Beginning JavaScript with DOM Scripting and Ajax, 2nd Edition

Chapter 2 ■ Data and Decisions
You call the methods of the Math object using the following format:
Math.methodOfMathObject( aNumber ):
alert( "The value of pi is " + Math.PI );
We’ll look at a few of the commonly used methods next. (You can find a complete reference by running a
search at https://developer.mozilla.org/en-US/docs/Web_Development.) We’ll look at the methods for rounding
numbers and generating random numbers here.
Rounding Numbers
You saw earlier that the parseInt() function will make a fractional number whole by removing everything after the
decimal point (so 24.999 becomes 24). Pretty often, you’ll want more mathematically accurate calculations—if you’re
working with financial calculations, for example—and for these, you can use one of the Math object’s three rounding
functions: round(), ceil(), and floor(). This is how they work:
• round(): Rounds a number up when the decimal is . 5 or greater
• ceil()(as in ceiling): Always rounds up, so 23.75 becomes 24, as does 23.25
• floor(): Always rounds down, so 23.75 becomes 23, as does 23.25
Here they are at work in a simple example:
var numberToRound = prompt( "Please enter a number", "" )
document.write( "round( ) = " + Math.round( numberToRound ) );
document.write( "" );
document.write( "floor( ) = " + Math.floor( numberToRound ) );
document.write( "" );
document.write( "ceil( ) = " + Math.ceil( numberToRound ) );
Even though you used prompt() to obtain a value from the user, which as you saw earlier returns a string, the
number returned is still treated as a number. This is because the rounding methods do the conversion for you as long
as the string contains something that can be converted to a number.
If you enter 23.75, you get the following result:
round() = 24
floor() = 23
ceil() = 24
If you enter –23.75, you get
round() = -24
floor() = -24
ceil() = -23
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