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.
//