Typescript generic type check not working as expected

Question

I have made a simple test fixture:

export interface ITest1 {}
export interface ITest2 {}
export interface ITestGeneric {}

export function test() {
  le         


        

Answer 1

This is because typescript uses structural compatibility to determine if two types are compatible. In your case since ITestGeneric has no members, it is basically compatible with anything. If you start adding properties, incompatibilities will quickly appear:

export interface ITest1 { t1: string}
export interface ITest2 { t2: number}
export interface ITestGeneric { value: T}

export function test() {
    let p: ITestGeneric = {} // error
    let q: ITestGeneric = p; // error
}

You can read more about type compatibility in typescript here