Readonly

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Most of the problems under my TypeHero Challenges folder were either obtained from typehero.dev or from type-challenges repo. Purpose of these articles are just to document my approaches for my easy reference. Please visit the respective links for more info.

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Problem Description

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

Solution

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