↑
Main Page

Global

When the interpreter finds an

eval()

call, it interprets the argument into actual ECMAScript statements

and then inserts it into place. This means that variables can be referenced inside of an

eval()

call that is

defined outside of its argument:

var msg = “hello world”;

eval(“alert(msg)”);

Here, the variable

msg

is defined outside of the context of the

eval()

call, yet the alert is still displayed

with the text

“hello world”

because the second line is replaced with a real line of code. Likewise, you

can define a function or variables inside of an

eval()

call that can be referenced by the code outside of

itself:

eval(“function sayHi() { alert(‘hi’); }”);

sayHi();

Here, the

sayHi()

function is defined inside of an

eval()

call. Because that call is replaced with the

actual function, it is possible to call

sayHi()

on the following line.

The

Global

object doesn’t just have methods, it also has properties. Remember those special values

undefined

NaN

, and

Infinity

? They are all properties of the

Global

object. Additionally, all native

object constructors are also properties of the

Global

object. The following table describes all the proper-

ties in more detail.

Property

Description

undefined

The literal for the Undefined type.

NaN

The special Number value for

Not a Number

Infinity

The special Number value for an infinite value.

Object

Constructor for

Object

Array

Constructor for

Array

Function

Constructor for

Function

Boolean

Constructor for

Boolean

String

Constructor for

String

Number

Constructor for

Number

Date

Constructor for

Date

RegExp

Constructor for

RegExp

Table continued on following page

This capability is very powerful, but also very dangerous. Use extreme caution with

eval()

, especially when passing user-entered data into it. A mischievous user could

insert values that could compromise your site or application security (this is called

code injection).

Object Basics

06_579088 ch03.qxd 3/28/05 11:36 AM Page 83