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Closures

function doAdd(iNum) {

alert(iNum + 10);

}

alert(doAdd.toString());

This code outputs the exact text of the

doAdd()

function (see Figure 2-9).

Figure 2-9

Closures

One of the most misunderstood aspects of ECMAScript is its support for

closures.

Closures are functions

whose lexical representation includes variables that aren’t evaluated, meaning that functions are capable

of using variables defined outside of the function itself. Using global variables in ECMAScript is a sim-

ple example of a closure. Consider the following example:

var sMessage = “Hello World!”;

function sayHelloWorld() {

alert(sMessage);

}

sayHelloWorld();

In this code, the variable

sMessage

isn’t evaluated for the function

sayHelloWorld()

while the scripts is

being loaded into memory. The function

captures

sMessage

for later use, which is to say that the interpreter

knows to check the value of

sMessage

when the function is called. When

sayHelloWorld()

is called (on

the last line), the value of

sMessage

is assigned and the message

“Hello World!”

is displayed.

Closures can get more complicated, as when you are defining a function inside of another function, as

shown here:

var iBaseNum = 10;

function addNumbers(iNum1, iNum2) {

function doAddition() {

There are a couple of other methods of the

Function

class that are more relevant to

the discussion of objects and so are described in the next chapter.

ECMAScript Basics

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