搜索
首页web前端js教程Mastering TypeScript: Understanding the Power of extends

Mastering TypeScript: Understanding the Power of extends

The extends keyword in TypeScript is a Swiss Army knife of sorts. It's used in multiple contexts, including inheritance, generics, and conditional types. Understanding how to use extends effectively can lead to more robust, reusable, and type-safe code.

Inheritance using extends

One of the primary uses of extends is in inheritance, allowing you to create new interfaces or classes that build upon existing ones.

interface User {
  firstName: string;
  lastName: string;
  email: string;
}

interface StaffUser extends User {
  roles: string[];
  department: string;
}

const regularUser: User = {
  firstName: "John",
  lastName: "Doe",
  email: "john@example.com"
};

const staffMember: StaffUser = {
  firstName: "Jane",
  lastName: "Smith",
  email: "jane@company.com",
  roles: ["Manager", "Developer"],
  department: "Engineering"
};

In this example, StaffUser extends User, inheriting all its properties and adding new ones. This allows us to create more specific types based on more general ones.

Class Inheritance

The extends keyword is also used for class inheritance:

class Animal {
  constructor(public name: string) {}

  makeSound(): void {
    console.log("Some generic animal sound");
  }
}

class Dog extends Animal {
  constructor(name: string, public breed: string) {
    super(name);
  }

  makeSound(): void {
    console.log("Woof! Woof!");
  }

  fetch(): void {
    console.log(`${this.name} is fetching the ball!`);
  }
}

const myDog = new Dog("Buddy", "Golden Retriever");
myDog.makeSound(); // Output: Woof! Woof!
myDog.fetch(); // Output: Buddy is fetching the ball!

Here, Dog extends Animal, inheriting its properties and methods, and also adding its own.

Type Constraints in Generics

The extends keyword is crucial when working with generics, allowing us to constrain the types that can be used with a generic function or class.

interface Printable {
  print(): void;
}

function printObject<T extends Printable>(obj: T) {
  obj.print();
}

class Book implements Printable {
  print() {
    console.log("Printing a book.");
  }
}

class Magazine implements Printable {
  print() {
    console.log("Printing a magazine.");
  }
}

const myBook = new Book();
const myMagazine = new Magazine();

printObject(myBook);      // Output: Printing a book.
printObject(myMagazine);  // Output: Printing a magazine.
// printObject(42);       // Error, number doesn't have a 'print' method
  1. interface Printable: Here, we define an interface named Printable. This interface declares a contract that any class implementing it must adhere to. Tha contract specifies that any class implementing Printable must provide a method named print that takes no arguments and returns void
  2. function printObject(obj: T): This is a generic function named printObject. It takes a single argument named obj, which is type T. The type parameter T is constrained to types that extend (implement) the Printable interface can bef used as the argument to this function.
  3. class Book implements Printable and class Magazine implements Printable: Here, we define two classes, Book and Magazine, both of which implement the Printable interface. This means that these classes must provide a print method as required by the contract of the Printable interface.
  4. const myBook = new Book(); and const myMagazine = new Magazine();: We create instances of the Book and Magazine classes.
  5. printObject(myBook); and printObject(myMagazine);: We call the printObject function with the instances of Book and Magazine. Since both Book and Magazine classes implement the Printable interface, they fulfill the constraint of the T extends Printable type parameter. Inside the function, the print method of the appropriate class is called, resulting in the expected output.
  6. // printObject(42);: If we try to call printObject with a type that doesn't implement the Printable interface, such as the number 42, TypeScript will raise an error. This is because the type constraint is not satisfied, as number doesn't have a print method as required by the Printable interface.

In summary, the extends keyword in the context of function printObject(obj: T) is used to ensure that the type T used as the argument adheres to the contract defined by the Printable interface. This ensures that only types with a print method can be used with the printObject function, enforcing a specific behavior and contract for the function's usage.

Conditional Types

T extends U ? X : Y
  • T is the type that being checked
  • U is the condition type that T is being checked against.
  • X is the type that the conditional type evaluates to if T extends (is assignable to) U
  • Y is the type that the conditional type evaluates to if T does not extend U
type ExtractNumber<T> = T extends number ? T : never;

type NumberOrNever = ExtractNumber<number>; // number
type StringOrNever = ExtractNumber<string>; // never

Here, the ExtractNumber type takes a type parameter T. The conditional type checks whether T extends the number type. if does, the type resolves to T (which is number type). If it doesn't, the type resolves to never.

The extends Keyword with Union Types

Now, let's consider the expression A | B | C extends A. This might seem counterintuitive at first, but in TypeScript, this condition is actually false. Here's why:

  1. In TypeScript, when you use extends with a union type on the left side, it's equivalent to asking: "Is every possible type in this union assignable to the type on the right?"
  2. In other words, A | B | C extends A is asking: "Can A be assigned to A, AND can B be assigned to A, AND can C be assigned to A?"
  3. While A can certainly be assigned to A, B and C might not be assignable to A (unless they are subtypes of A), so the overall result is false.
type Fruit = "apple" | "banana" | "cherry";
type CitrusFruit = "lemon" | "orange";

type IsCitrus<T> = T extends CitrusFruit ? true : false;

type Test1 = IsCitrus<"lemon">; // true
type Test2 = IsCitrus<"apple">; // false
type Test3 = IsCitrus<Fruit>; // false

In this example, IsCitrus is false because not all fruits in the Fruit union are CitrusFruit.

Best Practices and Tips

  • Use extends for meaningful relationships: Only use inheritance when there's a clear "is-a" relationship between types.
  • Prefer composition over inheritance: In many cases, composition (using interfaces and type intersections) can be more flexible than class inheritance.
  • Be cautious with deep inheritance chains: Deep inheritance can make code harder to understand and maintain.
  • Leverage conditional types for flexible APIs: Use conditional types with extends to create APIs that adapt based on input types.
  • Use extends in generics to create reusable, type-safe functions: This allows you to write functions that work with a variety of types while still maintaining type safety

以上是Mastering TypeScript: Understanding the Power of extends的详细内容。更多信息请关注PHP中文网其他相关文章!

声明
本文内容由网友自发贡献,版权归原作者所有,本站不承担相应法律责任。如您发现有涉嫌抄袭侵权的内容,请联系admin@php.cn
在JavaScript中替换字符串字符在JavaScript中替换字符串字符Mar 11, 2025 am 12:07 AM

JavaScript字符串替换方法详解及常见问题解答 本文将探讨两种在JavaScript中替换字符串字符的方法:在JavaScript代码内部替换和在网页HTML内部替换。 在JavaScript代码内部替换字符串 最直接的方法是使用replace()方法: str = str.replace("find","replace"); 该方法仅替换第一个匹配项。要替换所有匹配项,需使用正则表达式并添加全局标志g: str = str.replace(/fi

自定义Google搜索API设置教程自定义Google搜索API设置教程Mar 04, 2025 am 01:06 AM

本教程向您展示了如何将自定义的Google搜索API集成到您的博客或网站中,提供了比标准WordPress主题搜索功能更精致的搜索体验。 令人惊讶的是简单!您将能够将搜索限制为Y

示例颜色json文件示例颜色json文件Mar 03, 2025 am 12:35 AM

本文系列在2017年中期进行了最新信息和新示例。 在此JSON示例中,我们将研究如何使用JSON格式将简单值存储在文件中。 使用键值对符号,我们可以存储任何类型的

10个jQuery语法荧光笔10个jQuery语法荧光笔Mar 02, 2025 am 12:32 AM

增强您的代码演示:开发人员的10个语法荧光笔 在您的网站或博客上共享代码片段是开发人员的常见实践。 选择合适的语法荧光笔可以显着提高可读性和视觉吸引力。 t

构建您自己的Ajax Web应用程序构建您自己的Ajax Web应用程序Mar 09, 2025 am 12:11 AM

因此,在这里,您准备好了解所有称为Ajax的东西。但是,到底是什么? AJAX一词是指用于创建动态,交互式Web内容的一系列宽松的技术。 Ajax一词,最初由Jesse J创造

8令人惊叹的jQuery页面布局插件8令人惊叹的jQuery页面布局插件Mar 06, 2025 am 12:48 AM

利用轻松的网页布局:8个基本插件 jQuery大大简化了网页布局。 本文重点介绍了简化该过程的八个功能强大的JQuery插件,对于手动网站创建特别有用

10 JavaScript和JQuery MVC教程10 JavaScript和JQuery MVC教程Mar 02, 2025 am 01:16 AM

本文介绍了关于JavaScript和JQuery模型视图控制器(MVC)框架的10多个教程的精选选择,非常适合在新的一年中提高您的网络开发技能。 这些教程涵盖了来自Foundatio的一系列主题

什么是这个&#x27;在JavaScript?什么是这个&#x27;在JavaScript?Mar 04, 2025 am 01:15 AM

核心要点 JavaScript 中的 this 通常指代“拥有”该方法的对象,但具体取决于函数的调用方式。 没有当前对象时,this 指代全局对象。在 Web 浏览器中,它由 window 表示。 调用函数时,this 保持全局对象;但调用对象构造函数或其任何方法时,this 指代对象的实例。 可以使用 call()、apply() 和 bind() 等方法更改 this 的上下文。这些方法使用给定的 this 值和参数调用函数。 JavaScript 是一门优秀的编程语言。几年前,这句话可

See all articles

热AI工具

Undresser.AI Undress

Undresser.AI Undress

人工智能驱动的应用程序,用于创建逼真的裸体照片

AI Clothes Remover

AI Clothes Remover

用于从照片中去除衣服的在线人工智能工具。

Undress AI Tool

Undress AI Tool

免费脱衣服图片

Clothoff.io

Clothoff.io

AI脱衣机

AI Hentai Generator

AI Hentai Generator

免费生成ai无尽的。

热门文章

R.E.P.O.能量晶体解释及其做什么(黄色晶体)
2 周前By尊渡假赌尊渡假赌尊渡假赌
仓库:如何复兴队友
4 周前By尊渡假赌尊渡假赌尊渡假赌
Hello Kitty Island冒险:如何获得巨型种子
4 周前By尊渡假赌尊渡假赌尊渡假赌

热工具

Dreamweaver CS6

Dreamweaver CS6

视觉化网页开发工具

SecLists

SecLists

SecLists是最终安全测试人员的伙伴。它是一个包含各种类型列表的集合,这些列表在安全评估过程中经常使用,都在一个地方。SecLists通过方便地提供安全测试人员可能需要的所有列表,帮助提高安全测试的效率和生产力。列表类型包括用户名、密码、URL、模糊测试有效载荷、敏感数据模式、Web shell等等。测试人员只需将此存储库拉到新的测试机上,他就可以访问到所需的每种类型的列表。

安全考试浏览器

安全考试浏览器

Safe Exam Browser是一个安全的浏览器环境,用于安全地进行在线考试。该软件将任何计算机变成一个安全的工作站。它控制对任何实用工具的访问,并防止学生使用未经授权的资源。

EditPlus 中文破解版

EditPlus 中文破解版

体积小,语法高亮,不支持代码提示功能

mPDF

mPDF

mPDF是一个PHP库,可以从UTF-8编码的HTML生成PDF文件。原作者Ian Back编写mPDF以从他的网站上“即时”输出PDF文件,并处理不同的语言。与原始脚本如HTML2FPDF相比,它的速度较慢,并且在使用Unicode字体时生成的文件较大,但支持CSS样式等,并进行了大量增强。支持几乎所有语言,包括RTL(阿拉伯语和希伯来语)和CJK(中日韩)。支持嵌套的块级元素(如P、DIV),