Object

创建源

new Object();

参数

属性概览

方法概览

属性

描述

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

示例

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

方法

语法

描述

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.

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')) 示例 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)

参看

toString

语法

安全性

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

描述

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

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

Array, Boolean, Connection, database, DbPool, Function, Number

User-defined toString methods

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 + "]"; } ;]

alert(theDog.toString())

示例

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

location.toString() is file:///C|/TEMP/myprog.html 示例 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] 示例 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

参看

unwatch

语法

描述

示例

valueOf

语法

描述

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.

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

表 4.2 Use valueOf for these object types

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.

watch

语法

描述

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.

示例

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