x = 7 is an expression that assigns x the value seven. This expression itself evaluates to seven. Such expressions use assignment operators. On the other hand, the expression 3 + 4 simply evaluates to seven; it does not perform an assignment. The operators used in such expressions are referred to simply as operators.
JavaScript has the following types of expressions:
null denotes a null value. In contrast, variables that have not been assigned a value are undefined and cause a runtime error if used as numbers or as numeric variables. Array elements that have not been assigned a value, however, evaluate to false. For example, the following code executes the function myFunction because the array element is not defined:
myArray=new Array()
if (!myArray["notThere"])
myFunction()
operand1 operator operand2For example,
3+4 or x*y.
A unary operator requires a single operand, either before or after the operator:
operator operandor
operand operatorFor example,
x++ or ++x.
In addition, JavaScript has one ternary operator, the conditional operator. A ternary operator requires three operands.
Table 9.1 Assignment operators
Comparison Operators
A comparison operator compares its operands and returns a logical value based on whether the comparison is true or not. The operands can be numerical or string values. When used on string values, the comparisons are based on the standard lexicographical ordering. They are described in Table 9.2.
Table 9.2 Comparison operators
| 1 In these examples, assume var1 has been assigned the value 3 and var2 had been assigned the value 4. |
Arithmetic Operators
Arithmetic operators take numerical values (either literals or variables) as their operands and return a single numerical value. The standard arithmetic operators are addition (+), subtraction (-), multiplication (*), and division (/). These operators work as they do in other programming languages.
In addition, JavaScript provides the arithmetic operators listed in Table 9.3.
Table 9.3 Arithmetic Operators
Bitwise Operators
Bitwise operators treat their operands as a set of bits (zeros and ones), rather than as decimal, hexadecimal, or octal numbers. For example, the decimal number nine has a binary representation of 1001. Bitwise operators perform their operations on such binary representations, but they return standard JavaScript numerical values.
The following table summarizes JavaScript's bitwise operators
Bitwise Logical Operators
Conceptually, the bitwise logical operators work as follows:
Table 9.5 Bitwise shift operators
Logical Operators
Logical operators take Boolean (logical) values as operands and return a Boolean value. They are described in Table 9.6.
| 1 Assume var1 is "Cat", var2 is "Dog", and var3 is false. |
Short-Circuit Evaluation
As logical expressions are evaluated left to right, they are tested for possible "short-circuit" evaluation using the following rules:
"my " + "string" returns the string "my string".
The shorthand assignment operator += can also be used to concatenate strings. For example, if the variable mystring has the value "alpha," then the expression mystring += "bet" evaluates to "alphabet" and assigns this value to mystring.
(condition) ? val1 : val2If
condition is true, the operator has the value of val1. Otherwise it has the value of val2. You can use the conditional operator anywhere you would use a standard operator.
For example,
status = (age >= 18) ? "adult" : "minor"This statement assigns the value "adult" to the variable
status if age is eighteen or more. Otherwise, it assigns the value "minor" to status.
,) simply evaluates both of its operands and returns the value of the second operand. This operator is primarily used inside a for loop, to allow multiple variables to be updated each time through the loop.
For example, if a is a 2-dimensional array with 10 elements on a side, the following code uses the comma operator to increment two variables at once. The code prints the values of the diagonal elements in the array:
for (var i=0, j=10; i <= 10; i++, j--)
document.writeln("a["+i+","+j+"]= " + a[i,j])
delete operator deletes an object's property or an element at a specified index in an array. Its syntax is:
delete objectName.propertywhere
delete objectname[index]
delete property
objectName is the name of an object, property is an existing property, and index is an integer representing the location of an element in an array
The third form is legal only within a with statement. If the deletion succeeds, the delete operator sets the property or element to undefined. delete always returns undefined.
new operator to create an instance of a user-defined object type or of one of the predefined object types Array, Boolean, Date, Function, Image, Number, Object, Option, RegExp, or String. On the server, you can also use it with DbPool, Lock, File, or SendMail. Use new as follows:
objectName = new objectType ( param1 [,param2] ...[,paramN] )For more information, see
new in the JavaScript Reference.
typeof
The typeof operator is used in either of the following ways:
1. typeof operand
The
2. typeof (operand)typeof operator returns a string indicating the type of the unevaluated operand. operand is the string, variable, keyword, or object for which the type is to be returned. The parentheses are optional.
Suppose you define the following variables:
var myFun = new Function("5+2")
The
var shape="round"
var size=1
var today=new Date()typeof operator returns the following results for these variables:
typeof myFun is object
For the keywords
typeof shape is string
typeof size is number
typeof today is object
typeof dontExist is undefinedtrue and null, the typeof operator returns the following results:
typeof true is boolean
For a number or string, the
typeof null is objecttypeof operator returns the following results:
typeof 62 is number
For property values, the
typeof 'Hello world' is stringtypeof operator returns the type of value the property contains:
typeof document.lastModified is string
For methods and functions, the
typeof window.length is number
typeof Math.LN2 is numbertypeof operator returns results as follows:
typeof blur is function
For predefined objects, the
typeof eval is function
typeof parseInt is function
typeof shape.split is functiontypeof operator returns results as follows:
typeof Date is function
typeof Function is function
typeof Math is function
typeof Option is function
typeof String is function void
The void operator is used in either of the following ways:
1. javascript:void (expression)
The void operator specifies an expression to be evaluated without returning a value.
2. javascript:void expressionexpression is a JavaScript expression to evaluate. The parentheses surrounding the expression are optional, but it is good style to use them.
You can use the void operator to specify an expression as a hypertext link. The expression is evaluated but is not loaded in place of the current document.
The following code creates a hypertext link that does nothing when the user clicks it. When the user clicks the link, void(0) evaluates to 0, but that has no effect in JavaScript.
<A HREF="javascript:void(0)">Click here to do nothing</A>
The following code creates a hypertext link that submits a form when the user clicks it.
<A HREF="javascript:void(document.form.submit())">
Click here to submit</A> Operator Precedence
The precedence of operators determines the order they are applied when evaluating an expression. You can override operator precedence by using parentheses.
Table 9.7 describes the precedence of operators, from lowest to highest.
Regular Expressions
JavaScript 1.2, available in Navigator 4.0, adds regular expressions to the language. Regular expressions are patterns used to match character combinations in strings. In JavaScript, regular expressions are also objects. For example, to search for all occurrences of 'the' in a string, you create a pattern consisting of 'the' and use the pattern to search for its match in a string. Regular expression patterns can be constructed using either object initializers (for example, /abc/) or the RegExp constructor function (for example, re = new RegExp("abc")). Object initializers are discussed in "Using Object Initializers".
These patterns are used with the exec and test methods of regular expressions, and with the match, replace, search, and split methods of String.
Creating a Regular Expression
You construct a regular expression in one of two ways:
re = /ab+c/
RegExp object, as in:re = new RegExp("ab+c")
compile method to compile a new regular expression for efficient reuse./abc/, or a combination of simple and special characters, such as /ab*c/ or /Chapter (\d+)\.\d*/. The last example includes parentheses which are used as a memory device. The match made with this part of the pattern is remembered for later use, as described in "Using Parenthesized Substring Matches".
Using Simple Patterns
Simple patterns are constructed of characters for which you want to find a direct match. For example, the pattern /abc/ matches character combinations in strings only when exactly the characters 'abc' occur together and in that order. Such a match would succeed in the strings "Hi, do you know your abc's?" and "The latest airplane designs evolved from slabcraft." In both cases the match is with the substring 'abc'. There is no match in the string "Grab crab" because it does not contain the substring 'abc'.
Using Special Characters
When the search for a match requires something more than a direct match, such as finding one or more b's, or finding whitespace, the pattern includes special characters. For example, the pattern /ab*c/ matches any character combination in which a single 'a' is followed by zero or more 'b's (* means 0 or more occurrences of the preceding character) and then immediately followed by 'c'. In the string "cbbabbbbcdebc," the pattern matches the substring 'abbbbc'.
Table 9.8 provides a complete list and description of the special characters that can be used in regular expressions.
Table 9.8 Special characters in regular expressions.
Using Parentheses
Parentheses around any part of the regular expression pattern cause that part of the matched substring to be remembered. Once remembered, the substring can be recalled for other use, as described in "Using Parenthesized Substring Matches".
For example, the pattern /Chapter (\d+)\.\d*/ illustrates additional escaped and special characters and indicates that part of the pattern should be remembered. It matches precisely the characters 'Chapter ' followed by one or more numeric characters (\d means any numeric character and + means 1 or more times), followed by a decimal point (which in itself is a special character; preceding the decimal point with \ means the pattern must look for the literal character '.'), followed by any numeric character 0 or more times (\d means numeric character, * means 0 or more times). In addition, parentheses are used to remember the first matched numeric characters.
This pattern is found in "Open Chapter 4.3, paragraph 6" and '4' is remembered. The pattern is not found in "Chapters 3 and 4", because that string does not have a period after the '3'.
Working With Regular Expressions
Regular expressions are used with the regular expression methods test and exec and with the String methods match, replace, search, and split. These methods are explained in detail in the JavaScript Reference.
When you want to know whether a pattern is found in a string, use the test or search method; for more information (but slower execution) use the exec or match methods. If you use exec or match and if the match succeeds, these methods return an array and update properties of the associated regular expression object and also of the predefined regular expression object, RegExp. If the match fails, the exec method returns null (which converts to false).
In the following example, the script uses the exec method to find a match in a string.
<SCRIPT LANGUAGE="JavaScript1.2">
If you do not need to access the properties of the regular expression, an alternative way of creating
myRe=/d(b+)d/g;
myArray = myRe.exec("cdbbdbsbz");
</SCRIPT>myArray is with this script:
<SCRIPT LANGUAGE="JavaScript1.2">
If you want to be able to recompile the regular expression, yet another alternative is this script:
myArray = /d(b+)d/g.exec("cdbbdbsbz");
</SCRIPT><SCRIPT LANGUAGE="JavaScript1.2">
With these scripts, the match succeeds and returns the array and updates the properties shown in Table 9.9.
myRe= new RegExp ("d(b+)d", "g:);
myArray = myRe.exec("cdbbdbsbz");
</SCRIPT>
Table 9.9 Results of regular expression execution.
RegExp.leftContext and RegExp.rightContext can be computed from the other values. RegExp.leftContext is equivalent to:
myArray.input.substring(0, myArray.index)
and RegExp.rightContext is equivalent to:
myArray.input.substring(myArray.index + myArray[0].length)
As shown in the second form of this example, you can use the a regular expression created with an object initializer without assigning it to a variable. If you do, however, every occurrence is a new regular expression. For this reason, if you use this form without assigning it to a variable, you cannot subsequently access the properties of that regular expression. For example, assume you have this script:
<SCRIPT LANGUAGE="JavaScript1.2">
This script displays:
myRe=/d(b+)d/g;
myArray = myRe.exec("cdbbdbsbz");
document.writeln("The value of lastIndex is " + myRe.lastIndex);
</SCRIPT>The value of lastIndex is 5
However, if you have this script:
<SCRIPT LANGUAGE="JavaScript1.2">
It displays:
myArray = /d(b+)d/g.exec("cdbbdbsbz");
document.writeln("The value of lastIndex is " + /d(b+)d/g.lastIndex);
</SCRIPT>The value of lastIndex is 0
The occurrences of /d(b+)d/g in the two statements are different regular expression objects and hence have different values for their lastIndex property. If you need to access the properties of a regular expression created with an object initalizer, you should first assign it to a variable.
Using Parenthesized Substring Matches
Including parentheses in a regular expression pattern causes the corresponding submatch to be remembered. For example, /a(b)c/ matches the characters 'abc' and remembers 'b'. To recall these parenthesized substring matches, use the RegExp properties $1, ..., $9 or the Array elements [1], ..., [n].
The number of possible parenthesized substrings is unlimited. The predefined RegExp object holds up to the last nine and the returned array holds all that were found. The following examples illustrate how to use parenthesized substring matches.
Example 1. :
The following script uses the replace method to switch the words in the string. For the replacement text, the script uses the values of the $1 and $2 properties.
<SCRIPT LANGUAGE="JavaScript1.2">
This prints "Smith, John".
Example 2. :
In the following example,
re = /(\w+)\s(\w+)/;
str = "John Smith";
newstr = str.replace(re, "$2, $1");
document.write(newstr)
</SCRIPT>RegExp.input is set by the Change event. In the getInfo function, the exec method uses the value of RegExp.input as its argument. Note that RegExp must be prepended to its $ properties (because they appear outside the replacement string). (Example 3 is a more efficient, though possibly more cryptic, way to accomplish the same thing.)
<HTML>
<SCRIPT LANGUAGE="JavaScript1.2">
function getInfo(){
re = /(\w+)\s(\d+)/
re.exec();
window.alert(RegExp.$1 + ", your age is " + RegExp.$2);
}
</SCRIPT>Enter your first name and your age, and then press Enter.
<FORM>
<INPUT TYPE="text" NAME="NameAge" onChange="getInfo(this);">
</FORM></HTML>
Example 3. :
The following example is similar to Example 2. Instead of using the RegExp.$1 and RegExp.$2, this example creates an array and uses a[1] and a[2]. It also uses the shortcut notation for using the exec method.
<HTML>
<SCRIPT LANGUAGE="JavaScript1.2">
function getInfo(){
a = /(\w+)\s(\d+)/();
window.alert(a[1] + ", your age is " + a[2]);
}
</SCRIPT>Enter your first name and your age, and then press Enter.
<FORM>
<INPUT TYPE="text" NAME="NameAge" onChange="getInfo(this);">
</FORM></HTML>
Executing a Global Search and Ignoring Case
Regular expressions have two optional flags that allow for global and case insensitive searching. To indicate a global search, use the g flag. To indicate a case insensitive search, use the i flag. These flags can be used separately or together in either order, and are included as part of the regular expression.
To include a flag with the regular expression, use this syntax:
re = /pattern/[g|i|gi]
Note that the flags,
re = new RegExp("pattern", ['g'|'i'|'gi'])i and g, are an integral part of a regular expression. They cannot be added or removed later.
For example, re = /\w+\s/g creates a regular expression that looks for one or more characters followed by a space, and it looks for this combination throughout the string.
<SCRIPT LANGUAGE="JavaScript1.2">
This displays ["fee ", "fi ", "fo "]. In this example, you could replace the line:
re = /\w+\s/g;
str = "fee fi fo fum";
myArray = str.match(re);
document.write(myArray);
</SCRIPT>re = /\w+\s/g;
with:
re = new RegExp("\\w+\\s", "g");
and get the same result.
Examples
Changing the Order in an Input String
The following example illustrates the formation of regular expressions and the use of string.split() and string.replace().
It cleans a roughly-formatted input string containing names (first name first) separated by blanks, tabs and exactly one semicolon.
Finally, it reverses the name order (last name first) and sorts the list.
<SCRIPT LANGUAGE="JavaScript1.2">
// The name string contains multiple spaces and tabs,
// and may have multiple spaces between first and last names.
names = new String ( "Harry Trump ;Fred Barney; Helen Rigby ;\
Bill Abel ;Chris Hand ")document.write ("---------- Original String" + "<BR>" + "<BR>")
document.write (names + "<BR>" + "<BR>")// Prepare two regular expression patterns and array storage.
// Split the string into array elements.// pattern: possible white space then semicolon then possible white space
pattern = /\s*;\s*/// Break the string into pieces separated by the pattern above and
// and store the pieces in an array called nameList
nameList = names.split (pattern)// new pattern: one or more characters then spaces then characters.
// Use parentheses to "memorize" portions of the pattern.
// The memorized portions are referred to later.
pattern = /(\w+)\s+(\w+)/// New array for holding names being processed.
bySurnameList = new Array;// Display the name array and populate the new array
// with comma-separated names, last first.
//
// The replace method removes anything matching the pattern
// and replaces it with the memorized string--second memorized portion
// followed by comma space followed by first memorized portion.
//
// The variables $1 and $2 refer to the portions
// memorized while matching the pattern.document.write ("---------- After Split by Regular Expression" + "<BR>")
for ( i = 0; i < nameList.length; i++) {
document.write (nameList[i] + "<BR>")
bySurnameList[i] = nameList[i].replace (pattern, "$2, $1")
}// Display the new array.
document.write ("---------- Names Reversed" + "<BR>")
for ( i = 0; i < bySurnameList.length; i++) {
document.write (bySurnameList[i] + "<BR>")
}// Sort by last name, then display the sorted array.
bySurnameList.sort()
document.write ("---------- Sorted" + "<BR>")
for ( i = 0; i < bySurnameList.length; i++) {
document.write (bySurnameList[i] + "<BR>")
}document.write ("---------- End" + "<BR>")
</SCRIPT>
Using Special Characters to Verify Input
In the following example, a user enters a phone number. When the user presses Enter, the script checks the validity of the number. If the number is valid (matches the character sequence specified by the regular expression), the script posts a window thanking the user and confirming the number. If the number is invalid, the script posts a window telling the user that the phone number isn't valid.
The regular expression looks for zero or one open parenthesis \(?, followed by three digits \d{3}, followed by zero or one close parenthesis \)?, followed by one dash, forward slash, or decimal point and when found, remember the character ([-\/\.]), followed by three digits \d{3}, followed by the remembered match of a dash, forward slash, or decimal point \1, followed by four digits \d{4}.
The Change event activated when the user presses Enter, sets the value of RegExp.input.
<HTML>
<SCRIPT LANGUAGE = "JavaScript1.2">re = /\(?\d{3}\)?([-\/\.])\d{3}\1\d{4}/
function testInfo() {
OK = re.exec()
if (!OK)
window.alert (RegExp.input +
" isn't a phone number with area code!")
else
window.alert ("Thanks, your phone number is " + OK[0])
}</SCRIPT>
Enter your phone number (with area code) and then press Enter.
<FORM>
<INPUT TYPE="text" NAME="Phone" onChange="testInfo(this);">
</FORM></HTML>
Last Updated: 11/26/97 09:25:49