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Function
Specifies a string of JavaScript code to be compiled as a function.
创建源
The Function constructor:
new Function (arg1, arg2, ... argN, functionBody)
参数
描述
Function objects are evaluated each time they are used. This is less efficient than declaring a function and calling it within your code, because declared functions are compiled.
In addition to defining functions as described here, you can also use the function statement, as described in the JavaScript Guide.
属性概览
方法概览
Specifying a variable value with a Function object
The following code assigns a function to the variable setBGColor. This function sets the current document's background color.
var setBGColor = new Function("document.bgColor='antiquewhite'")
To call the Function object, you can specify the variable name as if it were a function. The following code executes the function specified by the setBGColor variable:
var colorChoice="antiquewhite"
if (colorChoice=="antiquewhite") {setBGColor()}
You can assign the function to an event handler in either of the following ways:
document.form1.colorButton.onclick=setBGColor
<INPUT NAME="colorButton" TYPE="button"
VALUE="Change background color"
onClick="setBGColor()">
Creating the variable setBGColor shown above is similar to declaring the following function:
function setBGColor() {
document.bgColor='antiquewhite'
}
Assigning a function to a variable is similar to declaring a function, but they have differences:
- When you assign a function to a variable using var setBGColor = new Function("..."), setBGColor is a variable for which the current value is a reference to the function created with new Function().
- When you create a function using function setBGColor() {...}, setBGColor is not a variable, it is the name of a function.
Specifying参数in a Function object
The following code specifies a Function object that takes two arguments.
var multFun = new Function("x", "y", "return x * y")
The string arguments "x" and "y" are formal argument names that are used in the function body, "return x * y".
The following code shows several ways to call the function multFun:
var theAnswer = multFun(7,6)
document.write("15*2 = " + multFun(15,2))
<INPUT NAME="operand1" TYPE="text" VALUE="5" SIZE=5>
<INPUT NAME="operand2" TYPE="text" VALUE="6" SIZE=5>
<INPUT NAME="result" TYPE="text" VALUE="" SIZE=10>
<INPUT NAME="buttonM" TYPE="button" VALUE="Multiply"
onClick="document.form1.result.value=
multFun(document.form1.operand1.value,
document.form1.operand2.value)">
You cannot call the function multFun in an object's event handler property, because event handler properties cannot take arguments. For example, you cannot call the function multFun by setting a button's onclick property as follows:
document.form1.button1.onclick=multFun(5,10)
Specifying an event handler with a Function object
The following code assigns a function to a window's onFocus event handler (the event handler must be spelled in all lowercase):
window.onfocus = new Function("document.bgColor='antiquewhite'")
Once you have a reference to a function object, you can use it like a function and it will convert from an object to a function:
window.onfocus()
Event handlers do not take arguments, so you cannot declare any arguments in the Function constructor for an event handler.
示例
示例 1. The following example creates onFocus and onBlur event handlers for a frame. This code exists in the same file that contains the FRAMESET tag. Note that this is the only way to create onFocus and onBlur event handlers for a frame, because you cannot specify the event handlers in the FRAME tag.
frames[0].onfocus = new Function("document.bgColor='antiquewhite'")
frames[0].onblur = new Function("document.bgColor='lightgrey'")
示例 2. You can determine whether a function exists by comparing the function name to null. In the following example, func1 is called if the function noFunc does not exist; otherwise func2 is called. Notice that the window name is needed when referring to the function name noFunc.
if (window.noFunc == null)
func1()
else func2()
属性
arguments
An array corresponding to the arguments passed to a function.
描述
You can call a function with more arguments than it is formally declared to accept by using the arguments array. This technique is useful if a function can be passed a variable number of arguments. You can use arguments.length to determine the number of arguments passed to the function, and then treat each argument by using the arguments array.
The arguments array is available only within a function declaration. Attempting to access the arguments array outside a function declaration results in an error.
The this keyword does not refer to the currently executing function, so you must refer to functions and Function objects by name, even within the function body. In JavaScript 1.2, arguments includes these additional properties:
- formal arguments--each formal argument of a function is a property of the arguments array.
- local variables--each local variable of a function is a property of the arguments array.
- caller--a property whose value is the arguments array of the outer function. If there is no outer function, the value is undefined.
- callee--a property whose value is the function reference.
For example, the following script demonstrates several of the arguments properties:
<SCRIPT>
function b(z) {
document.write(arguments.z + "<BR>")
document.write (arguments.caller.x + "<BR>")
return 99
}
function a(x, y) {
return b(534)
}
document.write (a(2,3) + "<BR>")
</SCRIPT>
This displays:
534
2
99
534 is the actual parameter to b, so it is the value of arguments.z.
2 is a's actual x parameter, so (viewed within b) it is the value of arguments.caller.x.
99 is what a(2,3) returns.
示例
This example defines a function that creates HTML lists. The only formal argument for the function is a string that is "U" if the list is to be unordered (bulleted), or "O" if the list is to be ordered (numbered). The function is defined as follows:
function list(type) {
document.write("<" + type + "L>")
for (var i=1; i<list.arguments.length; i++) {
document.write("<LI>" + list.arguments[i])
document.write("</" + type + "L>")
}
}
You can pass any number of arguments to this function, and it displays each argument as an item in the type of list indicated. For example, the following call to the function
list("U", "One", "Two", "Three")
results in this output:
<UL>
<LI>One
<LI>Two
<LI>Three
</UL>
In server-side JavaScript, you can display the same output by calling the write function instead of using document.write.
arity
When the LANGUAGE attribute of the SCRIPT tag is "JavaScript1.2", this property indicates the number of arguments expected by a function.
描述
arity is external to the function, and indicates how many arguments the function expects. By contrast, arguments.length provides the number of arguments actually passed to the function.
示例
The following example demonstrates the use of arity and arguments.length.
<SCRIPT LANGUAGE = "JavaScript1.2">
function addNumbers(x,y){
document.write("length = " + arguments.length + "<BR>")
z = x + y
}
document.write("arity = " + addNumbers.arity + "<BR>")
addNumbers(3,4,5)
</SCRIPT>
This script writes:
arity = 2
length = 3
caller
Returns the name of the function that invoked the currently executing function.
描述
The caller property is available only within the body of a function. If used outside a function declaration, the caller property is null.
If the currently executing function was invoked by the top level of a JavaScript program, the value of caller is null.
The this keyword does not refer to the currently executing function, so you must refer to functions and Function objects by name, even within the function body.
The caller property is a reference to the calling function, so
- If you use it in a string context, you get the result of calling functionName.toString. That is, the decompiled canonical source form of the function.
- You can also call the calling function, if you know what arguments it might want. Thus, a called function can call its caller without knowing the name of the particular caller, provided it knows that all of its callers have the same form and fit, and that they will not call the called function again unconditionally (which would result in infinite recursion).
示例
The following code checks the value of a function's caller property.
function myFunc() {
if (myFunc.caller == null) {
alert("The function was called from the top!")
} else alert("This function's caller was " + myFunc.caller)
}
参看
Function.arguments
prototype
A value from which instances of a particular class are created. Every object that can be created by calling a constructor function has an associated prototype property.
描述
You can add new properties or methods to an existing class by adding them to the prototype associated with the constructor function for that class. The语法 for adding a new property or method is:
fun.prototype.name = value
where
If you add a new property to the prototype for an object, then all objects created with that object's constructor function will have that new property, even if the objects existed before you created the new property. For example, assume you have the following statements:
var array1 = new Array();
var array2 = new Array(3);
Array.prototype.description=null;
array1.description="Contains some stuff"
array2.description="Contains other stuff"
After you set a property for the prototype, all subsequent objects created with Array will have the property:
anotherArray=new Array()
anotherArray.description="Currently empty"
示例
The following example creates a method, str_rep, and uses the statement String.prototype.rep = str_rep to add the method to all String objects. All objects created with new String() then have that method, even objects already created. The example then creates an alternate method and adds that to one of the String objects using the statement s1.rep = fake_rep. The str_rep method of the remaining String objects is not altered.
var s1 = new String("a")
var s2 = new String("b")
var s3 = new String("c")
// Create a repeat-string-N-times method for all String objects
function str_rep(n) {
var s = "", t = this.toString()
while (--n >= 0) s += t
return s
}
String.prototype.rep = str_rep
// Display the results
document.write("<P>s1.rep(3) is " + s1.rep(3)) // "aaa"
document.write("<BR>s2.rep(5) is " + s2.rep(5)) // "bbbbb"
document.write("<BR>s3.rep(2) is " + s3.rep(2)) // "cc"
// Create an alternate method and assign it to only one String variable
function fake_rep(n) {
return "repeat " + this + n + " times."
}
s1.rep = fake_rep
document.write("<P>s1.rep(1) is " + s1.rep(1)) // "repeat a 1 times."
document.write("<BR>s2.rep(4) is " + s2.rep(4)) // "bbbb"
document.write("<BR>s3.rep(6) is " + s3.rep(6)) // "cccccc"
This example produces the following output:
s1.rep(3) is aaa
s2.rep(5) is bbbbb
s3.rep(2) is cc
s1.rep(1) is repeat a1 times.
s2.rep(4) is bbbb
s3.rep(6) is cccccc
The function in this example also works on String objects not created with the String constructor. The following code returns "zzz".
"z".rep(3)
方法
toString
Returns a string representing the specified object.
语法
toString()
参数
无。
描述
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.
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.
For Function objects, the built-in toString method decompiles the function back into the JavaScript source that defines the function. This string includes the function keyword, the argument list, curly braces, and function body.
For example, assume you have the following code that 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")
Any time Dog is used in a string context, JavaScript automatically calls the toString function, which returns the following string:
function Dog(name, breed, color, sex) { this.name = name; this.breed =
breed; this.color = color; this.sex = sex; }
For information on defining your own toString method, see the Object.toString method.
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