@@ -22,7 +22,7 @@ Most of this book is focused on the JavaScript language itself. Chapter 11 docum
This book covers low-level fundamentals first, and then builds on those to more advanced and higher-level abstractions. The chapters are intended to be read more or less in order. But learning a new programming language is never a linear process, and describing a language is not linear either: each language feature is related to other features, and this book is full of cross-references—sometimes backward and sometimes forward—to related material. This introductory chapter makes a quick first pass through the language, introducing key features that will make it easier to understand the in-depth treatment in the chapters that follow. If you are already a practicing JavaScript programmer, you can probably skip this chapter. (Although you might enjoy reading Example 1-1 at the end of the chapter before you move on.)
1.1 Exploring JavaScript
## 1.1 Exploring JavaScript
When learning a new programming language, it’s important to try the examples in the book, then modify them and try them again to test your understanding of the language. To do that, you need a JavaScript interpreter.
The easiest way to try out a few lines of JavaScript is to open up the web developer tools in your web browser (with F12, Ctrl-Shift-I, or Command-Option-I) and select the Console tab. You can then type code at the prompt and see the results as you type. Browser developer tools often appear as panes at the bottom or right of the browser window, but you can usually detach them as separate windows (as pictured in Figure 1-1), which is often quite convenient.
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@@ -52,7 +52,7 @@ undefined
true
> (x > 3) || (y < 3)
false
1.2 Hello World
## 1.2 Hello World
When you are ready to start experimenting with longer chunks of code, these line-by-line interactive environments may no longer be suitable, and you will probably prefer to write your code in a text editor. From there, you can copy and paste to the JavaScript console or into a Node session. Or you can save your code to a file (the traditional filename extension for JavaScript code is .js) and then run that file of JavaScript code with Node:
$ node snippet.js
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@@ -69,7 +69,7 @@ Then load hello.html into your web browser using a file:// URL like this one:
file:///Users/username/javascript/hello.html
Open the developer tools window to see the greeting in the console.
1.3 A Tour of JavaScript
## 1.3 A Tour of JavaScript
This section presents a quick introduction, through code examples, to the JavaScript language. After this introductory chapter, we dive into JavaScript at the lowest level: Chapter 2 explains things like JavaScript comments, semicolons, and the Unicode character set. Chapter 3 starts to get more interesting: it explains JavaScript variables and the values you can assign to those variables.
Here’s some sample code to illustrate the highlights of those two chapters:
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@@ -306,7 +306,7 @@ Introduces the Node host environment, covering the fundamental programming model
Chapter 17, JavaScript Tools and Extensions
Covers tools and language extensions that are worth knowing about because they are widely used and may make you a more productive programmer.
1.4 Example: Character Frequency Histograms
## 1.4 Example: Character Frequency Histograms
This chapter concludes with a short but nontrivial JavaScript program. Example 1-1 is a Node program that reads text from standard input, computes a character frequency histogram from that text, and then prints out the histogram. You could invoke the program like this to analyze the character frequency of its own source code:
$ node charfreq.js < charfreq.js
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@@ -420,5 +420,5 @@ async function histogramFromStdin() {
// This one final line of code is the main body of the program.
// It makes a Histogram object from standard input, then prints the histogram.
This book explains JavaScript from the bottom up. This means that we start with low-level details like comments, identifiers, variables, and types; then build to expressions, statements, objects, and functions; and then cover high-level language abstractions like classes and modules. I take the word definitive in the title of this book seriously, and the coming chapters explain the language at a level of detail that may feel off-putting at first. True mastery of JavaScript requires an understanding of the details, however, and I hope that you will make time to read this book cover to cover. But please don’t feel that you need to do that on your first reading. If you find yourself feeling bogged down in a section, simply skip to the next. You can come back and master the details once you have a working knowledge of the language as a whole.
JavaScript is a case-sensitive language. This means that language keywords, variables, function names, and other identifiers must always be typed with a consistent capitalization of letters. The while keyword, for example, must be typed “while,” not “While” or “WHILE.” Similarly, online, Online, OnLine, and ONLINE are four distinct variable names.
JavaScript ignores spaces that appear between tokens in programs. For the most part, JavaScript also ignores line breaks (but see §2.6 for an exception). Because you can use spaces and newlines freely in your programs, you can format and indent your programs in a neat and consistent way that makes the code easy to read and understand.
In addition to the regular space character (\u0020), JavaScript also recognizes tabs, assorted ASCII control characters, and various Unicode space characters as whitespace. JavaScript recognizes newlines, carriage returns, and a carriage return/line feed sequence as line terminators.
2.2 Comments
## 2.2 Comments
JavaScript supports two styles of comments. Any text between a // and the end of a line is treated as a comment and is ignored by JavaScript. Any text between the characters /* and */ is also treated as a comment; these comments may span multiple lines but may not be nested. The following lines of code are all legal JavaScript comments:
// This is a single-line comment.
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@@ -31,7 +31,7 @@ JavaScript supports two styles of comments. Any text between a // and the end of
* This is a multi-line comment. The extra * characters at the start of
* each line are not a required part of the syntax; they just look cool!
*/
2.3 Literals
## 2.3 Literals
A literal is a data value that appears directly in a program. The following are all literals:
12 // The number twelve
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@@ -43,7 +43,7 @@ false // The other Boolean value
null // Absence of an object
Complete details on numeric and string literals appear in Chapter 3.
2.4 Identifiers and Reserved Words
## 2.4 Identifiers and Reserved Words
An identifier is simply a name. In JavaScript, identifiers are used to name constants, variables, properties, functions, and classes and to provide labels for certain loops in JavaScript code. A JavaScript identifier must begin with a letter, an underscore (_), or a dollar sign ($). Subsequent characters can be letters, digits, underscores, or dollar signs. (Digits are not allowed as the first character so that JavaScript can easily distinguish identifiers from numbers.) These are all legal identifiers:
i
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$str
Like any language, JavaScript reserves certain identifiers for use by the language itself. These “reserved words” cannot be used as regular identifiers. They are listed in the next section.
2.4.1 Reserved Words
### 2.4.1 Reserved Words
The following words are part of the JavaScript language. Many of these (such as if, while, and for) are reserved keywords that must not be used as the names of constants, variables, functions, or classes (though they can all be used as the names of properties within an object). Others (such as from, of, get, and set) are used in limited contexts with no syntactic ambiguity and are perfectly legal as identifiers. Still other keywords (such as let) can’t be fully reserved in order to retain backward compatibility with older programs, and so there are complex rules that govern when they can be used as identifiers and when they cannot. (let can be used as a variable name if declared with var outside of a class, for example, but not if declared inside a class or with const.) The simplest course is to avoid using any of these words as identifiers, except for from, set, and target, which are safe to use and are already in common use.
as const export get null target void
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@@ -68,12 +68,12 @@ JavaScript also reserves or restricts the use of certain keywords that are not c
enum implements interface package private protected public
For historical reasons, arguments and eval are not allowed as identifiers in certain circumstances and are best avoided entirely.
2.5 Unicode
## 2.5 Unicode
JavaScript programs are written using the Unicode character set, and you can use any Unicode characters in strings and comments. For portability and ease of editing, it is common to use only ASCII letters and digits in identifiers. But this is a programming convention only, and the language allows Unicode letters, digits, and ideographs (but not emojis) in identifiers. This means that programmers can use mathematical symbols and words from non-English languages as constants and variables:
const π = 3.14;
const sí = true;
2.5.1 Unicode Escape Sequences
### 2.5.1 Unicode Escape Sequences
Some computer hardware and software cannot display, input, or correctly process the full set of Unicode characters. To support programmers and systems using older technology, JavaScript defines escape sequences that allow us to write Unicode characters using only ASCII characters. These Unicode escapes begin with the characters \u and are either followed by exactly four hexadecimal digits (using uppercase or lowercase letters A–F) or by one to six hexadecimal digits enclosed within curly braces. These Unicode escapes may appear in JavaScript string literals, regular expression literals, and identifiers (but not in language keywords). The Unicode escape for the character “é,” for example, is \u00E9; here are three different ways to write a variable name that includes this character:
let café = 1; // Define a variable using a Unicode character
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@@ -84,7 +84,7 @@ Early versions of JavaScript only supported the four-digit escape sequence. The
console.log("\u{1F600}"); // Prints a smiley face emoji
Unicode escapes may also appear in comments, but since comments are ignored, they are simply treated as ASCII characters in that context and not interpreted as Unicode.
2.5.2 Unicode Normalization
### 2.5.2 Unicode Normalization
If you use non-ASCII characters in your JavaScript programs, you must be aware that Unicode allows more than one way of encoding the same character. The string “é,” for example, can be encoded as the single Unicode character \u00E9 or as a regular ASCII “e” followed by the acute accent combining mark \u0301. These two encodings typically look exactly the same when displayed by a text editor, but they have different binary encodings, meaning that they are considered different by JavaScript, which can lead to very confusing programs:
const café = 1; // This constant is named "caf\u{e9}"
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@@ -93,7 +93,7 @@ café // => 1: this constant has one value
café // => 2: this indistinguishable constant has a different value
The Unicode standard defines the preferred encoding for all characters and specifies a normalization procedure to convert text to a canonical form suitable for comparisons. JavaScript assumes that the source code it is interpreting has already been normalized and does not do any normalization on its own. If you plan to use Unicode characters in your JavaScript programs, you should ensure that your editor or some other tool performs Unicode normalization of your source code to prevent you from ending up with different but visually indistinguishable identifiers.
2.6 Optional Semicolons
## 2.6 Optional Semicolons
Like many programming languages, JavaScript uses the semicolon (;) to separate statements (see Chapter 5) from one another. This is important for making the meaning of your code clear: without a separator, the end of one statement might appear to be the beginning of the next, or vice versa. In JavaScript, you can usually omit the semicolon between two statements if those statements are written on separate lines. (You can also omit a semicolon at the end of a program or if the next token in the program is a closing curly brace: }.) Many JavaScript programmers (and the code in this book) use semicolons to explicitly mark the ends of statements, even where they are not required. Another style is to omit semicolons whenever possible, using them only in the few situations that require them. Whichever style you choose, there are a few details you should understand about optional semicolons in JavaScript.
Consider the following code. Since the two statements appear on separate lines, the first semicolon could be omitted:
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@@ -142,5 +142,5 @@ This means that you must not insert a line break between return, break, or conti
The second exception involves the ++ and −− operators (§4.8). These operators can be prefix operators that appear before an expression or postfix operators that appear after an expression. If you want to use either of these operators as postfix operators, they must appear on the same line as the expression they apply to. The third exception involves functions defined using concise “arrow” syntax: the => arrow itself must appear on the same line as the parameter list.
2.7 Summary
## 2.7 Summary
This chapter has shown how JavaScript programs are written at the lowest level. The next chapter takes us one step higher and introduces the primitive types and values (numbers, strings, and so on) that serve as the basic units of computation for JavaScript programs.
