Table 4.1 JavaScript operators.
Table 4.2 Assignment operators
a[i++] += 5 //i is evaluated only once
a[i++] = a[i++] + 5 //i is evaluated twice
true or false.Table 4.3 Comparison operators
1
These examples assume that var1 has been assigned the value 3 and var2 has been assigned the value 4.
|
String, Number, Boolean, or Object operands as follows.
Number type value. First, a mathematical value is derived from the string numeric literal. Next, this value is rounded to nearest Number type value.Boolean, the Boolean operand is converted to 1 if it is true and +0 if it is false.String or Number value, using the valueOf and toString methods of the objects. If this attempt to convert the object fails, a runtime error is generated.==) to compare instances of JSObject. Use the JSObject.equals method for such comparisons. JSObject.equals is available for this purpose in all previous versions of JavaScript.
JavaScript 1.2.
The standard equality operators (== and !=) do not perform a type conversion before the comparison is made. The strict equality operators (=== and !==) are unavailable.
JavaScript 1.1 and earlier versions.
The standard equality operators (== and !=) perform a type conversion before the comparison is made. The strict equality operators (=== and !==) are unavailable.
1/2 //returns 0.5 in JavaScript
1/2 //returns 0 in Java
var1 % var2The modulus operator returns the first operand modulo the second operand, that is,
var1 modulo var2, in the preceding statement, where var1 and var2 are variables. The modulo function is the integer remainder of dividing var1 by var2. For example, 12 % 5 returns 2.
++ or ++var
This operator increments (adds one to) its operand and returns a value. If used postfix, with operator after operand (for example, x++), then it returns the value before incrementing. If used prefix with operator before operand (for example, ++x), then it returns the value after incrementing.
For example, if x is three, then the statement y = x++ sets y to 3 and increments x to 4. If x is 3, then the statement y = ++x increments x to 4 and sets y to 4.
-- or --var
This operator decrements (subtracts one from) its operand and returns a value. If used postfix (for example, x--), then it returns the value before decrementing. If used prefix (for example, --x), then it returns the value after decrementing.
For example, if x is three, then the statement y = x-- sets y to 3 and decrements x to 2. If x is 3, then the statement y = --x decrements x to 2 and sets y to 2.
y = -x negates the value of x and assigns that to y; that is, if x were 3, y would get the value -3 and x would retain the value 3.
9<<2 yields thirty-six, because 1001 shifted two bits to the left becomes 100100, which is thirty-six.
| Operator | Usage |
Description
expr1 && expr2
| expr1 || expr2
| !expr (Logical NOT) Returns false if its single operand can be converted to true; otherwise, returns true. |
|---|
false && anything is short-circuit evaluated to false.true || anything is short-circuit evaluated to true.
| Operator |
Behavior
|
|
| |
|---|
a1=true && true // t && t returns trueThe following code shows examples of the || (logical OR) operator.
a2=true && false // t && f returns false
a3=false && true // f && t returns false
a4=false && (3 == 4) // f && f returns false
a5="Cat" && "Dog" // t && t returns Dog
a6=false && "Cat" // f && t returns false
a7="Cat" && false // t && f returns false
o1=true || true // t || t returns trueThe following code shows examples of the ! (logical NOT) operator.
o2=false || true // f || t returns true
o3=true || false // t || f returns true
o4=false || (3 == 4) // f || f returns false
o5="Cat" || "Dog" // t || t returns Cat
o6=false || "Cat" // f || t returns Cat
o7="Cat" || false // t || f returns Cat
n1=!true // !t returns false
n2=!false // !f returns true
n3=!"Cat" // !t returns false
"my " + "string" returns the string "my string".mystring has the value "alpha," then the expression mystring += "bet" evaluates to "alphabet" and assigns this value to mystring.
if statement.condition ? expr1 : expr2
condition | |
expr1, expr2 |
condition is true, the operator returns the value of expr1; otherwise, it returns the value of expr2. For example, to display a different message based on the value of the isMember variable, you could use this statement:
document.write ("The fee is " + (isMember ? "$2.00" : "$10.00"))
expr1, expr2
expr1, expr2 |
for 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=9; i <= 9; i++, j--)
document.writeln("a["+i+","+j+"]= " + a[i,j])
delete objectName
delete objectName.property
delete objectName[index]
delete property // legal only within a with statement
objectName | |
property | |
index |
with statement, to delete a property from an object.
You can use the delete operator to delete variables declared implicitly but not those declared with the var statement.
If the delete operator succeeds, it sets the property or element to undefined. The delete operator returns true if the operation is possible; it returns false if the operation is not possible.
x=42Deleting array elements. When you delete an array element, the array length is not affected. For example, if you delete a[3], a[4] is still a[4] and a[3] is undefined. When the
var y= 43
myobj=new Number()
myobj.h=4 // create property h
delete x // returns true (can delete if declared implicitly)
delete y // returns false (cannot delete if declared with var)
delete Math.PI // returns false (cannot delete predefined properties)
delete myobj.h // returns true (can delete user-defined properties)
delete myobj // returns true (can delete objects)
delete operator removes an array element, that element is no longer in the array. In the following example, trees[3] is removed with delete.
trees=new Array("redwood","bay","cedar","oak","maple")
delete trees[3]
if (3 in trees) {
// this does not get executed
}
If you want an array element to exist but have an undefined value, use the undefined keyword instead of the delete operator. In the following example, trees[3] is assigned the value undefined, but the array element still exists:
trees=new Array("redwood","bay","cedar","oak","maple")
trees[3]=undefined
if (3 in trees) {
// this gets executed
}
in operator returns true if the specified property is in the specified object.propNameOrNumber in objectName
propNameOrNumber | A string or numeric expression representing a property name or array index. |
objectName |
in operator.
// Arrays
trees=new Array("redwood","bay","cedar","oak","maple")
0 in trees // returns true
3 in trees // returns true
6 in trees // returns false
"bay" in trees // returns false (you must specify the index number,
// not the value at that index)
"length" in trees // returns true (length is an Array property)
// Predefined objects
"PI" in Math // returns true
myString=new String("coral")
"length" in myString // returns true
// Custom objectsYou must specify an object on the right side of the
mycar = {make:"Honda",model:"Accord",year:1998}
"make" in mycar // returns true
"model" in mycar // returns true
in operator. For example, you can specify a string created with the String constructor, but you cannot specify a string literal.
color1=new String("green")
"length" in color1 // returns true
color2="coral"
"length" in color2 // generates an error (color is not a String object)
Using in with deleted or undefined properties.
If you delete a property with the delete operator, the in operator returns false for that property.
mycar = {make:"Honda",model:"Accord",year:1998}
delete mycar.make
"make" in mycar // returns falsetrees=new Array("redwood","bay","cedar","oak","maple")
delete trees[3]
3 in trees // returns false
If you set a property to undefined but do not delete it, the in operator returns true for that property.
mycar = {make:"Honda",model:"Accord",year:1998}
mycar.make=undefined
"make" in mycar // returns truetrees=new Array("redwood","bay","cedar","oak","maple")
trees[3]=undefined
3 in trees // returns true
For additional information about using the in operator with deleted array elements, see "delete" on page 267.
instanceof operator returns true if the specified object is of the specified object type.objectName instanceof objectType
objectName | |
objectType |
instanceof when you need to confirm the type of an object at runtime. For example, when catching exceptions, you can branch to different exception-handling code depending on the type of exception thrown.
You must specify an object on the right side of the instanceof operator. For example, you can specify a string created with the String constructor, but you cannot specify a string literal.
color1=new String("green")
color1 instanceof String // returns true
color2="coral"
color2 instanceof String // returns false (color is not a String object)
throw.
Example 1. The following code uses instanceof to determine whether theDay is a Date object. Because theDay is a Date object, the statements in the if statement execute.
theDay=new Date(1995, 12, 17)Example 2. The following code uses
if (theDay instanceof Date) {
// statements to execute
}
instanceof to demonstrate that String and Date objects are also of type Object (they are derived from Object).
myString=new String()
myDate=new Date()
myString instanceof String // returns true
myString instanceof Object // returns true
myString instanceof Date // returns false
myDate instanceof Date // returns trueExample 3. The following code creates an object type
myDate instanceof Object // returns true
myDate instanceof String // returns false
Car and an instance of that object type, mycar. The instanceof operator demonstrates that the mycar object is of type Car and of type Object.
function Car(make, model, year) {
this.make = make
this.model = model
this.year = year
}
mycar = new Car("Honda", "Accord", 1998)
a=mycar instanceof Car // returns true
b=mycar instanceof Object // returns true
objectName = new objectType (param1 [,param2] ...[,paramN])
car1.color = "black" adds a property color to car1, and assigns it a value of "black". However, this does not affect any other objects. To add the new property to all objects of the same type, you must add the property to the definition of the car object type.
You can add a property to a previously defined object type by using the Function.prototype property. This defines a property that is shared by all objects created with that function, rather than by just one instance of the object type. The following code adds a color property to all objects of type car, and then assigns a value to the color property of the object car1. For more information, see prototype
Car.prototype.color=null
car1.color="black"
birthday.description="The day you were born"
car, and you want it to have properties for make, model, and year. To do this, you would write the following function:
function car(make, model, year) {
this.make = make
this.model = model
this.year = year
}
Now you can create an object called mycar as follows:
mycar = new car("Eagle", "Talon TSi", 1993)
This statement creates mycar and assigns it the specified values for its properties. Then the value of mycar.make is the string "Eagle", mycar.year is the integer 1993, and so on.
You can create any number of car objects by calls to new. For example,
kenscar = new car("Nissan", "300ZX", 1992)
Example 2: Object property that is itself another object. Suppose you define an object called person as follows:
function person(name, age, sex) {
this.name = name
this.age = age
this.sex = sex
}
And then instantiate two new person objects as follows:
rand = new person("Rand McNally", 33, "M")
ken = new person("Ken Jones", 39, "M")
Then you can rewrite the definition of car to include an owner property that takes a person object, as follows:
function car(make, model, year, owner) {
this.make = make;
this.model = model;
this.year = year;
this.owner = owner;
}
To instantiate the new objects, you then use the following:
car1 = new car("Eagle", "Talon TSi", 1993, rand);
car2 = new car("Nissan", "300ZX", 1992, ken)
Instead of passing a literal string or integer value when creating the new objects, the above statements pass the objects rand and ken as the parameters for the owners. To find out the name of the owner of car2, you can access the following property:
car2.owner.name
this refers to the calling object.this[.propertyName]validate validates an object's value property, given the object and the high and low values:
function validate(obj, lowval, hival) {
if ((obj.value < lowval) || (obj.value > hival))
alert("Invalid Value!")
}
You could call validate in each form element's onChange event handler, using this to pass it the form element, as in the following example:
<B>Enter a number between 18 and 99:</B>
<INPUT TYPE = "text" NAME = "age" SIZE = 3
onChange="validate(this, 18, 99)">
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.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. void (expression)The void operator specifies an expression to be evaluated without returning a value.
2. void expression
expression is a JavaScript expression to evaluate. The parentheses surrounding the expression are optional, but it is good style to use them.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>
Last Updated: 10/29/98 21:19:40