Question

Implement the built-in Readonly<T> generic without using it.
Constructs a type with all properties of T set to readonly, meaning the properties of the constructed type cannot be reassigned.
For example

interface Todo {
  title: string
  description: string
}
const todo: MyReadonly<Todo> = {
  title: "Hey",
  description: "foobar"
}
todo.title = "Hello" // Error: cannot reassign a readonly property
todo.description = "barFoo" // Error: cannot reassign a readonly property

Answer

type MyReadonly<T> = {
    readonly [P in keyof T]: T[P]
}