HTML and CSS Reference
In-Depth Information
.
Output
FIGURE 14.3
Values passed to
functions are
copies.
Functions are called using the function name, followed by parentheses. If you are passing
arguments to a function, they are included in the parentheses in a comma-separated list.
Even if you're not using arguments, the parentheses are still required. This is true
whether you're calling a function you wrote yourself or a function that's built in to
JavaScript.
Data Types
I've mentioned JavaScript's type system, but I haven't talked much about JavaScript data
types. JavaScript supports the following types of values:
Strings, like "Teach Yourself Web Publishing".
n
Boolean values (true or false).
n
Numbers, integer or decimal.
n
null
, which is used to represent an unknown or missing value.
n
undefined
, the value associated with variables that have been declared but have
not yet had a value assigned to them.
n
This is the full set of primitive data types that JavaScript supports. JavaScript attempts to
convert data to whatever type it needs in a given context. So if you take a Boolean value
and use it in a context where a string is expected, JavaScript will convert it to a string. In
some cases, this automatic conversion process can lead to odd results. For example, if
you try to use a value that's not a number in a context where a number is expected,
JavaScript will return a special value,
NaN
, which is short for “not a number”:
var squareRoot = Math.sqrt(“a string”); // The value of squareRoot is NaN
Boolean values represent a state of either true or false. You've already seen some exam-
ples that involve boolean values. For example,
if
statements and
while
loops require
conditional expressions that return a Boolean value. Any JavaScript value or expression
can ultimately be converted to a Boolean. The values that are treated as false are the
number zero, empty strings,
null
,
undefined
, and
NaN
. Everything else is true.
