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Chapter 4
Values, Variables, and Literals

This chapter discusses values that JavaScript recognizes and describes the fundamental building blocks of JavaScript expressions: variables and literals.

This chapter contains the following sections:


Values

JavaScript recognizes the following types of values:

This relatively small set of types of values, or data types, enables you to perform useful functions with your applications. There is no explicit distinction between integer and real-valued numbers. Nor is there an explicit date data type in JavaScript. However, you can use the Date object and its methods to handle dates.

Objects and functions are the other fundamental elements in the language. You can think of objects as named containers for values, and functions as procedures that your application can perform.

Data Type Conversion

JavaScript is a dynamically typed language. That means you do not have to specify the data type of a variable when you declare it, and data types are converted automatically as needed during script execution. So, for example, you could define a variable as follows:

var answer = 42
And later, you could assign the same variable a string value, for example,

answer = "Thanks for all the fish..."
Because JavaScript is dynamically typed, this assignment does not cause an error message.

In expressions involving numeric and string values with the + operator, JavaScript converts numeric values to strings. For example, consider the following statements:

x = "The answer is " + 42 // returns "The answer is 42"
y = 42 + " is the answer" // returns "42 is the answer"
In statements involving other operators, JavaScript does not convert numeric values to strings. For example:

"37" - 7 // returns 30
"37" + 7 // returns 377

Variables

You use variables as symbolic names for values in your application. You give variables names by which you refer to them and which must conform to certain rules.

A JavaScript identifier, or name, must start with a letter or underscore ("_"); subsequent characters can also be digits (0-9). Because JavaScript is case sensitive, letters include the characters "A" through "Z" (uppercase) and the characters "a" through "z" (lowercase).

Some examples of legal names are Number_hits, temp99, and _name.

Declaring Variables

You can declare a variable in two ways:

Evaluating Variables

A variable or array element that has not been assigned a value has the value undefined. The result of evaluating an unassigned variable depends on how it was declared:

The following code demonstrates evaluating unassigned variables.

function f1() {
   return y - 2;
}
f1() //Causes runtime error
function f2() {
   return var y - 2;
}
f2() //returns NaN
You can use undefined to determine whether a variable has a value. In the following code, the variable input is not assigned a value, and the if statement evaluates to true.

var input;
if(input === undefined){
   doThis();
} else {
   doThat();
}
The undefined value behaves as false when used as a Boolean value. For example, the following code executes the function myFunction because the array element is not defined:

myArray=new Array()
if (!myArray[0])
   myFunction()
When you evaluate a null variable, the null value behaves as 0 in numeric contexts and as false in Boolean contexts. For example:

var n = null
n * 32 //returns 0

Variable Scope

When you set a variable identifier by assignment outside of a function, it is called a global variable, because it is available everywhere in the current document. When you declare a variable within a function, it is called a local variable, because it is available only within the function.

Using var to declare a global variable is optional. However, you must use var to declare a variable inside a function.

You can access global variables declared in one window or frame from another window or frame by specifying the window or frame name. For example, if a variable called phoneNumber is declared in a FRAMESET document, you can refer to this variable from a child frame as parent.phoneNumber.


Literals

You use literals to represent values in JavaScript. These are fixed values, not variables, that you literally provide in your script. This section describes the following types of literals:

Array Literals

An array literal is a list of zero or more expressions, each of which represents an array element, enclosed in square brackets ([]). When you create an array using an array literal, it is initialized with the specified values as its elements, and its length is set to the number of arguments specified.

The following example creates the coffees array with three elements and a length of three:

coffees = ["French Roast", "Columbian", "Kona"]
NOTE: An array literal is a type of object initializer. See "Using Object Initializers" on page 139.
If an array is created using a literal in a top-level script, JavaScript interprets the array each time it evaluates the expression containing the array literal. In addition, a literal used in a function is created each time the function is called.

Array literals are also Array objects. See "Array Object" on page 145 for details on Array objects.

Extra Commas in Array Literals

You do not have to specify all elements in an array literal. If you put two commas in a row, the array is created with spaces for the unspecified elements. The following example creates the fish array:

fish = ["Lion", , "Angel"]
This array has two elements with values and one empty element (fish[0] is "Lion", fish[1] is undefined, and fish[2] is "Angel"):

If you include a trailing comma at the end of the list of elements, the comma is ignored. In the following example, the length of the array is three. There is no myList[3]. All other commas in the list indicate a new element.

myList = ['home', , 'school', ];
In the following example, the length of the array is four, and myList[0] is missing.

myList = [ , 'home', , 'school'];
In the following example, the length of the array is four, and myList[3] is missing. Only the last comma is ignored. This trailing comma is optional.

myList = ['home', , 'school', , ];

Boolean Literals

The Boolean type has two literal values: true and false.

Do not confuse the primitive Boolean values true and false with the true and false values of the Boolean object. The Boolean object is a wrapper around the primitive Boolean data type. See "Boolean Object" on page 148 for more information.

Floating-Point Literals

A floating-point literal can have the following parts:

The exponent part is an "e" or "E" followed by an integer, which can be signed (preceded by "+" or "-"). A floating-point literal must have at least one digit and either a decimal point or "e" (or "E").

Some examples of floating-point literals are 3.1415, -3.1E12, .1e12, and 2E-12

Integers

Integers can be expressed in decimal (base 10), hexadecimal (base 16), and octal (base 8). A decimal integer literal consists of a sequence of digits without a leading 0 (zero). A leading 0 (zero) on an integer literal indicates it is in octal; a leading 0x (or 0X) indicates hexadecimal. Hexadecimal integers can include digits (0-9) and the letters a-f and A-F. Octal integers can include only the digits 0-7.

Some examples of integer literals are: 42, 0xFFF, and -345.

Object Literals

An object literal is a list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}). You should not use an object literal at the beginning of a statement. This will lead to an error.

The following is an example of an object literal. The first element of the car object defines a property, myCar; the second element, the getCar property, invokes a function (Cars("honda")); the third element, the special property, uses an existing variable (Sales).

var Sales = "Toyota";
function CarTypes(name) {
   if(name == "Honda")
      return name;
   else
      return "Sorry, we don't sell " + name + ".";
}
car = {myCar: "Saturn", getCar: CarTypes("Honda"), special: Sales}
document.write(car.myCar); // Saturn
document.write(car.getCar); // Honda
document.write(car.special); // Toyota
Additionally, you can use an index for the object, the index property (for example, 7), or nest an object inside another. The following example uses these options. These features, however, may not be supported by other ECMA-compliant browsers.

car = {manyCars: {a: "Saab", b: "Jeep"}, 7: "Mazda"}
document.write(car.manyCars.b); // Jeep
document.write(car[7]); // Mazda

String Literals

A string literal is zero or more characters enclosed in double (") or single (') quotation marks. A string must be delimited by quotation marks of the same type; that is, either both single quotation marks or both double quotation marks. The following are examples of string literals:

You can call any of the methods of the String object on a string literal value--JavaScript automatically converts the string literal to a temporary String object, calls the method, then discards the temporary String object. You can also use the String.length property with a string literal.

You should use string literals unless you specifically need to use a String object. See "String Object" on page 155 for details on String objects.

Using Special Characters in Strings

In addition to ordinary characters, you can also include special characters in strings, as shown in the following example.

"one line \n another line"
The following table lists the special characters that you can use in JavaScript strings.

Table 4.1 JavaScript special characters
Character Meaning
\b

Backspace

\f

Form feed

\n

New line

\r

Carriage return

\t

Tab

\'

Apostrophe or single quote

\"

Double quote

\\

Backslash character (\)

\XXX

The character with the Latin-1 encoding specified by up to three octal digits XXX between 0 and 377. For example, \251 is the octal sequence for the copyright symbol.

\xXX

The character with the Latin-1 encoding specified by the two hexadecimal digits XX between 00 and FF. For example, \xA9 is the hexadecimal sequence for the copyright symbol.

Escaping Characters

For characters not listed in Table 4.1, a preceding backslash is ignored, with the exception of a quotation mark and the backslash character itself.

You can insert a quotation mark inside a string by preceding it with a backslash. This is known as escaping the quotation mark. For example,

var quote = "He read \"The Cremation of Sam McGee\" by R.W. Service."
document.write(quote)
The result of this would be

He read "The Cremation of Sam McGee" by R.W. Service.

To include a literal backslash inside a string, you must escape the backslash character. For example, to assign the file path c:\temp to a string, use the following:

var home = "c:\\temp"

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Last Updated: 11/12/98 15:29:12

Copyright 1998 Netscape Communications Corporation

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