↑
Main Page

ECMAScript

var doAdd = new Function(“iNum”, “alert(iNum + 10) “);

var alsoDoAdd = doAdd;

doAdd(10); //outputs “20”

alsoDoAdd(10); //outputs “20”

Here, the variable

doAdd

is defined as a function, and then

alsoDoAdd

is declared to point to the same

function. Both can then be used to execute the function’s code and output the same result

, “20”

. So if a

function name is just a variable pointing to a function, is it possible to pass a function as an argument to

another function? Yes!

function callAnotherFunc(fnFunction, vArgument) {

fnFunction(vArgument);

}

var doAdd = new Function(“iNum”, “alert(iNum + 10)”);

callAnotherFunc(doAdd, 10); //outputs “20”

In this example,

callAnotherFunction()

accepts two arguments: a function to call and an argument

to pass to the function. This code passes the

doAdd()

function into

callAnotherFunction()

with an

argument of

, outputting

“20”

Because functions are reference types, they can also have properties and methods. The one property

defined in ECMAScript is

length

, which indicates the number of arguments that a function expects.

Example:

function doAdd(iNum) {

alert(iNum + 10);

}

function sayHi() {

alert(“Hi”);

}

alert(doAdd.length); //outputs “1”

alert(sayHi.length); //outputs “0”

The function

doAdd()

defines one argument to pass in, so its

length

;

sayHi()

defines no argu-

ments, so its length is

. Remember, ECMAScript functions can accept any number of arguments (up to

255) regardless of how many are defined. The

length

property just gives a convenient way to check

how many arguments are expected by default.

Function objects also have the standard

valueOf()

and

toString()

methods shared by all objects.

Both of these methods return the source code for the function and are particularly useful in debugging.

For example:

Even though it’s possible to create a function using the

Function

constructor, it’s

best to avoid it because it’s slower than defining the function in the traditional man-

ner. However, all functions are considered instances of

Function

Chapter 2

05_579088 ch02.qxd 3/28/05 11:35 AM Page 64