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掌握JavaScript全貌：You Don't Know JS系列入门指南

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"‘You Don't Know JS’系列是一套深入解析JavaScript编程的教程，由前端顾问Jenn Lukas编撰，旨在帮助读者从初级到高级全面掌握这门语言。这套书籍特别关注那些许多经验丰富的开发者可能忽视的复杂部分，即使对JavaScript是新手或已经使用多年的人来说，完全理解和精通JavaScript也是一项挑战。在《Up&Going》这一系列的第一本书中，作者为编程基础薄弱但想深入学习JavaScript的人提供了基础。通过学习编程的基本要素，如运算符、类型系统、变量、条件语句和循环结构，读者将建立起坚实的编程基础，为后续的深度学习做好准备。这本书的目标是让你从初步接触JavaScript到逐步迈向真正的编程高手。在这本书中，读者可以期待以下内容： 1. **核心概念掌握**：理解并掌握编程语言的基础构建块，这些是所有高级编程技巧的基础。 2. **运算符和类型**：学习JavaScript中不同类型的运算符以及它们如何处理数据。 3. **变量管理**：学会创建、赋值、管理和作用域规则，这是编写有效代码的关键。 4. **条件与控制流程**：条件语句如if-else和switch的使用，以及循环（for, while, do-while）如何控制程序执行流程。 5. **循序渐进的学习路径**：《Up&Going》作为系列的起点，为后续更深入的书籍打下坚实基础，帮助你逐步深化对JavaScript的理解。通过《You Don't Know JS》系列，无论是初学者还是进阶者，都能在提升编程技能和理解语言本质的同时，培养出对代码的深入洞察力，从而创作出更高质量的代码，并在个人职业生涯中不断成长。系列作者Kyles Simpson在Twitter(@oreillymedia)和Facebook(facebook.com/oreilly)上也有活跃的互动，可以提供额外的学习资源和支持。"

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CHAPTER 1

Into Programming

Welcome to the You Don’t Know JS (YDKJS) series.

Up & Going is an introduction to several basic concepts of program‐

ming—of course we lean toward JavaScript (often abbreviated JS)

specifically—and how to approach and understand the rest of the

titles in this series. Especially if you’re just getting into programming

and/or JavaScript, this book will briefly explore what you need to get

up and going.

This book starts off explaining the basic principles of programming

at a very high level. It’s mostly intended if you are starting YDKJS

with little to no prior programming experience, and are looking to

these books to help get you started along a path to understanding

programming through the lens of JavaScript.

Chapter 1 should be approached as a quick overview of the things

you’ll want to learn more about and practice to get into program‐

ming. There are also many other fantastic programming introduc‐

tion resources that can help you dig into these topics further, and I

encourage you to learn from them in addition to this chapter.

Once you feel comfortable with general programming basics, Chap‐

ter 2 will help guide you to a familiarity with JavaScript’s flavor of

programming. Chapter 2 introduces what JavaScript is about, but

again, it’s not a comprehensive guide—that’s what the rest of the

YDKJS books are for!

www.it-ebooks.info

If you’re already fairly comfortable with JavaScript, first check out

Chapter 3 as a brief glimpse of what to expect from YDKJS, then

jump right in!

Code

Let’s start from the beginning.

A program, often referred to as source code or just code, is a set of

special instructions to tell the computer what tasks to perform. Usu‐

ally code is saved in a text file, although with JavaScript you can also

type code directly into a developer console in a browser, which we’ll

cover shortly.

The rules for valid format and combinations of instructions is called

a computer language, sometimes referred to as its syntax, much the

same as English tells you how to spell words and how to create valid

sentences using words and punctuation.

Statements

In a computer language, a group of words, numbers, and operators

that performs a specific task is a statement. In JavaScript, a statement

might look as follows:

a = b * 2;

The characters a and b are called variables (see “Variables” on page

14), which are like simple boxes you can store any of your stuff in.

In programs, variables hold values (like the number 42) to be used

by the program. Think of them as symbolic placeholders for the val‐

ues themselves.

By contrast, the 2 is just a value itself, called a literal value, because it

stands alone without being stored in a variable.

The = and * characters are operators (see “Operators” on page 8)—

they perform actions with the values and variables such as assign‐

ment and mathematic multiplication.

Most statements in JavaScript conclude with a semicolon (;) at the

end.

The statement a = b * 2; tells the computer, roughly, to get the

current value stored in the variable b, multiply that value by 2, then

store the result back into another variable we call a.

2 | Chapter 1: Into Programming

www.it-ebooks.info

Programs are just collections of many such statements, which

together describe all the steps that it takes to perform your pro‐

gram’s purpose.

Expressions

Statements are made up of one or more expressions. An expression is

any reference to a variable or value, or a set of variable(s) and

value(s) combined with operators.

For example:

a = b * 2;

This statement has four expressions in it:

• 2 is a literal value expression.

•

b is a variable expression, which means to retrieve its current

value.

•

b * 2 is an arithmetic expression, which means to do the multi‐

plication.

• a = b * 2 is an assignment expression, which means to assign

the result of the b * 2 expression to the variable a (more on

assignments later).

A general expression that stands alone is also called an expression

statement, such as the following:

b * 2;

This flavor of expression statement is not very common or useful, as

generally it wouldn’t have any effect on the running of the program

—it would retrieve the value of b and multiply it by 2, but then

wouldn’t do anything with that result.

A more common expression statement is a call expression statement

(see “Functions” on page 22), as the entire statement is the function

call expression itself:

alert( a );

Expressions | 3

www.it-ebooks.info

Executing a Program

How do those collections of programming statements tell the com‐

puter what to do? The program needs to be executed, also referred to

as running the program.

Statements like a = b * 2 are helpful for developers when reading

and writing, but are not actually in a form the computer can directly

understand. So a special utility on the computer (either an inter‐

preter or a compiler) is used to translate the code you write into com‐

mands a computer can understand.

For some computer languages, this translation of commands is typi‐

cally done from top to bottom, line by line, every time the program

is run, which is usually called interpreting the code.

For other languages, the translation is done ahead of time, called

compiling the code, so when the program runs later, what’s running

is actually the already compiled computer instructions ready to go.

It’s typically asserted that JavaScript is interpreted, because your Java‐

Script source code is processed each time it’s run. But that’s not

entirely accurate. The JavaScript engine actually compiles the pro‐

gram on the fly and then immediately runs the compiled code.

For more information on JavaScript compiling,

see the first two chapters of the Scope & Closures

title of this series.

Try It Yourself

This chapter is going to introduce each programming concept with

simple snippets of code, all written in JavaScript (obviously!).

It cannot be emphasized enough: while you go through this chapter

—and you may need to spend the time to go over it several times—

you should practice each of these concepts by typing the code your‐

self. The easiest way to do that is to open up the developer tools con‐

sole in your nearest browser (Firefox, Chrome, IE, etc.).

4 | Chapter 1: Into Programming

www.it-ebooks.info

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