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.
JavaScript 1.0:
JavaScript 1.1, NES 2.0: added
JavaScript 1.2: deprecated | |
Object constructor:
new Object()
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Description
Deprecated. Evaluates a string of JavaScript code in the context of the specified object.
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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
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; }
JavaScript 1.2, NES 3.0: deprecated as method of objects; retained as top-level function |
eval(string)
string |
eval as a method of Object and every object derived from Object is deprecated. Use the top-level eval function.
eval is a method of Object and every object derived from Object.
eval
Function.prototype.toString()
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)By default, the
document.write("The dog is " + theDog)
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()The behavior of the
o.toString // returns [object Object]
toString method depends on whether you specify LANGUAGE="JavaScript1.2" in the <SCRIPT> tag:
LANGUAGE="JavaScript1.2" in the <SCRIPT> tag, the toString method returns an object literal.LANGUAGE="JavaScript1.2" in the <SCRIPT> tag, the toString method returns [object type], as with other JavaScript versions.toString method of Object to return an appropriate value. JavaScript calls this method whenever it needs to convert an object to a string.
Some built-in client-side and server-side JavaScript objects do not override the toString method of Object. For example, 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">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 = dogToStringWith 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 [An object's
name is Gabby;
breed is Lab;
color is chocolate;
sex is girl;
]
toString method is usually invoked by JavaScript, but you can invoke it yourself as follows:
var dogString = theDog.toString()
document.write("location.toString() is " + location.toString() + "<BR>")
The output is as follows:
location.toString() is file:///C|/TEMP/myprog.htmlExample 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
Object.valueOf
watch method.unwatch(prop)
Object.
watch.
valueOf()
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 thetoStringmethod, which is different fromStringobjects converting to string primitives usingvalueOf. All string objects have a string conversion, if only"[objecttype]". But many objects do not convert to number, boolean, or function.
parseInt, Object.toString
watch(prop, handler)
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 JavaScript Debugger.
<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
Last Updated: 11/13/98 10:23:32