0007 · Readonly

Challenge Link

Problem

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.

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] }

Explanation

This solution is almost identical to Pick, but with the readonly modifier applied to every property.

Step by step:

1. keyof T — produces a union of all keys of T
2. [K in keyof T] — iterates over every key K in T
3. readonly — marks each property as read-only
4. T[K] — preserves the original value type of each property

The result is a new object type where all properties are read-only. Attempting to assign a value to any property will cause a TypeScript compile error.

Why readonly matters: TypeScript’s readonly modifier is a compile-time check only — the JavaScript runtime doesn’t enforce it. But it’s extremely useful for expressing intent (e.g., immutable state, frozen config objects) and catching accidental mutations early.

Key concepts:

• Mapped Types
• Readonly modifier

TIP

A normal array (T[]) is a subtype of a readonly array (readonly T[]), because a mutable array can do everything a readonly array can (plus mutation). This is the Liskov Substitution Principle applied to TypeScript’s type system.

中文解析

核心思路：

实现 MyReadonly<T> 的关键是使用映射类型（Mapped Types），对 T 的每个属性加上 readonly 修饰符。

// 解法
type MyReadonly<T> = { readonly [K in keyof T]: T[K] }
//                   ^^^^^^^^  映射类型修饰符，让每个属性变为只读

逐步分析：

1. keyof T — 获取类型 T 所有键的联合类型
2. [K in keyof T] — 遍历 T 的每一个键 K
3. readonly — 给每个属性加上只读修饰
4. T[K] — 保留原属性的值类型不变

注意： readonly 只是 TypeScript 编译时检查，运行时的 JavaScript 并不会阻止赋值。若需要运行时不可变，应使用 Object.freeze()。

考察知识点：

• 映射类型（Mapped Types）
• keyof 操作符
• readonly 修饰符