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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

Navigator 2.0: toString method
Navigator 3.0, LiveWire 1.0: added eval and valueOf methods; constructor property
Navigator 3.0: removed eval method

Created by

The Object constructor:

new Object();

Parameters

None

Property Summary

constructor
Specifies the function that creates an object's prototype.

prototype
Allows the addition of properties to all objects.

Method Summary

eval
Evaluates a string of JavaScript code in the context of the specified 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.

Properties

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

Navigator 3.0, LiveWire 1.0

Description

All objects inherit a constructor property from their prototype:

o = new Object  // or o = {} in Navigator 4.0
o.constructor == Object
a = new Array // or a = [] in Navigator 4.0
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; }

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

Navigator 3.0

Methods

eval

Evaluates a string of JavaScript code in the context of this object.

Method of

Object

Implemented in

Navigator 3.0, LiveWire 1.0
Navigator 4.0, Netscape Server 3.0: removed as method of objects; retained as global function.

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 argument of the eval method is a string. If the string represents an expression, eval evaluates the expression. If the argument represents one or more JavaScript statements, eval performs the statements. Do not call eval to evaluate an arithmetic expression; JavaScript evaluates arithmetic expressions automatically.

If you construct an arithmetic expression as a string, you can use eval to evaluate it at a later time. For example, suppose you have a variable x. You can postpone evaluation of an expression involving x by assigning the string value of the expression, say "3 * x + 2", to a variable, and then calling eval at a later point in your script.

eval is also a global function, not associated with any object.

NOTE: In Navigator 2.0, eval was a top-level function. In Navigator 3.0 eval was also a method of every object. The ECMA-262 standard for JavaScript made eval available only as a top-level function. For this reason, in Navigator 4.0, eval is once again a top-level function. In Navigator 4.02, obj.eval(str) is equivalent in all scopes to with(obj)eval(str), except of course that the latter is a statement, not an expression.

Examples

Example 1. The following example creates breed as a property of the object myDog, and also as a variable. The first write statement uses eval('breed') without specifying an object; the string "breed" is evaluated without regard to any object, and the write method displays "Shepherd", which is the value of the breed variable. The second write statement uses myDog.eval('breed') which specifies the object myDog; the string "breed" is evaluated with regard to the myDog object, and the write method displays "Lab", which is the value of the breed property of the myDog object.

function Dog(name,breed,color) {
   this.name=name
   this.breed=breed
   this.color=color
}
myDog = new Dog("Gabby")
myDog.breed="Lab"
var breed='Shepherd'
document.write("<P>" + eval('breed'))
document.write("<BR>" + myDog.eval('breed'))
Example 2. The following example uses eval within a function that defines an object type, stone. The statement flint = new stone("x=42") creates the object flint with the properties x, y, z, and z2. The write statements display the values of these properties as 42, 43, 44, and 45, respectively.

function stone(str) {
   this.eval("this."+str)
   this.eval("this.y=43")
   this.z=44
   this["z2"] = 45
}
flint = new stone("x=42")
document.write("<BR>flint.x is " + flint.x)
document.write("<BR>flint.y is " + flint.y)
document.write("<BR>flint.z is " + flint.z)
document.write("<BR>flint.z2 is " + flint.z2)

See also

eval

toString

Returns a string representing the specified object.

Method of

Object

Implemented in

Navigator 2.0

Syntax

toString()
toString(radix)

Parameters

radix
(Optional) An integer between 2 and 16 specifying the base to use for representing numeric values.

Security

Navigator 3.0: This method is tainted by default for the following objects: Button, Checkbox, FileUpload, Hidden, History, Link, Location, Password, Radio, Reset, Select, Submit, Text, and Textarea.

For information on data tainting, see "JavaScript Security".

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)
You can use toString within your own code to convert an object into a string, and you can create your own function to be called in place of the default toString method.

Built-in toString methods
Every object type has a built-in toString method, which JavaScript calls whenever it needs to convert an object to a string. If an object has no string value and no user-defined toString method, toString returns "[object type]", where type is the object type or the name of the constructor function that created the object. For example, if for an Image object named sealife defined as shown below, sealife.toString() returns [object Image].

<IMG NAME="sealife" SRC="images\seaotter.gif" ALIGN="left" VSPACE="10">
Some built-in classes have special definitions for their toString methods. See the descriptions of this method for these objects:

Array, Boolean, Connection, database, DbPool, Function, Number
User-defined toString methods
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")
The following code creates dogToString, the function that will be used in place of 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 ["
   for (var prop in this)
      ret += " " + prop + " is " + this[prop] + ";"
   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; toString is function dogToString() { var ret = "Object " + this.name + " is ["; for (var prop in this) { ret += " " + prop + " is " + this[prop] + ";"; } return ret + "]"; } ;]
An object's toString method is usually invoked by JavaScript, but you can invoke it yourself as follows:

alert(theDog.toString())

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.valueOf

unwatch

Removes a watchpoint set with the watch method.

Method of

Object

Implemented in

Navigator 4.0, Netscape Server 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.

Example

See watch.

valueOf

Returns the primitive value of the specified object.

Method of

Object

Implemented in

Navigator 3.0

Syntax

valueOf()

Parameters

None

Description

Every object has a valueOf method that is automatically called when it is to be represented as a primitive value. If an object has no primitive value, valueOf returns the object itself.

You can use valueOf within your own code to convert an object into a primitive value, and you can create your own function to be called in place of the default valueOf method.

Every object type has a built-in valueOf method, which JavaScript calls whenever it needs 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.

Table 4.2 shows the object types for which the valueOf method is most useful. Most other objects have no primitive value.

Table 4.2 Use valueOf for these object types
Object type Value returned by valueOf
Number

Primitive numeric value associated with the object.

Boolean

Primitive boolean value associated with the object.

String

String associated with the object.

Function

Function reference associated with the object. For example, typeof funObj returns "object", but typeof funObj.valueOf() returns "function".

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

Navigator 4.0, Netscape Server 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.

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.

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: 10/31/97 12:30:31


Copyright 1997 Netscape Communications Corporation