Core JavaScript Reference 1.5: | 16 | WebReference

Core JavaScript Reference 1.5: | 16

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Core JavaScript Reference 1.5





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.htmll

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 Venkman, the new JavaScript Debugger for Netscape 7.x.

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 Venkman, the new JavaScript Debugger for Netscape 7.x.


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 September 28, 2000