Table 5.5 Logical operators
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Operator
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Usage
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Description
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&&
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expr1 && expr2
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(Logical AND) Returns expr1 if it can be converted to false; otherwise, returns expr2. Thus, when used with Boolean values, && returns true if both operands are true; otherwise, returns false.
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||
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expr1 || expr2
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(Logical OR) Returns expr1 if it can be converted to true; otherwise, returns expr2. Thus, when used with Boolean values, || returns true if either operand is true; if both are false, returns false.
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!
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!expr
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(Logical NOT) Returns false if its single operand can be converted to true; otherwise, returns true.
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Examples of expressions that can be converted to false are those that evaluate to null, 0, the empty string (""), or undefined.
Even though the && and || operators can be used with operands that are not Boolean values, they can still be considered Boolean operators since their return values can always be converted to Boolean values.
Short-Circuit Evaluation. As logical expressions are evaluated left to right, they are tested for possible "short-circuit" evaluation using the following rules:
-
-
false && anything is short-circuit evaluated to false.
-
true || anything is short-circuit evaluated to true.
The rules of logic guarantee that these evaluations are always correct. Note that the anything part of the above expressions is not evaluated, so any side effects of doing so do not take effect.
Backward Compatibility
JavaScript 1.0 and 1.1. The && and || operators behave as follows:
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Operator
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Behavior
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&&
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If the first operand (expr1) can be converted to false, the && operator returns false rather than the value of expr1.
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||
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If the first operand (expr1) can be converted to true, the || operator returns true rather than the value of expr1.
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Examples
The following code shows examples of the && (logical AND) operator.
a1=true && true // t && t returns true
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
The following code shows examples of the || (logical OR) operator.
o1=true || true // t || t returns true
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
The following code shows examples of the ! (logical NOT) operator.
n1=!true // !t returns false
n2=!false // !f returns true
n3=!"Cat" // !t returns false
String Operators
In addition to the comparison operators, which can be used on string values, the concatenation operator (+) concatenates two string values together, returning another string that is the union of the two operand strings. For example, "my " + "string" returns the string "my string".
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Implemented in
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JavaScript 1.0
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ECMA version
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ECMA-262
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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.
Special Operators
?: (Conditional operator)
The conditional operator is the only JavaScript operator that takes three operands. This operator is frequently used as a shortcut for the if statement.
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Implemented in
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JavaScript 1.0
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ECMA version
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ECMA-262
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Syntax
condition ? expr1 : expr2
Parameters
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condition
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An expression that evaluates to true or false.
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expr1, expr2
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Expressions with values of any type.
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Description
If 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"))
, (Comma operator)
The comma operator evaluates both of its operands and returns the value of the second operand.
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Implemented in
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JavaScript 1.0
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ECMA version
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ECMA-262
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Syntax
expr1, expr2
Parameters
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expr1, expr2
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Any expressions.
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Description
You can use the comma operator when you want to include multiple expressions in a location that requires a single expression. The most common usage of this operator is to supply multiple parameters in a 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
The delete operator deletes an object, an object's property, or an element at a specified index in an array.
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Implemented in
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JavaScript 1.2, NES 3.0
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ECMA version
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ECMA-262
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Syntax
delete objectName
delete objectName.property
delete objectName[index]
delete property // legal only within a with statement
Parameters
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objectName
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The name of an object.
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property
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The property to delete.
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index
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An integer representing the array index to delete.
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Description
The fourth form is legal only within a 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=42
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)
Deleting 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 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
}
function
The function operator defines an anonymous function inside an expression.
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Implemented in
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JavaScript 1.5
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Syntax
{var | const} variableName = function(parameters) {functionBody};
Description
The following examples shows how the function operator is used.
This example declares an unnamed function inside an expression. It sets x to a function that returns the square of its argument:
var x = function(y) {return y*y};
The next example declares array a as an array of three functions:
var a = [function(y) {return y}, function y {return y*y}, function (y) [return y*y*y}];
For this array, a[0](5) returns 5, a[1](5) returns 25, and a[2](5) returns 125.
in
The in operator returns true if the specified property is in the specified object.
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Implemented in
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JavaScript 1.4
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Syntax
propNameOrNumber in objectName
Parameters
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propNameOrNumber
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A string or numeric expression representing a property name or array index.
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objectName
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Name of an object.
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Description
The following examples show some uses of the 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 objects
mycar = {make:"Honda",model:"Accord",year:1998}
"make" in mycar // returns true
"model" in mycar // returns true
You must specify an object on the right side of the 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 false
trees=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 true
trees=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.
instanceof
The instanceof operator returns true if the specified object is of the specified object type.
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Implemented in
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JavaScript 1.4
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Syntax
objectName instanceof objectType
Parameters
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objectName
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Name of the object to compare to objectType.
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objectType
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Object type.
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Description
Use 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)
Examples
See also the examples for 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)
if (theDay instanceof Date) {
// statements to execute
}
Example 2. The following code uses 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 true
myDate instanceof Object // returns true
myDate instanceof String // returns false
Example 3. The following code creates an object type 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
new
The new operator creates an instance of a user-defined object type or of one of the built-in object types that has a constructor function.
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Implemented in
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JavaScript 1.0
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ECMA version
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ECMA-262
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Syntax
objectName = new objectType (param1 [,param2] ...[,paramN])
Parameters
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objectName
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Name of the new object instance.
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objectType
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Object type. It must be a function that defines an object type.
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param1...paramN
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Property values for the object. These properties are parameters defined for the objectType function.
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Description
Creating a user-defined object type requires two steps:
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Define the object type by writing a function.
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Create an instance of the object with new.
To define an object type, create a function for the object type that specifies its name, properties, and methods. An object can have a property that is itself another object. See the examples below.
You can always add a property to a previously defined object. For example, the statement 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"
Examples
Example 1: Object type and object instance. Suppose you want to create an object type for cars. You want this type of object to be called 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
The this keyword refers to the current object. In general, in a method this refers to the calling object.
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Implemented in
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JavaScript 1.0
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ECMA version
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ECMA-262
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Syntax
this[.propertyName]
Examples
Suppose a function called 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
The typeof operator is used in either of the following ways:
1. typeof operand
2. typeof (operand)
The 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.
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Implemented in
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JavaScript 1.1
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ECMA version
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ECMA-262
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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 object
typeof shape is string
typeof size is number
typeof today is object
typeof dontExist is undefined
For the keywords true and null, the typeof operator returns the following results:
typeof true is boolean
typeof null is object
For a number or string, the typeof operator returns the following results:
typeof 62 is number
typeof 'Hello world' is string
For property values, the typeof operator returns the type of value the property contains:
typeof document.lastModified is string
typeof window.length is number
typeof Math.LN2 is number
For methods and functions, the typeof operator returns results as follows:
typeof blur is function
typeof eval is function
typeof parseInt is function
typeof shape.split is function
For predefined objects, the 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
void
The void operator is used in either of the following ways:
1. void (expression)
2. void expression
The void operator specifies an expression to be evaluated without returning a value. expression is a JavaScript expression to evaluate. The parentheses surrounding the expression are optional, but it is good style to use them.
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Implemented in
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JavaScript 1.1
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ECMA version
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ECMA-262
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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>
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