Beginning JavaScript with DOM Scripting and Ajax, 2nd Edition

Chapter 2 ■ Data and Decisions
Let’s see how these functions work in practice:
var userEnteredNumber = prompt( "Please enter a number", "" );
document.write( typeof( userEnteredNumber ) );
document.write( "" );
document.write( parseFloat( userEnteredNumber ) );
document.write( "" );
document.write( parseInt( userEnteredNumber ) );
userEnteredNumber = Number( userEnteredNumber )
document.write( "" );
document.write( userEnteredNumber );
document.write( "" );
document.write( typeof( userEnteredNumber ) );
Try entering the value 23.50. You should get this output:
string
23.5
23
23.5
number
The data entered is read as a string in the first line. Then parseFloat() converts 23.50 from a string to a floating
point number, and in the next line, parseInt() strips out the fractional part (without rounding up or down). The
variable is then converted to a number using the Number() function and stored in the userEnteredNumber variable itself
(overwriting the string held there). On the final line, you see that userEnteredNumber’s data type is indeed number.
Try entering 23.50abc at the user prompt:
string
23.5
23
NaN
number
The results are similar, but this time Number() has returned NaN. The parseFloat() and parseInt() functions still
return a number because they work from left to right, converting as much of the string to a number as they can, and
then stop when they hit a nonnumeric value. The Number() function rejects any string that contains nonnumerical
characters. (Digits, a valid decimal place, and + and – signs are allowed, but nothing else.)
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