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JavaScriptSource Staff Jul 24, 2006
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Core JavaScript Reference 1.5



Chapter 5   Chapter 5 Operators


JavaScript has assignment, comparison, arithmetic, bitwise, logical, string, and special operators. This chapter describes the operators and contains information about operator precedence.

The following table summarizes the JavaScript operators.


Table 5.1    JavaScript operators.  

Operator category

Operator

Description

Arithmetic Operators

+

 

(Addition) Adds 2 numbers.  

++

 

(Increment) Adds one to a variable representing a number (returning either the new or old value of the variable).  

-

 

(Unary negation, subtraction) As a unary operator, negates the value of its argument. As a binary operator, subtracts 2 numbers.  

--

 

(Decrement) Subtracts one from a variable representing a number (returning either the new or old value of the variable).  

*

 

(Multiplication) Multiplies 2 numbers.  

/

 

(Division) Divides 2 numbers.  

%

 

(Modulus) Computes the integer remainder of dividing 2 numbers.  

String Operators

+  

(String addition) Concatenates 2 strings.  

+=  

Concatenates 2 strings and assigns the result to the first operand.  

Logical Operators

&&  

(Logical AND) Returns the first operand if it can be converted to false; otherwise, returns the second operand. Thus, when used with Boolean values, && returns true if both operands are true; otherwise, returns false.  

||  

(Logical OR) Returns the first operand if it can be converted to true; otherwise, returns the second operand. Thus, when used with Boolean values, || returns true if either operand is true; if both are false, returns false.  

!  

(Logical NOT) Returns false if its single operand can be converted to true; otherwise, returns true.  

Bitwise Operators

&

 

(Bitwise AND) Returns a one in each bit position if bits of both operands are ones.  

^

 

(Bitwise XOR) Returns a one in a bit position if bits of one but not both operands are one.  

|

 

(Bitwise OR) Returns a one in a bit if bits of either operand is one.  

~

 

(Bitwise NOT) Flips the bits of its operand.  

<<

 

(Left shift) Shifts its first operand in binary representation the number of bits to the left specified in the second operand, shifting in zeros from the right.  

>>

 

(Sign-propagating right shift) Shifts the first operand in binary representation the number of bits to the right specified in the second operand, discarding bits shifted off.  

>>>

 

(Zero-fill right shift) Shifts the first operand in binary representation the number of bits to the right specified in the second operand, discarding bits shifted off, and shifting in zeros from the left.  

Assignment Operators

=

 

Assigns the value of the second operand to the first operand.  

+=

 

Adds 2 numbers and assigns the result to the first.  

-=

 

Subtracts 2 numbers and assigns the result to the first.  

*=

 

Multiplies 2 numbers and assigns the result to the first.  

/=

 

Divides 2 numbers and assigns the result to the first.  

%=

 

Computes the modulus of 2 numbers and assigns the result to the first.  

&=

 

Performs a bitwise AND and assigns the result to the first operand.  

^=

 

Performs a bitwise XOR and assigns the result to the first operand.  

|=

 

Performs a bitwise OR and assigns the result to the first operand.  

<<=

 

Performs a left shift and assigns the result to the first operand.  

>>=

 

Performs a sign-propagating right shift and assigns the result to the first operand.  

>>>=

 

Performs a zero-fill right shift and assigns the result to the first operand.  

Comparison Operators

==  

Returns true if the operands are equal.  

!=  

Returns true if the operands are not equal.  

===  

Returns true if the operands are equal and of the same type.  

!==  

Returns true if the operands are not equal and/or not of the same type.  

>  

Returns true if the left operand is greater than the right operand.  

>=  

Returns true if the left operand is greater than or equal to the right operand.  

<  

Returns true if the left operand is less than the right operand.  

<=  

Returns true if the left operand is less than or equal to the right operand.  

Special Operators

?:

 

Performs a simple "if...then...else".  

,

 

Evaluates two expressions and returns the result of the second expression.  

delete

 

Deletes an object, an object's property, or an element at a specified index in an array.  

function

 

Defines an anonymous function.  

in

 

Returns true if the specified property is in the specified object.  

instanceof

 

Returns true if the specified object is of the specified object type.  

new

 

Creates an instance of a user-defined object type or of one of the built-in object types.  

this

 

Keyword that you can use to refer to the current object.  

typeof

 

Returns a string indicating the type of the unevaluated operand.  

void

 

Specifies an expression to be evaluated without returning a value.  



Assignment Operators



An assignment operator assigns a value to its left operand based on the value of its right operand.

Implemented in  

JavaScript 1.0  

ECMA version  

ECMA-262  

The basic assignment operator is equal (=), which assigns the value of its right operand to its left operand. That is, x = y assigns the value of y to x. The other assignment operators are usually shorthand for standard operations, as shown in the following table.


Table 5.2    Assignment operators

Shorthand operator

Meaning

x += y

 

x = x + y

 

x -= y

 

x = x - y

 

x *= y

 

x = x * y

 

x /= y

 

x = x / y

 

x %= y

 

x = x % y

 

x <<= y

 

x = x << y

 

x >>= y

 

x = x >> y

 

x >>>= y

 

x = x >>> y

 

x &= y

 

x = x & y

 

x ^= y

 

x = x ^ y

 

x |= y

 

x = x | y

 

In unusual situations, the assignment operator is not identical to the Meaning expression in Table 5.2. When the left operand of an assignment operator itself contains an assignment operator, the left operand is evaluated only once. For example:

a[i++] += 5 //i is evaluated only once
a[i++] = a[i++] + 5 //i is evaluated twice



Comparison Operators



A comparison operator compares its operands and returns a logical value based on whether the comparison is true.

Implemented in  

JavaScript 1.0

JavaScript 1.3: Added the === and !== operators.

JavaScript 1.4: Deprecated == for comparison of two JSObject objects. Use the JSObject.equals method.  

ECMA version  

ECMA-262 includes all comparison operators except === and !==.
ECMA-262 Edition 3 adds === and !==.
 

The operands can be numerical or string values. Strings are compared based on standard lexicographical ordering, using Unicode values.

A Boolean value is returned as the result of the comparison.

  • Two strings are equal when they have the same sequence of characters, same length, and same characters in corresponding positions.

  • Two numbers are equal when they are numerically equal (have the same number value). NaN is not equal to anything, including NaN. Positive and negative zeros are equal.

  • Two objects are equal if they refer to the same Object.

  • Two Boolean operands are equal if they are both true or false.

  • Null and Undefined types are == (but not ===).
The following table describes the comparison operators.


Table 5.3    Comparison operators

Operator

Description

Examples returning true1

Equal (==)  

Returns true if the operands are equal. If the two operands are not of the same type, JavaScript attempts to convert the operands to an appropriate type for the comparison.  

3 == var1
"3" == var1
3 == '3'

 

Not equal (!=)  

Returns true if the operands are not equal. If the two operands are not of the same type, JavaScript attempts to convert the operands to an appropriate type for the comparison.  

var1 != 4
var1 != "3"

 

Strict equal (===)  

Returns true if the operands are equal and of the same type.  

3 === var1

 

Strict not equal (!==)  

Returns true if the operands are not equal and/or not of the same type.  

var1 !== "3"
3 !== '3'

 

Greater than (>)  

Returns true if the left operand is greater than the right operand.  

var2 > var1

 

Greater than or equal (>=)  

Returns true if the left operand is greater than or equal to the right operand.  

var2 >= var1
var1 >= 3

 

Less than (<)  

Returns true if the left operand is less than the right operand.  

var1 < var2

 

Less than or equal (<=)  

Returns true if the left operand is less than or equal to the right operand.  

var1 <= var2
var2 <= 5

 

1 These examples assume that var1 has been assigned the value 3 and var2 has been assigned the value 4.


Using the Equality Operators

The standard equality operators (== and !=) compare two operands without regard to their type. The strict equality operators (=== and !==) perform equality comparisons on operands of the same type. Use strict equality operators if the operands must be of a specific type as well as value or if the exact type of the operands is important. Otherwise, use the standard equality operators, which allow you to compare the identity of two operands even if they are not of the same type.

When type conversion is needed, JavaScript converts String, Number, Boolean, or Object operands as follows.

  • When comparing a number and a string, the string is converted to a number value. JavaScript attempts to convert the string numeric literal to a Number type value. First, a mathematical value is derived from the string numeric literal. Next, this value is rounded to nearest Number type value.

  • If one of the operands is Boolean, the Boolean operand is converted to 1 if it is true and +0 if it is false.

  • If an object is compared with a number or string, JavaScript attempts to return the default value for the object. Operators attempt to convert the object to a primitive value, a 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.
You cannot use the standard equality operator (==) to compare instances of JSObject. Use the JSObject.equals method for such comparisons.


Backward Compatibility
The behavior of the standard equality operators (== and !=) depends on the JavaScript version.

JavaScript 1.3 and earlier versions. You can use either the standard equality operator (==) or JSObject.equals to compare instances of JSObject.

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.



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 (/).

Implemented in  

JavaScript 1.0  

ECMA version  

ECMA-262  

These operators work as they do in most other programming languages, except the / operator returns a floating-point division in JavaScript, not a truncated division as it does in languages such as C or Java. For example:

1/2 //returns 0.5 in JavaScript
1/2 //returns 0 in Java


% (Modulus)

The modulus operator is used as follows:

var1 % var2

The 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.


++ (Increment)

The increment operator is used as follows:

var ++ 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.


-- (Decrement)

The decrement operator is used as follows:

var -- 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.


- (Unary Negation)

The unary negation operator precedes its operand and negates it. For example, 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.



Bitwise Operators



Bitwise operators treat their operands as a set of 32 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:


Table 5.4    Bitwise operators

Operator

Usage

Description

Bitwise AND  

a & b

 

Returns a one in each bit position for which the corresponding bits of both operands are ones.  

Bitwise OR  

a | b

 

Returns a one in each bit position for which the corresponding bits of either or both operands are ones.  

Bitwise XOR  

a ^ b

 

Returns a one in each bit position for which the corresponding bits of either but not both operands are ones.  

Bitwise NOT  

~ a

 

Inverts the bits of its operand.  

Left shift  

a << b

 

Shifts a in binary representation b bits to left, shifting in zeros from the right.  

Sign-propagating right shift  

a >> b

 

Shifts a in binary representation b bits to right, discarding bits shifted off.  

Zero-fill right shift  

a >>> b

 

Shifts a in binary representation b bits to the right, discarding bits shifted off, and shifting in zeros from the left.  



Implemented in  

JavaScript 1.0  

ECMA version  

ECMA-262  

"> Bitwise Logical Operators



Implemented in  

JavaScript 1.0  

ECMA version  

ECMA-262  



Conceptually, the bitwise logical operators work as follows:

  • The operands are converted to thirty-two-bit integers and expressed by a series of bits (zeros and ones).

  • Each bit in the first operand is paired with the corresponding bit in the second operand: first bit to first bit, second bit to second bit, and so on.

  • The operator is applied to each pair of bits, and the result is constructed bitwise.
For example, the binary representation of nine is 1001, and the binary representation of fifteen is 1111. So, when the bitwise operators are applied to these values, the results are as follows:
  • 15 & 9 yields 9 (1111 & 1001 = 1001)

  • 15 | 9 yields 15 (1111 | 1001 = 1111)

  • 15 ^ 9 yields 6 (1111 ^ 1001 = 0110)




Implemented in  

JavaScript 1.0  

ECMA version  

ECMA-262  

"> Bitwise Shift Operators



Implemented in  

JavaScript 1.0  

ECMA version  

ECMA-262  



The bitwise shift operators take two operands: the first is a quantity to be shifted, and the second specifies the number of bit positions by which the first operand is to be shifted. The direction of the shift operation is controlled by the operator used.

Shift operators convert their operands to thirty-two-bit integers and return a result of the same type as the left operator.


<< (Left Shift)

This operator shifts the first operand the specified number of bits to the left. Excess bits shifted off to the left are discarded. Zero bits are shifted in from the right.

For example, 9<<2 yields thirty-six, because 1001 shifted two bits to the left becomes 100100, which is thirty-six.


>> (Sign-Propagating Right Shift)

This operator shifts the first operand the specified number of bits to the right. Excess bits shifted off to the right are discarded. Copies of the leftmost bit are shifted in from the left.

For example, 9>>2 yields two, because 1001 shifted two bits to the right becomes 10, which is two. Likewise, -9>>2 yields -3, because the sign is preserved.


>>> (Zero-Fill Right Shift)

This operator shifts the first operand the specified number of bits to the right. Excess bits shifted off to the right are discarded. Zero bits are shifted in from the left.

For example, 19>>>2 yields four, because 10011 shifted two bits to the right becomes 100, which is four. For non-negative numbers, zero-fill right shift and sign-propagating right shift yield the same result.



Logical Operators



Logical operators are typically used with Boolean (logical) values; when they are, they return a Boolean value. However, the && and || operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they may return a non-Boolean value.



Implemented in  

JavaScript 1.0  

ECMA version  

ECMA-262  

The logical operators are described in the following table.


Table 5.5    Logical operators

Operator

Usage

Description

&&  

expr1 && expr2

 

(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.  

||  

expr1 || expr2

 

(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.  

!  

!expr

 

(Logical NOT) Returns false if its single operand can be converted to true; otherwise, returns true.  

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:




Operator

Behavior

&&  

If the first operand (expr1) can be converted to false, the && operator returns false rather than the value of expr1.  

||  

If the first operand (expr1) can be converted to true, the || operator returns true rather than the value of expr1.  


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".



Implemented in  

JavaScript 1.0  

ECMA version  

ECMA-262  

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.



Implemented in  

JavaScript 1.0  

ECMA version  

ECMA-262  


Syntax
condition ? expr1 : expr2


Parameters



condition

 

An expression that evaluates to true or false.  

expr1, expr2

 

Expressions with values of any type.  


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.



Implemented in  

JavaScript 1.0  

ECMA version  

ECMA-262  


Syntax
expr1, expr2


Parameters



expr1, expr2

 

Any expressions.  


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.



Implemented in  

JavaScript 1.2, NES 3.0  

ECMA version  

ECMA-262  


Syntax
delete objectName
delete objectName.property
delete objectName[index]
delete property // legal only within a with statement


Parameters



objectName

 

The name of an object.  

property

 

The property to delete.  

index

 

An integer representing the array index to delete.  


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.



Implemented in  

JavaScript 1.5  


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.



Implemented in  

JavaScript 1.4  


Syntax
propNameOrNumber in objectName


Parameters



propNameOrNumber

 

A string or numeric expression representing a property name or array index.  

objectName

 

Name of an object.  


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.



Implemented in  

JavaScript 1.4  


Syntax
objectName instanceof objectType


Parameters



objectName

 

Name of the object to compare to objectType.  

objectType

 

Object type.  


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.



Implemented in  

JavaScript 1.0  

ECMA version  

ECMA-262  


Syntax
objectName = new objectType (param1 [,param2] ...[,paramN])


Parameters



objectName

 

Name of the new object instance.  

objectType

 

Object type. It must be a function that defines an object type.  

param1...paramN

 

Property values for the object. These properties are parameters defined for the objectType function.  


Description
Creating a user-defined object type requires two steps:

  1. Define the object type by writing a function.

  2. 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.



Implemented in  

JavaScript 1.0  

ECMA version  

ECMA-262  


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.



Implemented in  

JavaScript 1.1  

ECMA version  

ECMA-262  

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.



Implemented in  

JavaScript 1.1  

ECMA version  

ECMA-262  

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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Last Updated September 28, 2000

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