Conceptually, there are two types of expressions: those that assign a value to a variable, and those that simply have a value. For example, the expression 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()
JavaScript has both binary and unary operators. A binary operator requires two operands, one before the operator and one after the operator:
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.
The other assignment operators are shorthand for standard operations, as shown in Table 9.1.
Table 9.1 Assignment operators
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. |
In addition, JavaScript provides the arithmetic operators listed in Table 9.3.
Table 9.3 Arithmetic Operators
The following table summarizes JavaScript's bitwise operators
Shift operators convert their operands to thirty-two-bit integers and return a result of the same type as the left operator.
The shift operators are listed in Table 9.5.
Table 9.5 Bitwise shift operators
| 1 Assume var1 is "Cat", var2 is "Dog", and var3 is false. |
false && anything is short-circuit evaluated to false.true || anything is short-circuit evaluated to true."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.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 operator is used in either of the following ways:1. typeof operandThe
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")
var shape="round"
var size=1
var today=new Date()
The typeof operator returns the following results for these variables:typeof myFun is objectFor the keywords
typeof shape is string
typeof size is number
typeof today is object
typeof dontExist is undefined
true and null, the typeof operator returns the following results:typeof true is booleanFor a number or string, the
typeof null is object
typeof operator returns the following results:typeof 62 is numberFor property values, the
typeof 'Hello world' is string
typeof operator returns the type of value the property contains:typeof document.lastModified is stringFor methods and functions, the
typeof window.length is number
typeof Math.LN2 is number
typeof operator returns results as follows:typeof blur is functionFor predefined objects, the
typeof eval is function
typeof parseInt is function
typeof shape.split is function
typeof 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
1. javascript:void (expression)The void operator specifies an expression to be evaluated without returning a value.
2. javascript:void expression
expression 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.
<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>
Table 9.7 describes the precedence of operators, from lowest to highest.
/abc/) or the RegExp constructor function (for example, re = new RegExp("abc")). These patterns are used with the exec and test methods of regular expressions, and with the match, replace, search, and split methods of String.re = /ab+c/Literal notation provides compilation of the regular expression when the script is evaluated. When the regular expression will remain constant use literal notation for better performance.
RegExp object, as in:re = new RegExp("ab+c")
Using the constructor function provides runtime compilation of the regular expression. Use the constructor function when you know the regular expression pattern will be changing, or you don't know the pattern and are getting it from another source, such as user input. Once you have a defined regular expression, if the regular expression is used throughout the script, and if its source changes, you can use the 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"./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'. /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.
test and exec and with the String methods match, replace, search, and split. These methods are explained in detail in the JavaScript Reference.
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.
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 literal form of a regular expression without assigning it to a variable. If you do, however, every occurrence of the literal is a new regular expression. For this reason, if you use the literal 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 5However, 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 0The 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 literal regular expression, you should first assign it to a variable./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".
re = /(\w+)\s(\w+)/;
str = "John Smith";
newstr = str.replace(re, "$2, $1");
document.write(newstr)
</SCRIPT>
Example 2. :
In the following example, 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>
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.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>
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: 10/31/97 10:37:57