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 methodNavigator 3.0, LiveWire 1.0: added eval and valueOf methods; constructor propertyNavigator 3.0: removed eval method
|
Object constructor:new Object();
| Specifies the function that creates an object's prototype. |
| Allows the addition of properties to all objects. |
| Property of |
Object
|
| Implemented in | Navigator 3.0, LiveWire 1.0 |
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
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; }
Function.prototype.
| Property of |
Object
|
| Implemented in | Navigator 3.0 |
| 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. |
eval(string)
string | Any string representing a JavaScript expression, statement, or sequence of statements. The expression can include variables and properties of existing objects. |
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,evalwas a top-level function. In Navigator 3.0evalwas 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,evalis once again a top-level function. In Navigator 4.02,obj.eval(str)is equivalent in all scopes towith(obj)eval(str), except of course that the latter is a statement, not an expression.
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)
eval
| Method of |
Object
|
| Implemented in | Navigator 2.0 |
toString()
toString(radix)
radix | (Optional) An integer between 2 and 16 specifying the base to use for representing numeric values. |
Button, Checkbox, FileUpload, Hidden, History, Link, Location, Password, Radio, Reset, Select, Submit, Text, and Textarea. For information on data tainting, see "JavaScript Security".
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)You can use
document.write("The dog is " + theDog)
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.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
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 = 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 [ 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())
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.
| Method of |
Object
|
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
unwatch(prop)
prop | The name of a property of the object. |
watch.
| Method of |
Object
|
| Implemented in | Navigator 3.0 |
valueOf()
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.
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
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
| Method of |
Object
|
| Implemented in | Navigator 4.0, Netscape Server 3.0 |
watch(prop, handler)
prop
The name of a property of the object.
handler
A function to call.
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.
<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
Last Updated: 10/31/97 12:30:31