This section is a tour of JavaScript language, and also a tour of Part I of this book. After this introductory chapter, we dive into JavaScript at the lowest level: Chapter 2, Lexical Structure, explains things like JavaScript comments, semicolons, and the Unicode character set.
Chapter 3, Types, Values, and Variable, 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 thing following double slashes is an English-language comment.
// Read the comments carefully: they explain the JavaScript code.
// variable is a symbolic name for a value.
// Variables are declared with the var keyword:
var 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; // Just one Number type for Integers and reals.
x = "hello world"; // Strings of text in quotation marks.
x = 'JavaScript'; // Single quote marks also delimit strings.
x = true; // Boolean values.
x = false; // The other Boolean value.
x = null; // undefined is like null.
Two other very important types that JavaScript programs can manipulate are objects and arrays. These are the subject of Chapter 6, Objects, and Chapter 7, Arrays, but they are so important thaht you'll see them many times before you reach those chapters.
//JavaScript's most important data type is the object.
//An object is a collection of name/value pairs, or a string to value map.
var book = { // Objects are enclosed in curly braces.
topic: "JavaScript", // The property "topic" has value "JavaScript".
fat: true // The property "fat" has value true.
}; // The curly brace marks the end of object.
// Access the properties of an object with . or []:
book.topic // => "JavaScript"
book["fat"] // => true: another way to access property values.
book.author = "Flanagan"; // Create new properties by assignment.
book.contents = {}; // {} is an empty object with no properties.
// JavaScript also supports arrays (numerically indexed lists) of values.
var 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.
var empty = []; // [] is an empty array with no elements.
empty.length //=> 0
// Arrays and objects can hold other arrays and objects.
var points = [ // An array with 2 elements.
{x:0, y:0}, // Each element is an object.
{x:1, y:1}
];
var 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 above for listing array elements within square brakets or mapping object property names to property values inside curly braces is known as an initialize expression, and it is just one of the topic of Chapter 4, Expressions and Operators. An expression is a phrase of JavaScript that can be evaluated to produce a value. The use of . and [] to refer to the value of an object property or array element is an expression, for example. You may have noticed in the code above that when an expression stands alone on a line, the comment that follows it begins with an arrow(=>) and the value of the expression. This a a convention that you'll see throughtout this book.