Core JavaScript Guide 1.5: 2 Values, Variables, and Literals | WebReference

Core JavaScript Guide 1.5: 2 Values, Variables, and Literals

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Core JavaScript Guide 1.5



Chapter 2   Values, Variables, and Literals


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

This chapter contains the following sections:



Values

JavaScript recognizes the following types of values:

  • Numbers, such as 42 or 3.14159

  • Logical (Boolean) values, either true or false

  • Strings, such as "Howdy!"

  • null, a special keyword denoting a null value; null is also a primitive value. Because JavaScript is case-sensitive, null is not the same as Null, NULL, or any other variant

  • undefined, a top-level property whose value is undefined; undefined is also a primitive value

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. The names of variables, called identifiers, conform to certain rules.

A JavaScript identifier must start with a letter, underscore (_), or dollar sign ($); 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).

Starting with JavaScript 1.5, you can use ISO 8859-1 or Unicode letters such as å and ü in identifiers. You can also use the \uXXXX Unicode escape sequences listed on page 34 as characters in identifiers.

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


Declaring Variables

You can declare a variable in two ways:

  • By simply assigning it a value. For example, x = 42

  • With the keyword var. For example, var x = 42


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:

  • If the unassigned variable was declared without var, the evaluation results in a runtime error.

  • If the unassigned variable was declared with var, the evaluation results in the undefined value, or NaN in numeric contexts.

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.



Constants



You can create a read-only, named constant with the const keyword. The syntax of a constant identifier is the same as for a variable identifier: it must start with a letter or underscore and can contain alphabetic, numeric, or underscore characters.

const prefix = '212';

A constant cannot change value through assignment or be re-declared while the script is running.

The scope rules for constants are the same as those for variables, except that the const keyword is always required, even for global constants. If the keyword is omitted, the identifier is assumed to represent a var.

You cannot declare a constant at the same scope as a function or variable with the same name as the function or variable. For example:

//THIS WILL CAUSE AN ERROR
function f{};
const f = 5;

//THIS WILL CAUSE AN ERROR ALSO
function f{
const g=5;
var g;

//statements

}



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 93.

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 100 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] and myList[2]are missing.

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

In the following example, the length of the array is four, and myList[1]and myList[3]are 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 103 for more information.


Floating-Point Literals

A floating-point literal can have the following parts:

  • A decimal integer

  • A decimal point (".")

  • A fraction (another decimal number)

  • An exponent

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.

Octal integer literals are deprecated and have been removed from the ECMA-262, Edition 3 standard. JavaScript 1.5 still supports them for backward compatibility.

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 or not behave as you expect, because the { will be interpreted as the beginning of a block.

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 (CarTypes("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 a numeric or string literal for the name of a property or nest an object inside another. The following example uses these options.

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:

  • "blah"

  • 'blah'

  • "1234"

  • "one line \n another line"

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 110 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 2.1 &nbsp&nbsp JavaScript special characters

Character

Meaning

\b

 

Backspace 

\f

 

Form feed 

\n

 

New line 

\r

 

Carriage return 

\t

 

Tab 

\v

 

Vertical 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. 

\uXXXX

 

The Unicode character specified by the four hexadecimal digits XXXX. For example, \u00A9 is the Unicode sequence for the copyright symbol. See Unicode Escape Sequences. 


Escaping Characters

For characters not listed in Table 2.1, a preceding backslash is ignored, but this usage is deprecated and should be avoided.

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"



Unicode



Unicode is a universal character-coding standard for the interchange and display of principal written languages. It covers the languages of Americas, Europe, Middle East, Africa, India, Asia, and Pacifica, as well as historic scripts and technical symbols. Unicode allows for the exchange, processing, and display of multilingual texts, as well as the use of common technical and mathematical symbols. It hopes to resolve internationalization problems of multilingual computing, such as different national character standards. Not all modern or archaic scripts, however, are currently supported.

The Unicode character set can be used for all known encoding. Unicode is modeled after the ASCII (American Standard Code for Information Interchange) character set. It uses a numerical value and name for each character. The character encoding specifies the identity of the character and its numeric value (code position), as well as the representation of this value in bits. The 16-bit numeric value (code value) is defined by a hexadecimal number and a prefix U, for example, U+0041 represents A. The unique name for this value is LATIN CAPITAL LETTER A.

JavaScript versions prior to 1.3. Unicode is not supported in versions of JavaScript prior to 1.3.


Unicode Compatibility with ASCII and ISO

Unicode is compatible with ASCII characters and is supported by many programs. The first 128 Unicode characters correspond to the ASCII characters and have the same byte value. The Unicode characters U+0020 through U+007E are equivalent to the ASCII characters 0x20 through 0x7E. Unlike ASCII, which supports the Latin alphabet and uses a 7-bit character set, Unicode uses a 16-bit value for each character. It allows for tens of thousands of characters. It also supports an extension mechanism, UTF-16, that allows for the encoding of one million more characters by using 16-bit character pairs. UTF turns the encoding to actual bits.

Unicode is fully compatible with the International Standard ISO/IEC 10646-1; 1993, which is a subset of ISO 10646, and supports the ISO UCS-2 (Universal Character Set) that uses two-octets (two bytes or 16 bits).

JavaScript and Navigator support for Unicode means you can use non-Latin, international, and localized characters, plus special technical symbols in JavaScript programs. Unicode provides a standard way to encode multilingual text. Since Unicode is compatible with ASCII, programs can use ASCII characters. You can use non-ASCII Unicode characters in the comments, string literals, identifiers, and regular expressions of JavaScript.


Unicode Escape Sequences

You can use the Unicode escape sequence in string literals, regular expressions, and identifiers. The escape sequence consists of six ASCII characters: \u and a four-digit hexadecimal number. For example, \u00A9 represents the copyright symbol. Every Unicode escape sequence in JavaScript is interpreted as one character.

The following code returns the copyright symbol and the string "Netscape Communications".

x="\u00A9 Netscape Communications"

The following table lists frequently used special characters and their Unicode value.


Table 2.2 &nbsp&nbsp Unicode values for special characters

Category

Unicode value

Name

Format name

White space values 

\u0009 

Tab 

<TAB> 

 

\u000B 

Vertical Tab 

<VT> 

 

\u000C 

Form Feed 

<FF> 

 

\u0020 

Space 

<SP> 

Line terminator values 

\u000A 

Line Feed 

<LF> 

 

\u000D 

Carriage Return 

<CR> 

Additional Unicode escape sequence values 

\u0008 

Backspace 

<BS> 

 

\u0009 

Horizontal Tab 

<HT> 

 

\u0022 

Double Quote 

" 

 

\u0027 

Single Quote 

' 

 

\u005C 

Backslash 

\ 

The JavaScript use of the Unicode escape sequence is different from Java. In JavaScript, the escape sequence is never interpreted as a special character first. For example, a line terminator escape sequence inside a string does not terminate the string before it is interpreted by the function. JavaScript ignores any escape sequence if it is used in comments. In Java, if an escape sequence is used in a single comment line, it is interpreted as an Unicode character. For a string literal, the Java compiler interprets the escape sequences first. For example, if a line terminator escape character (\u000A) is used in Java, it terminates the string literal. In Java, this leads to an error, because line terminators are not allowed in string literals. You must use \n for a line feed in a string literal. In JavaScript, the escape sequence works the same way as \n.


Displaying Characters with Unicode

You can use Unicode to display the characters in different languages or technical symbols. For characters to be displayed properly, a client such as Netscape Navigator 4.x or Netscape 6 needs to support Unicode. Moreover, an appropriate Unicode font must be available to the client, and the client platform must support Unicode. Often, Unicode fonts do not display all the Unicode characters. Some platforms, such as Windows 95, provide a partial support for Unicode.

To receive non-ASCII character input, the client needs to send the input as Unicode. Using a standard enhanced keyboard, the client cannot easily input the additional characters supported by Unicode. Sometimes, the only way to input Unicode characters is by using Unicode escape sequences.

For more information on Unicode, see the Unicode Consortium Web site and The Unicode Standard, Version 2.0, published by Addison-Wesley, 1996.


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Last Updated September 28, 2000