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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.

Core object

Implemented in

JavaScript 1.0: toString method

JavaScript 1.1, NES 2.0: added eval and valueOf methods; constructor property

JavaScript 1.2: deprecated eval method

JavaScript 1.3: added toSource method

JavaScript 1.4: removed eval method

ECMA version

ECMA-262

Created by

The Object constructor:

new Object()

Parameters

None

Property Summary

Property Description
constructor

Specifies the function that creates an object's prototype.

prototype

Allows the addition of properties to all objects.

Method Summary

Method Description
eval

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

toSource

Returns an object literal representing the specified object; you can use this value to create a new object.

toString

Returns a string representing the specified object.

unwatch

Removes a watchpoint from a property of the object.

valueOf

Returns the primitive value of the specified object.

watch

Adds a watchpoint to a property of the object.


constructor

Specifies the function that creates an object's prototype. Note that the value of this property is a reference to the function itself, not a string containing the function's name.

Property of

Object

Implemented in

JavaScript 1.1, NES 2.0

ECMA version

ECMA-262

Description

All objects inherit a constructor property from their prototype:

o = new Object  // or o = {} in JavaScript 1.2
o.constructor == Object
a = new Array // or a = [] in JavaScript 1.2
a.constructor == Array
n = new Number(3)
n.constructor == Number
Even though you cannot construct most HTML objects, you can do comparisons. For example,

document.constructor == Document
document.form3.constructor == Form

Examples

The following example creates a prototype, Tree, and an object of that type, theTree. The example then displays the constructor property for the object theTree.

function Tree(name) {
   this.name=name
}
theTree = new Tree("Redwood")
document.writeln("<B>theTree.constructor is</B> " +
   theTree.constructor + "<P>")
This example displays the following output:

theTree.constructor is function Tree(name) { this.name = name; }

eval

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

Method of

Object

Implemented in

JavaScript 1.1, NES 2.0

JavaScript 1.2, NES 3.0: deprecated as method of objects; retained as top-level function

JavaScript 1.4: removed as method of objects

Syntax

eval(string)

Parameters

string

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

Description

The eval method is no 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


prototype

Represents the prototype for this class. You can use the prototype to add properties or methods to all instances of a class. For more information, see Function.prototype.

Property of

Object

Implemented in

JavaScript 1.1

ECMA version

ECMA-262


toSource

Returns a string representing the source code of the object.

Method of

Object

Implemented in

JavaScript 1.3

Syntax

toSource()

Parameters

None

Description

The toSource method returns the following values:

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.

Examples

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"}

See also

Object.toString


toString

Returns a string representing the specified object.

Method of

Object

Implemented in

JavaScript 1.0

ECMA version

ECMA-262

Syntax

toString()

Description

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]
Built-in toString methods. Every built-in core JavaScript object overrides the toString method of Object to return an appropriate value. JavaScript calls this method whenever it needs to convert an object to a string.

Overriding the default toString method. You can create a function to be called in place of the default toString method. The toString method takes no arguments and should return a string. The toString method you create can be any value you want, but it will be most useful if it carries information about the object.

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")
If you call the toString method on this custom object, it returns the default value inherited from Object:

theDog.toString() //returns [object Object]
The following code creates dogToString, the function that will be used to override the default toString method. This function generates a string containing each property, of the form "property = value;".

function dogToString() {
   var ret = "Dog " + this.name + " is [\n"
   for (var prop in this)
      ret += " " + prop + " is " + this[prop] + ";\n"
   return ret + "]"
}
The following code assigns the user-defined function to the object's toString method:

Dog.prototype.toString = dogToString
With the preceding code in place, any time theDog is used in a string context, JavaScript automatically calls the dogToString function, which returns the following string:

Dog Gabby is [
  name is Gabby;
  breed is Lab;
  color is chocolate;
  sex is girl;
]
An object's toString method is usually invoked by JavaScript, but you can invoke it yourself as follows:

var dogString = theDog.toString()

Backward Compatibility

JavaScript 1.2. The behavior of the toString method depends on whether you specify LANGUAGE="JavaScript1.2" in the <SCRIPT> tag:

Examples

Example 1: The location object. The following example prints the string equivalent of the current location.

document.write("location.toString() is " + location.toString() + "<BR>")
The output is as follows:

location.toString() is file:///C|/TEMP/myprog.html
Example 2: Object with no string value. 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]
Example 3: The radix parameter. 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), "<BR>")
}
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

See also

Object.toSource, Object.valueOf


unwatch

Removes a watchpoint set with the watch method.

Method of

Object

Implemented in

JavaScript 1.2, NES 3.0

Syntax

unwatch(prop)

Parameters

prop

The name of a property of the object.

Description

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

By default, this method is inherited by every object descended from Object.

Example

See watch.


valueOf

Returns the primitive value of the specified object.

Method of

Object

Implemented in

JavaScript 1.1

ECMA version

ECMA-262

Syntax

valueOf()

Parameters

None

Description

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

parseInt, Object.toString


watch

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

Method of

Object

Implemented in

JavaScript 1.2, NES 3.0

Syntax

watch(prop, handler)

Parameters

prop

The name of a property of the object.

handler

A function to call.

Description

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 Getting Started with Netscape 4.x JavaScript Debugger.

Example

<script language="JavaScript1.2">
o = {p:1}
o.watch("p",
   function (id,oldval,newval) {
      document.writeln("o." + id + " changed from "
         + oldval + " to " + newval)
      return newval
   })
o.p = 2
o.p = 3
delete o.p
o.p = 4
o.unwatch('p')
o.p = 5
</script>
This script displays the following:

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


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Last Updated: 10/29/98 21:09:03

Copyright 1998 Netscape Communications Corporation