JavaScript 1.2: added
JavaScript 1.3: added | |
Function constructor:
new Function ([arg1[, arg2[, ... argN]],] functionBody)The
function statement (see "function" on page 622 for details):
function name([param[, param[, ... param]]]) {
statements
}
Function objects created with the Function constructor 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.
To return a value, the function must have a return statement that specifies the value to return.
All parameters are passed to functions by value; the value is passed to the function, but if the function changes the value of the parameter, this change is not reflected globally or in the calling function. However, if you pass an object as a parameter to a function and the function changes the object's properties, that change is visible outside the function, as shown in the following example:
function myFunc(theObject) {
theObject.make="Toyota"
}
mycar = {make:"Honda", model:"Accord", year:1998}
x=mycar.make // returns Honda
myFunc(mycar) // pass object mycar to the function
y=mycar.make // returns Toyota (prop was changed by the function)
The this keyword does not refer to the currently executing function, so you must refer to Function objects by name, even within the function body.
Accessing a function's arguments with the arguments array.
You can refer to a function's arguments within the function by using the arguments array. See arguments.
Specifying arguments with the Function constructor.
The following code creates a Function object that takes two arguments.
var multiply = new Function("x", "y", "return x * y")
The arguments "x" and "y" are formal argument names that are used in the function body, "return x * y".
The preceding code assigns a function to the variable multiply. To call the Function object, you can specify the variable name as if it were a function, as shown in the following examples.
var theAnswer = multiply(7,6)
var myAge = 50Assigning a function to a variable with the Function constructor. Suppose you create the variable
if (myAge >=39) {myAge=multiply (myAge,.5)}
multiply using the Function constructor, as shown in the preceding section:
var multiply = new Function("x", "y", "return x * y")
This is similar to declaring the following function:
function multiply(x,y) {
return x*y
}
Assigning a function to a variable using the Function constructor is similar to declaring a function with the function statement, but they have differences:
var multiply = new Function("..."), multiply is a variable for which the current value is a reference to the function created with new Function().function multiply() {...}, multiply is not a variable, it is the name of a function.function addSquares (a,b) {
function square(x) {
return x*x
}
return square(a) + square(b)
}
a=addSquares(2,3) // returns 13
b=addSquares(3,4) // returns 25
c=addSquares(4,5) // returns 41
When a function contains a nested function, you can call the outer function and specify arguments for both the outer and inner function:
function outside(x) {
function inside(y) {
return x+y
}
return inside
}
result=outside(3)(5) // returns 8
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'")
If a function is assigned to a variable, you can assign the variable to an event handler. The following code assigns a function to the variable setBGColor.
var setBGColor = new Function("document.bgColor='antiquewhite'")
You can use this variable to assign a function to an event handler in either of the following ways:
document.form1.colorButton.onclick=setBGColor
<INPUT NAME="colorButton" TYPE="button"Once you have a reference to a
VALUE="Change background color"
onClick="setBGColor()">
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 a
Function constructor for an event handler. For example, you cannot call the function multiply by setting a button's onclick property as follows:
document.form1.button1.onclick=multFun(5,10)
// This function returns a string padded with leading zerosThe following statements call the
function padZeros(num, totalLen) {
var numStr = num.toString() // Initialize return value
// as string
var numZeros = totalLen - numStr.length // Calculate no. of zeros
if (numZeros > 0) {
for (var i = 1; i <= numZeros; i++) {
numStr = "0" + numStr
}
}
return numStr
}
padZeros function.
result=padZeros(42,4) // returns "0042"Example 2. You can determine whether a function exists by comparing the function name to null. In the following example,
result=padZeros(42,2) // returns "42"
result=padZeros(5,4) // returns "0005"
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)Example 3. The following example creates
func1()
else func2()
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'")
apply(thisArg[, argArray])
thisArg | |
argArray |
this object when calling an existing function. this refers to the current object, the calling object. With apply, you can write a method once and then inherit it in another object, without having to rewrite the method for the new object.
apply is very similar to call, except for the type of arguments it supports. You can use an arguments array instead of a named set of parameters. With apply, you can use an array literal, for example, apply(this, [name, value]), or an Array object, for example, apply(this, new Array(name, value)).
You can also use arguments for the argArray parameter. arguments is a local variable of a function. It can be used for all unspecified arguments of the called object. Thus, you do not have to know the arguments of the called object when you use the apply method. You can use arguments to pass all the arguments to the called object. The called object is then responsible for handling the arguments.
apply to chain constructors for an object, similar to Java. In the following example, the constructor for the product object is defined with two parameters, name and value. Another object, prod_dept, initializes its unique variable (dept) and calls the constructor for product in its constructor to initialize the other variables. In this example, the parameter arguments is used for all arguments of the product object's constructor.
function product(name, value){
this.name = name;
if(value > 1000)
this.value = 999;
else
this.value = value;
}function prod_dept(name, value, dept){
this.dept = dept;
product.apply(product, arguments);
}prod_dept.prototype = new product();
// since 5 is less than 100 value is set
cheese = new prod_dept("feta", 5, "food");
// since 5000 is above 1000, value will be 999
car = new prod_dept("honda", 5000, "auto");
Function.call
| |
JavaScript 1.2: added
JavaScript 1.3: deprecated | |
arguments array. This array contains an entry for each argument passed to the function. For example, if a function is passed three arguments, you can refer to the arguments as follows:
arguments[0]The
arguments[1]
arguments[2]
arguments array can also be preceded by the function name:
myFunc.arguments[0]The
myFunc.arguments[1]
myFunc.arguments[2]
arguments array is available only within a function body. Attempting to access the arguments array outside a function declaration results in an error.
You can use the arguments array if you call a function with more arguments than it is formally declared to accept. This technique is useful for functions that 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 process each argument by using the arguments array. (To determine the number of arguments declared when a function was defined, use the Function.length property.)
The arguments array has the following properties:
| Property |
Description
Specifies the function body of the currently executing function. Specifies the name of the function that invoked the currently executing function. (Deprecated)
| |
|---|
arguments array. For example, if a function myFunc has a local variable named myLocalVar, you can refer to the variable as arguments.myLocalVar.arguments array. For example, if a function myFunc has two arguments named arg1 and arg2, you can refer to the arguments as arguments.arg1 and arguments.arg2. (You can also refer to them as arguments[0] and arguments[1].)function myConcat(separator) {
result="" // initialize list
// iterate through arguments
for (var i=1; i<arguments.length; i++) {
result += arguments[i] + separator
}
return result
}
You can pass any number of arguments to this function, and it creates a list using each argument as an item in the list.
// returns "red, orange, blue, "
myConcat(", ","red","orange","blue")
// returns "elephant; giraffe; lion; cheetah;"
myConcat("; ","elephant","giraffe","lion", "cheetah")
// returns "sage. basil. oregano. pepper. parsley. "Example 2. This example defines a function that creates HTML lists. The only formal argument for the function is a string that is
myConcat(". ","sage","basil","oregano", "pepper", "parsley")
"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>") // begin list
// iterate through arguments
for (var i=1; i<arguments.length; i++) {
document.write("<LI>" + arguments[i])
}
document.write("</" + type + "L>") // end list
}
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>
callee property is available only within the body of a function.
The this keyword does not refer to the currently executing function. Use the callee property to refer to a function within the function body.
callee property.
function myFunc() {
return arguments.callee
}
The following value is returned:
function myFunc() { return arguments.callee; }
Function.arguments
caller is no longer used.
The caller property is available only within the body of a function.
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
functionName.toString. That is, the decompiled canonical source form of the function.caller property.
function myFunc() {
if (arguments.caller == null) {
return ("The function was called from the top!")
} else return ("This function's caller was " + arguments.caller)
}
Function.arguments
arguments.length provides the number of arguments actually passed to a function. By contrast, the Function.length property indicates how many arguments a function expects.
Function.length and arguments.length.
function addNumbers(x,y){
if (arguments.length == addNumbers.length) {
return (x+y)
}
else return 0
}
If you pass more than two arguments to this function, the function returns 0:
result=addNumbers(3,4,5) // returns 0
result=addNumbers(3,4) // returns 7
result=addNumbers(103,104) // returns 207
Function.arguments
arity is external to the function, and indicates how many arguments a function expects. By contrast, arguments.length provides the number of arguments actually passed to a function.
arity and arguments.length.
function addNumbers(x,y){
if (arguments.length == addNumbers.length) {
return (x+y)
}
else return 0
}
If you pass more than two arguments to this function, the function returns 0:
result=addNumbers(3,4,5) // returns 0
result=addNumbers(3,4) // returns 7
result=addNumbers(103,104) // returns 207
arguments.length, Function.length
call(thisArg[, arg1[, arg2[, ...]]])
thisArg | |
arg1, arg2, ... |
this object when calling an existing function. this refers to the current object, the calling object.
With call, you can write a method once and then inherit it in another object, without having to rewrite the method for the new object.
call to chain constructors for an object, similar to Java. In the following example, the constructor for the product object is defined with two parameters, name and value. Another object, prod_dept, initializes its unique variable (dept) and calls the constructor for product in its constructor to initialize the other variables.
function product(name, value){
this.name = name;
if(value > 1000)
this.value = 999;
else
this.value = value;
}function prod_dept(name, value, dept){
this.dept = dept;
product.call(this, name, value);
}prod_dept.prototype = new product();
// since 5 is less than 100 value is set
cheese = new prod_dept("feta", 5, "food");
// since 5000 is above 1000, value will be 999
car = new prod_dept("honda", 5000, "auto");
Function.apply
Object.constructor.
length is external to a function, and indicates how many arguments the function expects. By contrast, arguments.length is local to a function and provides the number of arguments actually passed to the function.
arguments.length.
arguments.length
prototype property.fun.prototype.name = valuewhere
fun | The name of the constructor function object you want to change. |
name | |
value |
var array1 = new Array();After you set a property for the prototype, all subsequent objects created with
var array2 = new Array(3);
Array.prototype.description=null;
array1.description="Contains some stuff"
array2.description="Contains other stuff"
Array will have the property:
anotherArray=new Array()
anotherArray.description="Currently empty"
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
s1a=s1.rep(3) // returns "aaa"
s2a=s2.rep(5) // returns "bbbbb"
s3a=s3.rep(2) // returns "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_repThe function in this example also works on
s1b=s1.rep(1) // returns "repeat a 1 times."
s2b=s2.rep(4) // returns "bbbb"
s3b=s3.rep(6) // returns "cccccc"
String objects not created with the String constructor. The following code returns "zzz".
"z".rep(3)
toSource()
toSource method returns the following values:
function Function() {
[native code]
}toSource returns the JavaScript source that defines the object as a string.
Function.toString, Object.valueOf
toString()
Function object overrides the toString method of the Object object; it does not inherit Object.toString. For Function objects, the toString method returns a string representation of the object.
JavaScript calls the toString method automatically when a Function is to be represented as a text value or when a Function is referred to in a string concatenation.
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; }
Object.toString
valueOf()
valueOf method returns the following values:
function Function() {
[native code]
}toSource returns the JavaScript source that defines the object as a string. The method is equivalent to the toString method of the function.
Function.toString, Object.valueOf
Last Updated: 05/28/99 11:59:30