWindow, and ClientRequest.

One specific question that arises is about the usage of typeof XX. Typically, typeof is understood to return the string name of a type. For example, BrowserView: typeof BrowserView; can be seen as equivalent to BrowserView: "BrowserView";

The confusion lies in understanding the purpose behind using typeof in these member definitions. What exactly does it signify or add to the definition?

Answer №1

When using a type annotation as shown above, the typeof operator specifies the type of a value. If used with a variable, it indicates the type of that specific variable. In the case of a class, it represents the constructor type and static methods, rather than the instance type.

class Bar {
  static method(){}
}

let bar: typeof Bar;
bar.method();
new bar()

let x = 1
let xx: typeof x

Answer №2

When working with Typescript, creating a class leads to the creation of two distinct interfaces:

  1. The interface specific to the class itself
  2. The interface for instances of the class

An issue arises because only the instance interface is named after the class. To access the class-specific interface for actions like new MyClass() or MyClass.staticProperty, you must utilize the typeof keyword.

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