前言

简单题：实现Readonly
TS Playground：www.typescriptlang.org/play?#code/…

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题目

7 - Readonly
-------
by Anthony Fu (@antfu) #easy #built-in #readonly #object-keys

### 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

> View on GitHub: <https://tsch.js.org/7>

/* _____________ Your Code Here _____________ */

type MyReadonly<T> = any

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<MyReadonly<Todo1>, Readonly<Todo1>>>,
]

interface Todo1 {
  title: string
  description: string
  completed: boolean
  meta: {
    author: string
  }
}

答案

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

总结
readonly

readonly修饰符，首先是一个关键字
对类中的属性成员进行修饰
修饰之后，该属性成员就不能修改，那么这个属性成员就成为类的参数属性
只能访问
在构造函数中是可以对只读属性进行修改
在类的普通方法中不能被修改

class Person {
    readonly name: string,
    constructor(name: string) {
        this.name = name;
    }
    say() {
        console.log(`我的名字叫${this.name}`);
    }
}
let person = new Person('Haha');

TypeScript 类型体操——实现Readonly

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