Chapter 1. Introduction to JavaScript

JavaScript is the programming language of the web. The overwhelming majority of websites use JavaScript, and all modern web browsers—on desktops, tablets, and phones—include JavaScript interpreters, making JavaScript the most-deployed programming language in history. Over the last decade, Node.js has enabled JavaScript programming outside of web browsers, and the dramatic success of Node means that JavaScript is now also the most-used programming language among software developers. Whether you’re starting from scratch or are already using JavaScript professionally, this book will help you master the language.

If you are already familiar with other programming languages, it may help you to know that JavaScript is a high-level, dynamic, interpreted programming language that is well-suited to object-oriented and functional programming styles. JavaScript’s variables are untyped. Its syntax is loosely based on Java, but the languages are otherwise unrelated. JavaScript derives its first-class functions from Scheme and its prototype-based inheritance from the little-known language Self. But you do not need to know any of those languages, or be familiar with those terms, to use this book and learn JavaScript.

The name “JavaScript” is quite misleading. Except for a superficial syntactic resemblance, JavaScript is completely different from the Java programming language. And JavaScript has long since outgrown its scripting-language roots to become a robust and efficient general-purpose language suitable for serious software engineering and projects with huge codebases.

To be useful, every language must have a platform, or standard library, for performing things like basic input and output. The core JavaScript language defines a minimal API for working with numbers, text, arrays, sets, maps, and so on, but does not include any input or output functionality. Input and output (as well as more sophisticated features, such as networking, storage, and graphics) are the responsibility of the “host environment” within which JavaScript is embedded.

The original host environment for JavaScript was a web browser, and this is still the most common execution environment for JavaScript code. The web browser environment allows JavaScript code to obtain input from the user’s mouse and keyboard and by making HTTP requests. And it allows JavaScript code to display output to the user with HTML and CSS.

Since 2010, another host environment has been available for JavaScript code. Instead of constraining JavaScript to work with the APIs provided by a web browser, Node gives JavaScript access to the entire operating system, allowing JavaScript programs to read and write files, send and receive data over the network, and make and serve HTTP requests. Node is a popular choice for implementing web servers and also a convenient tool for writing simple utility scripts as an alternative to shell scripts.

Most of this book is focused on the JavaScript language itself. Chapter 11 documents the JavaScript standard library, Chapter 15 introduces the web browser host environment, and Chapter 16 introduces the Node host environment.

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

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.

js7e 0101
Figure 1-1. The JavaScript console in Firefox’s Developer Tools

Another way to try out JavaScript code is to download and install Node from https://nodejs.org. Once Node is installed on your system, you can simply open a Terminal window and type node to begin an interactive JavaScript session like this one:

$ node
Welcome to Node.js v12.13.0.
Type ".help" for more information.
> .help
.break    Sometimes you get stuck, this gets you out
.clear    Alias for .break
.editor   Enter editor mode
.exit     Exit the repl
.help     Print this help message
.load     Load JS from a file into the REPL session
.save     Save all evaluated commands in this REPL session to a file

Press ^C to abort current expression, ^D to exit the repl
> let x = 2, y = 3;
undefined
> x + y
5
> (x === 2) && (y === 3)
true
> (x > 3) || (y < 3)
false

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

If you use Node in a noninteractive manner like this, it won’t automatically print out the value of all the code you run, so you’ll have to do that yourself. You can use the function console.log() to display text and other JavaScript values in your terminal window or in a browser’s developer tools console. So, for example, if you create a hello.js file containing this line of code:

console.log("Hello World!");

and execute the file with node hello.js, you’ll see the message “Hello World!” printed out.

If you want to see that same message printed out in the JavaScript console of a web browser, create a new file named hello.html, and put this text in it:

<script src="hello.js"></script>

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

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:

// Anything following double slashes is an English-language comment.
// Read the comments carefully: they explain the JavaScript code.

// A variable is a symbolic name for a value.
// Variables are declared with the let keyword:
let x;                     // Declare a variable named x.

// Values can be assigned to variables with an = sign
x = 0;                     // Now the variable x has the value 0
x                          // => 0: A variable evaluates to its value.

// JavaScript supports several types of values
x = 1;                     // Numbers.
x = 0.01;                  // Numbers can be integers or reals.
x = "hello world";         // Strings of text in quotation marks.
x = 'JavaScript';          // Single quote marks also delimit strings.
x = true;                  // A Boolean value.
x = false;                 // The other Boolean value.
x = null;                  // Null is a special value that means "no value."
x = undefined;             // Undefined is another special value like null.

Two other very important types that JavaScript programs can manipulate are objects and arrays. These are the subjects of Chapters 6 and 7, but they are so important that you’ll see them many times before you reach those chapters:

// JavaScript's most important datatype is the object.
// An object is a collection of name/value pairs, or a string to value map.
let book = {               // Objects are enclosed in curly braces.
    topic: "JavaScript",   // The property "topic" has value "JavaScript."
    edition: 7             // The property "edition" has value 7
};                         // The curly brace marks the end of the object.

// Access the properties of an object with . or []:
book.topic                 // => "JavaScript"
book["edition"]            // => 7: another way to access property values.
book.author = "Flanagan";  // Create new properties by assignment.
book.contents = {};        // {} is an empty object with no properties.

// Conditionally access properties with ?. (ES2020):
book.contents?.ch01?.sect1 // => undefined: book.contents has no ch01 property.

// JavaScript also supports arrays (numerically indexed lists) of values:
let primes = [2, 3, 5, 7]; // An array of 4 values, delimited with [ and ].
primes[0]                  // => 2: the first element (index 0) of the array.
primes.length              // => 4: how many elements in the array.
primes[primes.length-1]    // => 7: the last element of the array.
primes[4] = 9;             // Add a new element by assignment.
primes[4] = 11;            // Or alter an existing element by assignment.
let empty = [];            // [] is an empty array with no elements.
empty.length               // => 0

// Arrays and objects can hold other arrays and objects:
let points = [             // An array with 2 elements.
    {x: 0, y: 0},          // Each element is an object.
    {x: 1, y: 1}
];
let data = {                 // An object with 2 properties
    trial1: [[1,2], [3,4]],  // The value of each property is an array.
    trial2: [[2,3], [4,5]]   // The elements of the arrays are arrays.
};

The syntax illustrated here for listing array elements within square braces or mapping object property names to property values inside curly braces is known as an initializer expression, and it is just one of the topics of Chapter 4. An expression is a phrase of JavaScript that can be evaluated to produce a value. For example, the use of . and [] to refer to the value of an object property or array element is an expression.

One of the most common ways to form expressions in JavaScript is to use operators:

// Operators act on values (the operands) to produce a new value.
// Arithmetic operators are some of the simplest:
3 + 2                      // => 5: addition
3 - 2                      // => 1: subtraction
3 * 2                      // => 6: multiplication
3 / 2                      // => 1.5: division
points[1].x - points[0].x  // => 1: more complicated operands also work
"3" + "2"                  // => "32": + adds numbers, concatenates strings

// JavaScript defines some shorthand arithmetic operators
let count = 0;             // Define a variable
count++;                   // Increment the variable
count--;                   // Decrement the variable
count += 2;                // Add 2: same as count = count + 2;
count *= 3;                // Multiply by 3: same as count = count * 3;
count                      // => 6: variable names are expressions, too.

// Equality and relational operators test whether two values are equal,
// unequal, less than, greater than, and so on. They evaluate to true or false.
let x = 2, y = 3;          // These = signs are assignment, not equality tests
x === y                    // => false: equality
x !== y                    // => true: inequality
x < y                      // => true: less-than
x <= y                     // => true: less-than or equal
x > y                      // => false: greater-than
x >= y                     // => false: greater-than or equal
"two" === "three"          // => false: the two strings are different
"two" > "three"            // => true: "tw" is alphabetically greater than "th"
false === (x > y)          // => true: false is equal to false

// Logical operators combine or invert boolean values
(x === 2) && (y === 3)     // => true: both comparisons are true. && is AND
(x > 3) || (y < 3)         // => false: neither comparison is true. || is OR
!(x === y)                 // => true: ! inverts a boolean value

If JavaScript expressions are like phrases, then JavaScript statements are like full sentences. Statements are the topic of Chapter 5. Roughly, an expression is something that computes a value but doesn’t do anything: it doesn’t alter the program state in any way. Statements, on the other hand, don’t have a value, but they do alter the state. You’ve seen variable declarations and assignment statements above. The other broad category of statement is control structures, such as conditionals and loops. You’ll see examples below, after we cover functions.

A function is a named and parameterized block of JavaScript code that you define once, and can then invoke over and over again. Functions aren’t covered formally until Chapter 8, but like objects and arrays, you’ll see them many times before you get to that chapter. Here are some simple examples:

// Functions are parameterized blocks of JavaScript code that we can invoke.
function plus1(x) {        // Define a function named "plus1" with parameter "x"
    return x + 1;          // Return a value one larger than the value passed in
}                          // Functions are enclosed in curly braces

plus1(y)                   // => 4: y is 3, so this invocation returns 3+1

let square = function(x) { // Functions are values and can be assigned to vars
    return x * x;          // Compute the function's value
};                         // Semicolon marks the end of the assignment.

square(plus1(y))           // => 16: invoke two functions in one expression

In ES6 and later, there is a shorthand syntax for defining functions. This concise syntax uses => to separate the argument list from the function body, so functions defined this way are known as arrow functions. Arrow functions are most commonly used when you want to pass an unnamed function as an argument to another function. The preceding code looks like this when rewritten to use arrow functions:

const plus1 = x => x + 1;   // The input x maps to the output x + 1
const square = x => x * x;  // The input x maps to the output x * x
plus1(y)                    // => 4: function invocation is the same
square(plus1(y))            // => 16

When we use functions with objects, we get methods:

// When functions are assigned to the properties of an object, we call
// them "methods."  All JavaScript objects (including arrays) have methods:
let a = [];                // Create an empty array
a.push(1,2,3);             // The push() method adds elements to an array
a.reverse();               // Another method: reverse the order of elements

// We can define our own methods, too. The "this" keyword refers to the object
// on which the method is defined: in this case, the points array from earlier.
points.dist = function() { // Define a method to compute distance between points
    let p1 = this[0];      // First element of array we're invoked on
    let p2 = this[1];      // Second element of the "this" object
    let a = p2.x-p1.x;     // Difference in x coordinates
    let b = p2.y-p1.y;     // Difference in y coordinates
    return Math.sqrt(a*a + // The Pythagorean theorem
                     b*b); // Math.sqrt() computes the square root
};
points.dist()              // => Math.sqrt(2): distance between our 2 points

Now, as promised, here are some functions whose bodies demonstrate common JavaScript control structure statements:

// JavaScript statements include conditionals and loops using the syntax
// of C, C++, Java, and other languages.
function abs(x) {          // A function to compute the absolute value.
    if (x >= 0) {          // The if statement...
        return x;          // executes this code if the comparison is true.
    }                      // This is the end of the if clause.
    else {                 // The optional else clause executes its code if
        return -x;         // the comparison is false.
    }                      // Curly braces optional when 1 statement per clause.
}                          // Note return statements nested inside if/else.
abs(-10) === abs(10)       // => true

function sum(array) {      // Compute the sum of the elements of an array
    let sum = 0;           // Start with an initial sum of 0.
    for(let x of array) {  // Loop over array, assigning each element to x.
        sum += x;          // Add the element value to the sum.
    }                      // This is the end of the loop.
    return sum;            // Return the sum.
}
sum(primes)                // => 28: sum of the first 5 primes 2+3+5+7+11

function factorial(n) {    // A function to compute factorials
    let product = 1;       // Start with a product of 1
    while(n > 1) {         // Repeat statements in {} while expr in () is true
        product *= n;      // Shortcut for product = product * n;
        n--;               // Shortcut for n = n - 1
    }                      // End of loop
    return product;        // Return the product
}
factorial(4)               // => 24: 1*4*3*2

function factorial2(n) {   // Another version using a different loop
    let i, product = 1;    // Start with 1
    for(i=2; i <= n; i++)  // Automatically increment i from 2 up to n
        product *= i;      // Do this each time. {} not needed for 1-line loops
    return product;        // Return the factorial
}
factorial2(5)              // => 120: 1*2*3*4*5

JavaScript supports an object-oriented programming style, but it is significantly different than “classical” object-oriented programming languages. Chapter 9 covers object-oriented programming in JavaScript in detail, with lots of examples. Here is a very simple example that demonstrates how to define a JavaScript class to represent 2D geometric points. Objects that are instances of this class have a single method, named distance(), that computes the distance of the point from the origin:

class Point {              // By convention, class names are capitalized.
    constructor(x, y) {    // Constructor function to initialize new instances.
        this.x = x;        // This keyword is the new object being initialized.
        this.y = y;        // Store function arguments as object properties.
    }                      // No return is necessary in constructor functions.

    distance() {           // Method to compute distance from origin to point.
        return Math.sqrt(  // Return the square root of x² + y².
            this.x * this.x +  // this refers to the Point object on which
            this.y * this.y    // the distance method is invoked.
        );
    }
}

// Use the Point() constructor function with "new" to create Point objects
let p = new Point(1, 1);   // The geometric point (1,1).

// Now use a method of the Point object p
p.distance()               // => Math.SQRT2

This introductory tour of JavaScript’s fundamental syntax and capabilities ends here, but the book continues with self-contained chapters that cover additional features of the language:

Chapter 10, Modules

Shows how JavaScript code in one file or script can use JavaScript functions and classes defined in other files or scripts.

Chapter 11, The JavaScript Standard Library

Covers the built-in functions and classes that are available to all JavaScript programs. This includes important data stuctures like maps and sets, a regular expression class for textual pattern matching, functions for serializing JavaScript data structures, and much more.

Chapter 12, Iterators and Generators

Explains how the for/of loop works and how you can make your own classes iterable with for/of. It also covers generator functions and the yield statement.

Chapter 13, Asynchronous JavaScript

This chapter is an in-depth exploration of asynchronous programming in JavaScript, covering callbacks and events, Promise-based APIs, and the async and await keywords. Although the core JavaScript language is not asynchronous, asynchronous APIs are the default in both web browsers and Node, and this chapter explains the techniques for working with those APIs.

Chapter 14, Metaprogramming

Introduces a number of advanced features of JavaScript that may be of interest to programmers writing libraries of code for other JavaScript programmers to use.

Chapter 15, JavaScript in Web Browsers

Introduces the web browser host environment, explains how web browsers execute JavaScript code, and covers the most important of the many APIs defined by web browsers. This is by far the longest chapter in the book.

Chapter 16, Server-Side JavaScript with Node

Introduces the Node host environment, covering the fundamental programming model and the data structures and APIs that are most important to understand.

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

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
T: ########### 11.22%
E: ########## 10.15%
R: ####### 6.68%
S: ###### 6.44%
A: ###### 6.16%
N: ###### 5.81%
O: ##### 5.45%
I: ##### 4.54%
H: #### 4.07%
C: ### 3.36%
L: ### 3.20%
U: ### 3.08%
/: ### 2.88%

This example uses a number of advanced JavaScript features and is intended to demonstrate what real-world JavaScript programs can look like. You should not expect to understand all of the code yet, but be assured that all of it will be explained in the chapters that follow.

Example 1-1. Computing character frequency histograms with JavaScript
/**
 * This Node program reads text from standard input, computes the frequency
 * of each letter in that text, and displays a histogram of the most
 * frequently used characters. It requires Node 12 or higher to run.
 *
 * In a Unix-type environment you can invoke the program like this:
 *    node charfreq.js < corpus.txt
 */

// This class extends Map so that the get() method returns the specified
// value instead of null when the key is not in the map
class DefaultMap extends Map {
    constructor(defaultValue) {
        super();                          // Invoke superclass constructor
        this.defaultValue = defaultValue; // Remember the default value
    }

    get(key) {
        if (this.has(key)) {              // If the key is already in the map
            return super.get(key);        // return its value from superclass.
        }
        else {
            return this.defaultValue;     // Otherwise return the default value
        }
    }
}

// This class computes and displays letter frequency histograms
class Histogram {
    constructor() {
        this.letterCounts = new DefaultMap(0);  // Map from letters to counts
        this.totalLetters = 0;                  // How many letters in all
    }

    // This function updates the histogram with the letters of text.
    add(text) {
        // Remove whitespace from the text, and convert to upper case
        text = text.replace(/\s/g, "").toUpperCase();

        // Now loop through the characters of the text
        for(let character of text) {
            let count = this.letterCounts.get(character); // Get old count
            this.letterCounts.set(character, count+1);    // Increment it
            this.totalLetters++;
        }
    }

    // Convert the histogram to a string that displays an ASCII graphic
    toString() {
        // Convert the Map to an array of [key,value] arrays
        let entries = [...this.letterCounts];

        // Sort the array by count, then alphabetically
        entries.sort((a,b) => {              // A function to define sort order.
            if (a[1] === b[1]) {             // If the counts are the same
                return a[0] < b[0] ? -1 : 1; // sort alphabetically.
            } else {                         // If the counts differ
                return b[1] - a[1];          // sort by largest count.
            }
        });

        // Convert the counts to percentages
        for(let entry of entries) {
            entry[1] = entry[1] / this.totalLetters*100;
        }

        // Drop any entries less than 1%
        entries = entries.filter(entry => entry[1] >= 1);

        // Now convert each entry to a line of text
        let lines = entries.map(
            ([l,n]) => `${l}: ${"#".repeat(Math.round(n))} ${n.toFixed(2)}%`
        );

        // And return the concatenated lines, separated by newline characters.
        return lines.join("\n");
    }
}

// This async (Promise-returning) function creates a Histogram object,
// asynchronously reads chunks of text from standard input, and adds those chunks to
// the histogram. When it reaches the end of the stream, it returns this histogram
async function histogramFromStdin() {
    process.stdin.setEncoding("utf-8"); // Read Unicode strings, not bytes
    let histogram = new Histogram();
    for await (let chunk of process.stdin) {
        histogram.add(chunk);
    }
    return histogram;
}

// 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.
histogramFromStdin().then(histogram => { console.log(histogram.toString()); });

1.5 Summary

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.

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