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String type

A few special values are also defined as part of the Number type. The first two are

Number.MAX_VALUE

and

Number.MIN_VALUE

, which define the outer bounds of the Number value set. All ECMAScript

numbers must fall between these two values, without exception. A calculation can, however, result in

a number that does not fall in between these two numbers.

When a calculation results in a number greater than

Number.MAX_VALUE

, it is assigned a value of

Number.POSITIVE_INFINITY

, meaning that it has no numeric value anymore. Likewise a calculation

that results in a number less than

Number.MIN_VALUE

is assigned a value of

Number.NEGATIVE_

INFINITY

, which also has no numeric value. If a calculation returns an infinite value, the result cannot

be used in any further calculations.

There is actually a special value for infinity named (you guessed it)

Infinity

Number.POSITIVE_

INFINITY

has a value of

Infinity

, whereas

Number.NEGATIVE_INFINITY

has a value of

–Infinity

Because an infinite number can be positive or negative, a method can be used to determine if a number

is finite (instead of testing for each infinite number separately). The

isFinite()

method can be called

on any number to ensure that the number isn’t infinite. For example:

var iResult = iNum* some_really_large_number;

if (isFinite(iResult)) {

alert(“Number is finite.”);

} else {

alert(“Number is infinite.”);

}

The final special number value is

NaN

, which stands for

Not a Number

NaN

is an odd special value. In

general, this occurs when conversion from another type (String, Boolean, and so on) fails. For example,

trying to convert the word

blue

into a number value will fail because there is no numeric equivalent. Just

like the infinity values,

NaN

cannot be used in mathematical calculations. Another oddity of

NaN

is that it

is not equal to itself, meaning that the following will return

false

alert(NaN == NaN); //outputs “false”

For this reason, it is not recommended to use the

NaN

value itself. Instead, the function

isNaN()

will do

the job quite nicely:

alert(isNaN(“blue”)); //outputs “true”

alert(isNaN(“123”)); //outputs “false”

The String type

The String type is unique in that it is the only primitive type that doesn’t have a definite size. A string

can be used to store zero or more Unicode characters, represented by 16-bit integers (Unicode is an inter-

national character set that is discussed later in this book).

Each character in a string is given a position, starting with the first character in position 0, the second

character in position 1, and so on. This means that the position of the final character in a string is always

the length of the string minus 1 (see Figure 2-2).

Chapter 2

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