JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

The typeof Operator
If you‘re curious about the type of data you have, use the typeof operator to examine it. Applied
to a variable or literal, it returns a string indicating the type of its argument. The list of values
returned by typeof is given in Table 3-4.
Table 3-4: Values Returned by the typeof Operator
Type Result
Undefined undefined
Null object
Boolean boolean
Number number
String string
Object object
Function function
Type Conversion
Automatic type conversion is one of the most powerful features of JavaScript, as well as the
most dangerous for the sloppy programmer. Type conversion is the act of converting data of
one type into a different type. It occurs automatically in JavaScript when you change the type of
data stored in a variable:
var x = "3.14";
x = 3.14;
The type of x changes from string to number. Besides the automatic conversion inherent in
JavaScript, it is also possible for programmers to force the conversion using methods like
toString() or parseInt().
While it seems straightforward enough, the problem with type conversion also is that it often
occurs in less obvious ways, such as when you operate on data with dissimilar types. Consider
this code:
var x = "10" - 2;
This example subtracts a number from a string, which should seem very odd at first glance. Yet
JavaScript knows that subtraction requires two numbers, so it converts the string "10" into the
number 10, performs the subtraction, and stores the number 8 in x.
The truth is that automatic type conversion happens all over the place in JavaScript, any time
data is not of the type that might be required for some task. For example, we previously stated
that the type of the condition (the part between the parentheses) of flow control statements like
if/else is Boolean. This means that given a statement like this,
var x = "false"; // a string
if (x)
{