AJAX - The Complete Reference

556 Part IV: Appendixes

Data Types

JavaScript’s data types are broken down into primitive and composite types. Primitive types

hold simple values and are passed to functions by value. Composite types hold heterogeneous

data (primitive and/or composite values) and are passed to functions by reference. JavaScript

is weakly typed and will commonly convert values between types to make expressions work

which may lead to errors.

Primitive Types

Five primitive types are defined, only three of which can hold useful data. These data types

are summarized in Table A-5.

Type Description Examples

Boolean Takes on one of two values: true or false. Used both as

variable values and within loops and conditions as a literal.

true

false

null Has only one value. Indicates the absence of data, for example,

it can be placed in unspecified function argument.

null

number Includes both integer and floating point types. 64-bit IEEE 754

representation. Integer ops are usually carried out using only

32 bits. Magnitudes as large as ±1.7976 × 10308 and as small

as ±2.2250 × 10 −308. Integers are considered to have a range

of 2^31 –1 to –2^31 for computational purposes. Hexadecimal

and octal forms are supported but are stored in their decimal

equivalent. You may find a special value NaN (not a number)

in the case of numeric calculation problems such as type

conversion or (0/0). You also may reach a positive or negative

infinity value. All special number cases are toxic and will

override all other values in an expression. The Number and

Math objects contain these and are other useful constants.

5

1968.38

-4.567

string Zero or more Unicode (Latin-1 prior to Netscape 6/IE4)

characters delimited by either single or double quotes. There

is no meaning difference for the type of quotes, and they are

interchangeable. The quote flexibility is useful for including

script code within markup. JavaScript supports standard C-

like escaping with a \. Commonly, you may escape quotes

with \’ and \”. Also escaping the \ is commonly performed

using \\. The whole range of escaping including common text

characters like newlines (\n) or setting particular character

codes in Latin-1 (\044), or Unicode (\u00A9) is supported.

However, given that your output environment may be an XHTML

markup document, some whitespace indications like tabs and

newlines may appear not to work.

"I am string"

'So am I'

"Say \"what\"? "

'C'

"7"

""

''

"Newline \n

Time"

"\044\044\044"

"It's unicode time

\u00A9 2007 "

undefined Has only one value and indicates that data has not yet been

assigned. For example, undefined is the result of reading a

nonexistent object property.

undefined

TABLE A-5 Primitive JavaScript Data Types