How to intentionally define an "empty interface" in typescript

Question

TypeScript allows checking for checking unknown properties. The following

interface MyInterface {
  key: string
}

const myVar: MyInterface = {
  asda: 'asdfadf'
}

will fails with

Type '{ asda: string; }' is not assignable to type 'MyInterface'.
Object literal may only specify known properties, and 'asda' does not exist in type 'MyInterface'.

However, this statement will compile without any issue. Empty interface will accept any value

interface EmptyInterface {
}

const myVar: EmptyInterface = {
  asda: 'asdfadf'
}

However, what if I actually want to define type for an empty object that may not have any properties? How can I accomplish that in typescript?

Answer 1

To define an interface that never has any members, you can define an indexer that returns never

interface None { [n: string]: never } 
// OK
let d2 : None = {

}
let d3 : None = {
    x: "" // error
}