javascript入门_JavaScript入门手册（2020版）

javascript入门

JavaScript is one of the most popular programming languages in the world.

JavaScript是世界上最受欢迎的编程语言之一。

I believe it's a great choice for your first programming language ever.

我相信这是您有史以来第一门编程语言的绝佳选择。

We mainly use JavaScript to create

我们主要使用JavaScript创建

websites
网站
web applications
网络应用
server-side applications using Node.js
使用Node.js的服务器端应用程序

but JavaScript is not limited to these things, and it can also be used to

但是JavaScript不仅限于这些东西，它还可以用于

create mobile applications using tools like React Native
使用React Native等工具创建移动应用程序
create programs for microcontrollers and the internet of things
为微控制器和物联网创建程序
create smartwatch applications
创建smartwatch应用程序

It can basically do anything. It's so popular that everything new that shows up is going to have some kind of JavaScript integration at some point.

它基本上可以做任何事情。它是如此流行，以至于出现的所有新事物都将在某种程度上进行某种JavaScript集成。

JavaScript is a programming language that is:

JavaScript是一种编程语言，它是：

high level: it provides abstractions that allow you to ignore the details of the machine where it's running on. It manages memory automatically with a garbage collector, so you can focus on the code instead of managing memory like other languages like C would need, and provides many constructs which allow you to deal with highly powerful variables and objects.

高级别 ：它提供抽象，使您可以忽略正在运行的计算机的详细信息。它使用垃圾回收器自动管理内存，因此您可以专注于代码，而不必像C之类的其他语言那样管理内存，并提供许多构造，使您可以处理功能强大的变量和对象。
dynamic: opposed to static programming languages, a dynamic language executes at runtime many of the things that a static language does at compile time. This has pros and cons, and it gives us powerful features like dynamic typing, late binding, reflection, functional programming, object runtime alteration, closures and much more. Don't worry if those things are unknown to you - you'll know all of them by the end of the course.

动态的 ：与静态编程语言相反，动态语言在运行时执行静态语言在编译时执行的许多操作。这具有优缺点，并且为我们提供了强大的功能，例如动态类型输入，后期绑定，反射，函数式编程，对象运行时更改，闭包等等。如果您不知道这些事情，请不要担心-在课程结束时，您将了解所有这些内容。
dynamically typed: a variable does not enforce a type. You can reassign any type to a variable, for example, assigning an integer to a variable that holds a string.

动态类型的 ：变量不强制类型。您可以将任何类型重新分配给变量，例如，将整数分配给包含字符串的变量。
loosely typed: as opposed to strong typing, loosely (or weakly) typed languages do not enforce the type of an object, allowing more flexibility but denying us type safety and type checking (something that TypeScript - which builds on top of JavaScript - provides)

松散类型 ：与强类型相反，松散(或弱类)语言不会强制执行对象的类型，从而提供了更大的灵活性，但拒绝了我们进行类型安全性和类型检查(基于JavaScript的TypeScript提供的功能)
interpreted: it's commonly known as an interpreted language, which means that it does not need a compilation stage before a program can run, as opposed to C, Java or Go for example. In practice, browsers do compile JavaScript before executing it, for performance reasons, but this is transparent to you - there is no additional step involved.

解释型 ：它通常称为解释型语言，这意味着它可以在程序可以运行之前不需要编译阶段，例如，与C，Java或Go相对。实际上，出于性能原因，浏览器会在执行JavaScript之前先对其进行编译，但这对您来说是透明的-无需执行其他任何步骤。
multi-paradigm: the language does not enforce any particular programming paradigm, unlike Java for example, which forces the use of object-oriented programming, or C that forces imperative programming. You can write JavaScript using an object-oriented paradigm, using prototypes and the new (as of ES6) classes syntax. You can write JavaScript in a functional programming style, with its first-class functions, or even in an imperative style (C-like).

多范式 ：多语言不强制执行任何特定的编程范式，例如不同于Java，Java强制使用面向对象的编程，而C则强制执行命令式编程。您可以使用面向对象的范例，原型和新的(从ES6开始)类语法来编写JavaScript。您可以使用功能性编程风格，一流的功能甚至是命令式风格(类似于C)来编写JavaScript。

In case you're wondering, JavaScript has nothing to do with Java, it's a poor name choice but we have to live with it.

如果您想知道， JavaScript与Java无关 ，这是一个糟糕的选择，但我们必须忍受它。

手册摘要 (Summary of the handbook)

A little bit of history

一点历史
Just JavaScript

只是JavaScript
A brief intro to the syntax of JavaScript

JavaScript语法简介
Semicolons

分号
Values

价值观
Variables

变数
Types

种类
Expressions

表达方式
Operators

经营者
Precedence rules

优先规则
Comparison operators

比较运算符
Conditionals

有条件的
Arrays

数组
Strings

弦乐
Loops

循环
Functions

功能
Arrow functions

箭头功能
Objects

对象
Object Properties

对象属性
Object Methods

对象方法
Classes

班级
Inheritance

遗产
Asynchonous Programming and Callbacks

异步编程和回调
Promises

承诺
Async and Await

异步并等待
Variable scope

可变范围
Conclusion

结论

Update: You can now get a PDF and ePub version of this JavaScript Beginner's Handbook.

更新：您现在可以获得本JavaScript初学者手册的PDF和ePub版本。

一点历史 (A little bit of history)

Created in 1995, JavaScript has gone a very long way since its humble beginnings.

自1995年创建以来，JavaScript的不起眼发展就已经走了很长一段路。

It was the first scripting language that was supported natively by web browsers, and thanks to this it gained a competitive advantage over any other language and today it's still the only scripting language that we can use to build Web Applications.

它是Web浏览器本地支持的第一种脚本语言，并且由于它，它比其他任何语言都具有竞争优势，并且今天它仍然是我们可以用来构建Web应用程序的唯一脚本语言。

Other languages exist, but all must compile to JavaScript - or more recently to WebAssembly, but this is another story.

还存在其他语言，但是所有语言都必须编译为JavaScript，或者最近才可以编译为WebAssembly，但这是另一回事了。

In the begining, JavaScript was not nearly powerful as it is today, and it was mainly used for fancy animations and the marvel known at the time as Dynamic HTML.

刚开始时，JavaScript并不像今天那样强大，它主要用于精美的动画和当时称为Dynamic HTML的奇迹。

With the growing needs that the web platform demanded (and continues to demand), JavaScript had the responsibility to grow as well, to accommodate the needs of one of the most widely used ecosystems of the world.

随着Web平台不断增长的需求(并继续满足)，JavaScript 也有责任不断增长，以适应世界上使用最广泛的生态系统之一的需求。

JavaScript is also now widely used outside of the browser. The rise of Node.js in the last few years unlocked backend development, once the domain of Java, Ruby, Python, PHP, and more traditional server-side languages.

现在，JavaScript在浏览器之外也被广泛使用。过去几年，Node.js的兴起解锁了后端开发，而后端开发曾经是Java，Ruby，Python，PHP和更传统的服务器端语言的领域。

JavaScript is now also the language powering databases and many more applications, and it's even possible to develop embedded applications, mobile apps, TV apps, and much more. What started as a tiny language inside the browser is now the most popular language in the world.

现在，JavaScript也是语言为数据库和更多应用程序提供了动力，甚至有可能开发嵌入式应用程序，移动应用程序，电视应用程序等。从浏览器内部的一种很小的语言开始，到现在是世界上最受欢迎的语言。

只是JavaScript (Just JavaScript)

Sometimes it's hard to separate JavaScript from the features of the environment it is used in.

有时很难将JavaScript与使用环境的功能区分开。

For example, the console.log() line you can find in many code examples is not JavaScript. Instead, it's part of the vast library of APIs provided to us in the browser.

例如，您可以在许多代码示例中找到的console.log()行不是JavaScript。相反，它是浏览器中提供给我们的庞大API库的一部分。

In the same way, on the server it can be sometimes hard to separate the JavaScript language features from the APIs provided by Node.js.

同样，有时在服务器上有时很难将JavaScript语言功能与Node.js提供的API分开。

Is a particular feature provided by React or Vue? Or is it "plain JavaScript", or "vanilla JavaScript" as it's often called?

React或Vue是否提供特定功能？还是通常所说的“普通JavaScript”或“普通JavaScript”？

In this book I talk about JavaScript, the language.

在这本书中，我将讨论JavaScript，即语言。

Without complicating your learning process with things that are outside of it, and provided by external ecosystems.

不会使您的学习过程与外部生态系统所提供的外部事物复杂化。

JavaScript语法简介 (A brief intro to the syntax of JavaScript)

In this little introduction I want to tell you about 5 concepts:

在这个小介绍中，我想向您介绍5个概念：

white space
空白空间
case sensitivity
区分大小写
literals
文字
identifiers
身份标识
comments
注释

空格 (White space)

JavaScript does not consider white space meaningful. Spaces and line breaks can be added in any fashion you might like, at least in theory.

JavaScript认为空白没有意义。至少从理论上讲 ，可以以任何您喜欢的方式添加空格和换行符。

In practice, you will most likely keep a well defined style and adhere to what people commonly use, and enforce this using a linter or a style tool such as Prettier.

在实践中，您很可能会保持定义明确的样式并遵守人们常用的样式，并使用短绒棉布或样式工具(例如Prettier)来强制实施。

For example, I always use 2 space characters for each indentation.

例如，我总是对每个缩进使用2个空格字符。

区分大小写 (Case sensitive)

JavaScript is case sensitive. A variable named something is different than Something.

JavaScript区分大小写。名为something的变量不同于Something 。

The same goes for any identifier.

任何标识符也是如此。

文字 (Literals)

We define literal as a value that is written in the source code, for example, a number, a string, a boolean or also more advanced constructs, like Object Literals or Array Literals:

我们将文字定义为在源代码中写入的值，例如，数字，字符串，布尔值或其他更高级的构造，例如对象文字或数组文字：

5
'Test'
true
['a', 'b']
{color: 'red', shape: 'Rectangle'}

身份标识 (Identifiers)

An identifier is a sequence of characters that can be used to identify a variable, a function, or an object. It can start with a letter, the dollar sign $ or an underscore _, and it can contain digits. Using Unicode, a letter can be any allowed character, for example, an emoji