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Use local variables

Figure 19-1

Use local variables

Always use the

var

statement to define variables inside functions. Whenever you used

var

, a local vari-

able is created within the current scope. If you begin using a variable without first defining it with

var

the variable is created at the

window

scope, meaning that every time you use that variable the interpreter

has to search back up the scope tree to find it. For example, don’t do this:

function sayFirstName() {

sMyFirstName = “Nicholas”;

alert(sMyFirstName);

}

In this function, the variable

sMyFirstName

is assigned a value without using

var

; this variable is cre-

ated at the

window

scope. You can prove this to yourself by defining another function that also uses this

variable:

function sayFirstName() {

sMyFirstName = “Nicholas”;

alert(sMyFirstName);

}

function sayFirstNameToo() {

alert(sMyFirstName);

}

sayFirstName();

sayFirstNameToo();

In this example, you see two alerts displaying

“Nicholas”

. This happens because the first function call,

sayFirstName()

, creates the variable

sMyFirstName

at the window scope, which means the second

MyMiddleInitial = "C"

MyFirstName = "Nicholas"

(window)

fn3()

fn2()

MyLastName = "Zakas"

fn1()

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Chapter 19

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