JavaScript Core Reference: Object

Object is the primitive JavaScript object type. All JavaScript objects are descended from Object. That is, all JavaScript objects have the methods defined for Object.

Browser/User Agent Support

IE Mozilla Netscape Opera Safari
4.0+ 1.0+ 2.0+ 7.0+ 1.0+

IE	Mozilla	Netscape	Opera	Safari
4.0+	1.0+	2.0+	7.0+	1.0+

Constructors

Constructor IE Mozilla Netscape Opera Safari
Object Constructor
Creates a new instance of an Object.
4.0+ 1.0+ 2.0+ 7.0+ 1.0+

Constructor	IE	Mozilla	Netscape	Opera	Safari
Object Constructor Creates a new instance of an Object.	4.0+	1.0+	2.0+	7.0+	1.0+

Properties

Property IE Mozilla Netscape Opera Safari
constructor
Specifies the function that creates a prototype of an object.
4.0+ 1.0+ 3.0+ 7.0+ 1.0+

prototype
Represents the prototype for this class. You can use the prototype to add properties or methods to all instances of a class.
4.0+ 1.0+ 3.0+ 7.0+ 1.0+

Property	IE	Mozilla	Netscape	Opera	Safari
constructor Specifies the function that creates a prototype of an object.	4.0+	1.0+	3.0+	7.0+	1.0+

prototype Represents the prototype for this class. You can use the prototype to add properties or methods to all instances of a class.	4.0+	1.0+	3.0+	7.0+	1.0+

Methods

Method IE Mozilla Netscape Opera Safari
eval
Deprecated. Evaluates a string of JavaScript code in the context of an object.
4.0+ 1.0+ 3.0+ 7.0+ 1.0+

hasOwnProperty
Checks whether a property is inherited.
5.5+ 1.0+ 6.0+ 7.0+ no

isPrototypeOf
Returns true if the object is a prototype of another.
5.5+ 1.0+ 6.0+ 7.0+ no

propertyIsEnumerable
Returns true if the property can be enumerated in a for/in loop.
5.5+ 1.0+ 6.0+ 7.0+ no

toLocaleString
Returns a localized string representation of an object.
4.0+ 1.0+ 3.0+ 7.0+ 1.0+

toSource
Returns a string representing the source code of the object.
no 1.0+ 4.06+ no no

toString
Returns a string representing the specified object.
4.0+ 1.0+ 2.0+ 7.0+ 1.0+

unwatch
Removes a watchpoint set with the watch method.
no no 4.0+ 7.0+ no

valueOf
Returns the primitive value of the specified object
4.0+ 1.0+ 3.0+ 7.0+ 1.0+

watch
Watches for a property to be assigned a value and runs a function when that occurs.
no no 6.0+ 7.0+ no

Method	IE	Mozilla	Netscape	Opera	Safari
eval Deprecated. Evaluates a string of JavaScript code in the context of an object.	4.0+	1.0+	3.0+	7.0+	1.0+

hasOwnProperty Checks whether a property is inherited.	5.5+	1.0+	6.0+	7.0+	no

isPrototypeOf Returns true if the object is a prototype of another.	5.5+	1.0+	6.0+	7.0+	no

propertyIsEnumerable Returns true if the property can be enumerated in a for/in loop.	5.5+	1.0+	6.0+	7.0+	no

toLocaleString Returns a localized string representation of an object.	4.0+	1.0+	3.0+	7.0+	1.0+

toSource Returns a string representing the source code of the object.	no	1.0+	4.06+	no	no

toString Returns a string representing the specified object.	4.0+	1.0+	2.0+	7.0+	1.0+

unwatch Removes a watchpoint set with the watch method.	no	no	4.0+	7.0+	no

valueOf Returns the primitive value of the specified object	4.0+	1.0+	3.0+	7.0+	1.0+

watch Watches for a property to be assigned a value and runs a function when that occurs.	no	no	6.0+	7.0+	no

References

Availability

JavaScript objectsJavaScript 1.0 | JScript 1.0 | ECMAScript v1

Constructor Detail

ObjectNumberStringBoolean Object([String value])

Creates a new instance of an Object.

String value Specifies a JavaScript primitive type (String, Number, or Boolean) to be converted to a String, Number, or Boolean object. (optional)

Property Detail

Function constructor

Specifies the function that creates a prototype of an object.

Remarks
See Also: Object.toString
Availability: JavaScript 1.1 | JScript 2.0 | ECMAScript v1

Object prototype

Represents the prototype for this class. You can use the prototype to add properties or methods to all instances of a class.

Remarks
See Also: Function.prototype
Availability: JavaScript 1.1 | ECMA-262

Method Detail

eval(string string) : NumberObjectString

Deprecated. Evaluates a string of JavaScript code in the context of an object.

string string Any string representing a JavaScript expression, statement, or sequence of statements. The expression can include variables and properties of existing objects.

Remarks

The eval method is not longer available as a method of Object. Use the top-level eval function.

Backward Compatibility

JavaScript 1.2 and 1.3

eval as a method of Object and every object derived from Object is deprecated (but still available).

JavaScript 1.1

eval is a method of Object and every object derived from Object.

See Also

eval

Availability

JavaScript 1.1 | Deprecated as a method of objects; retained as a top-level function in JavaScript 1.2. | Removed as a method of objects in JavaScript 1.4

hasOwnProperty(String propname) : Boolean

Checks whether a property is inherited.

String propname The name of the property to check for.

See Also: Function.prototype | Object.propertyIsEnumerable
Availability: JavaScript 1.5 | JScript 5.5 | ECMAScript v3

isPrototypeOf(Object object) : Boolean

Returns true if the object is a prototype of another.

Object object The object to compare the original object to.

See Also: Function.prototype | Object.constructor
Availability: JavaScript 1.5 | JScript 5.5 | ECMAScript v3

propertyIsEnumerable(String property) : Boolean

Returns true if the property can be enumerated in a for/in loop.

String property Name of the property to check.

See Also: Function.prototype | Object.hasOwnProperty
Availability: JavaScript 1.5 | JScript 5.5 | ECMAScript v3

toLocaleString() : String

Returns a localized string representation of an object.

See Also: Array.toLocaleString | Date.toLocaleString | Number.toLocaleString | Object.toString
Availability: JavaScript 1.5 | JScript 5.5 | ECMAScript v3

toSource() : String

Returns a string representing the source code of the object.

Example: Using `toSource`

The following code defines the Dog object type and creates theDog, an object of type Dog:

function Dog(name,breed,color,sex) {
   this.name=name
   this.breed=breed
   this.color=color
   this.sex=sex
}
theDog = new Dog("Gabby","Lab","chocolate","girl")

Calling the toSource method of theDog displays the JavaScript source that defines the object:

theDog.toSource()
//returns ({name:"Gabby", breed:"Lab", color:"chocolate", sex:"girl"})

Remarks

The toSource method returns the following values:

For the built-in Object object, toSource returns the following string indicating that the source code is not available:

function Object() {
   [native code]
}

For instances of Object, toSource returns a string representing the source code.
For custom objects, toSource returns the JavaScript source that defines the object as a string.

This method is usually called internally by JavaScript and not explicitly in code. You can call toSource while debugging to examine the contents of an object.

See Also

toString

Availability

JavaScript 1.3

toString() : String

Returns a string representing the specified object.

The following example prints the string equivalent of the current location.

document.write("location.toString() is " + location.toString() + "
")

The output is as follows:

location.toString() is file:///C|/TEMP/myprog.htmll

Assume you have an Image object named sealife defined as follows:

<IMG NAME="sealife" SRC="images\seaotter.gif" ALIGN="left" VSPACE="10">

Because the Image object itself has no special toString method, sealife.toString() returns the following:

object Image

The following example prints the string equivalents of the numbers 0 through 9 in decimal and binary.

for (x = 0; x < 10; x++) {
   document.write("Decimal: ", x.toString(10), " Binary: ",
      x.toString(2), "
")
}

The preceding example produces the following output:

Decimal: 0 Binary: 0
Decimal: 1 Binary: 1
Decimal: 2 Binary: 10
Decimal: 3 Binary: 11
Decimal: 4 Binary: 100
Decimal: 5 Binary: 101
Decimal: 6 Binary: 110
Decimal: 7 Binary: 111
Decimal: 8 Binary: 1000
Decimal: 9 Binary: 1001

Remarks

Every object has a toString method that is automatically called when it is to be represented as a text value or when an object is referred to in a string concatenation. For example, the following examples require theDog to be represented as a string:

document.write(theDog)
document.write("The dog is " + theDog)

By default, the toString method is inherited by every object descended from Object. You can override this method for custom objects that you create. If you do not override toString in a custom object, toString returns [object type], where type is the object type or the name of the constructor function that created the object.

For example:

var o = new Object()
o.toString // returns [object Object]

Availability

JavaScript 1.0 | JScript 2.0 | ECMAScript v1

unwatch(String prop) : Object

Removes a watchpoint set with the watch method.

String prop The name of a property of the object.

Remarks: By default, this method is inherited by every object descended from Object.
See Also: watch
Availability: JavaScript 1.2

valueOf() : Object

Returns the primitive value of the specified object

Remarks

JavaScript calls the valueOf method to convert an object to a primitive value. You rarely need to invoke the valueOf method yourself; JavaScript automatically invokes it when encountering an object where a primitive value is expected.

By default, the valueOf method is inherited by every object descended from Object. Every built-in core object overrides this method to return an appropriate value. If an object has no primitive value, valueOf returns the object itself, which is displayed as:

object Object]

You can use valueOf within your own code to convert a built-in object into a primitive value. When you create a custom object, you can override Object.valueOf to call a custom method instead of the default Object method.

Overriding `valueOf` for custom objects

You can create a function to be called in place of the default valueOf method. Your function must take no arguments.

Suppose you have an object type myNumberType and you want to create a valueOf method for it. The following code assigns a user-defined function to the object's valueOf method:

myNumberType.prototype.valueOf = new Function(functionText);

With the preceding code in place, any time an object of type myNumberType is used in a context where it is to be represented as a primitive value, JavaScript automatically calls the function defined in the preceding code.

An object's valueOf method is usually invoked by JavaScript, but you can invoke it yourself as follows:

myNumber.valueOf();

Note

Objects in string contexts convert via the toString method, which is different from String objects converting to string primitives using valueOf. All string objects have a string conversion, if only "[object type]". But many objects do not convert to number, boolean, or function.

See Also

Object.toString

Availability

JavaScript 1.1 | JScript 2.0 | ECMAScript v1

watch(String prop, String handler) : Object

Watches for a property to be assigned a value and runs a function when that occurs.

String prop Name of the property to set.
String handler Name of the function to call.

Example: Using `watch` and `unwatch`

This script displays the following:

o.p changed from 1 to 2
o.p changed from 2 to 3
o.p changed from undefined to 4

Remarks

Watches for assignment to a property named prop in this object, calling handler(prop, oldval, newval) whenever prop is set and storing the return value in that property. A watchpoint can filter (or nullify) the value assignment, by returning a modified newval (or oldval).

If you delete a property for which a watchpoint has been set, that watchpoint does not disappear. If you later recreate the property, the watchpoint is still in effect.

To remove a watchpoint, use the unwatch method. By default, the watch method is inherited by every object descended from Object.

The JavaScript debugger has functionality similar to that provided by this method, as well as other debugging options. For information on the debugger, see Venkman.

In NES 3.0 and 4.x, handler is called from assignments in script as well as native code. In Firefox, handler is only called from assignments in script, not from native code. For example, window.watch('location', myHandler) will not call myHandler if the user clicks a link to an anchor within the current document. However, the following code will call myHandler: window.location += '#myAnchor';.

See Also

unwatch

Availability

JavaScript 1.2

String	prop	Name of the property to set.
String	handler	Name of the function to call.

Object

Browser/User Agent Support

Constructors

Properties

Methods

References

Availability

Constructor Detail

ObjectNumberStringBoolean Object([String value])

Property Detail

Function constructor

Object prototype

Method Detail

eval(string string) : NumberObjectString

Backward Compatibility

JavaScript 1.2 and 1.3

JavaScript 1.1

hasOwnProperty(String propname) : Boolean

isPrototypeOf(Object object) : Boolean

propertyIsEnumerable(String property) : Boolean

toLocaleString() : String

toSource() : String

Example: Using toSource

toString() : String

unwatch(String prop) : Object

valueOf() : Object

Overriding valueOf for custom objects

Note

watch(String prop, String handler) : Object

Example: Using watch and unwatch

Example: Using `toSource`

Overriding `valueOf` for custom objects

Example: Using `watch` and `unwatch`