Readonly

Learn how to build TypeScript's essential utility type: Readonly! 🔒

In this easy-level challenge, you'll implement the built-in Readonly<T> generic without using the actual Readonly utility. This type constructs a new type with all properties of T set to readonly, meaning the properties of the constructed type cannot be reassigned. It's a fundamental tool for creating immutable data structures in TypeScript.

You'll be creating a type that makes all properties of an object type read-only. This is extremely useful for preventing accidental mutations and ensuring data integrity in your TypeScript applications.

For this challenge, you will need to change the following code to make the tests pass (no type check errors).

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

Detailed Explanation

To implement the Readonly utility type, you need to understand mapped types and the readonly modifier.

The solution uses TypeScript's mapped types to iterate over all properties and add the readonly modifier:

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

Let's break this down:

• [Key in keyof T] iterates over each property key in type T
• readonly modifier is applied to each property
• T[Key] preserves the original type of each property
• The result is a new type where all properties are read-only