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: http://www.arcetri.astro.it/irlab/doc/library/javascript/clientreference/object.htm
Дата изменения: Thu Oct 7 13:13:36 1999 Дата индексирования: Sat Dec 22 14:23:52 2007 Кодировка: Поисковые слова: earth's atmosphere |
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
JavaScript 1.3: added | |
Object
constructor:
new Object()
Property |
Description
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Method |
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 1.2Even though you cannot construct most HTML objects, you can do comparisons. For example,
o.constructor == Object
a = new Array // or a = [] in JavaScript 1.2
a.constructor == Array
n = new Number(3)
n.constructor == Number
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 example displays the following output:
this.name=name
}
theTree = new Tree("Redwood")
document.writeln("<B>theTree.constructor is</B> " +
theTree.constructor + "<P>")
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
.toSource()
toSource
method returns the following values:
function Object() {
[native code]
}
Object
, toSource
returns a string representing the source code.toSource
returns the JavaScript source that defines the object as a string.toSource
while debugging to examine the contents of an object.
Dog
object type and creates theDog,
an object of type Dog
:
function Dog(name,breed,color,sex) {Calling the
this.name=name
this.breed=breed
this.color=color
this.sex=sex
}
theDog = new Dog("Gabby","Lab","chocolate","girl")
toSource
method of theDog
displays the JavaScript source that defines the object:
theDog.toSource()
//returns "{name:"Gabby", breed:"Lab", color:"chocolate", sex:"girl"}
Object.toString
toString()
Button
, Checkbox
, FileUpload
, Hidden
, History
, Link
, Location
, Password
, Radio
, Reset
, Select
, Submit
, Text
, and Textarea
. For information on data tainting, see the Client-Side JavaScript Guide.
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()Built-in toString methods. Every built-in core JavaScript object overrides the
o.toString // returns [object Object]
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() {The following code assigns the user-defined function to the object's
var ret = "Dog " + this.name + " is [\n"
for (var prop in this)
ret += " " + prop + " is " + this[prop] + ";\n"
return ret + "]"
}
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()
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.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++) {The preceding example produces the following output:
document.write("Decimal: ", x.toString(10), " Binary: ",
x.toString(2), "<BR>")
}
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.toSource
, 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 thetoString
method, which is different fromString
objects converting to string primitives usingvalueOf
. All string objects have a string conversion, if only"[object
type
]"
. 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: 05/28/99 12:00:04