JavaScript Core Objects - JavaScript by Example - page 244

Figure 9.24 The Math object, some properties and methods.

9.5.1 Rounding Up and Rounding Down

There are three Math methods available for rounding numbers up or down. They are the
ceil() , floor() , and round() methods (see Table 9.7 for examples). The differences among the
methods might be confusing because all three methods truncate the numbers after the dec-
imal point and return a whole number. If you recall, JavaScript also provides the parseInt()
function, but this function truncates the number after the decimal point, without rounding
either up or down.

The ceil() Method. The ceil() method rounds a number up to the next largest whole
number and then removes any numbers after the decimal point; thus, 5.02 becomes 6
because 6 is the next largest number, and -5.02 becomes -5 because -5 is larger than -6.

The floor() Method. The floor() method rounds a number down to the next lowest
whole number and then removes any numbers after the decimal point; thus, 5.02 now
becomes 5, and -5.02 becomes -6.

The round() Method. The round() method rounds up only if the decimal part of the
number is .5 or greater. Otherwise, it rounds down to the nearest integer; thus, 5.5 is
rounded up to 6, and 5.4 is rounded down to 5.

Table 9.7 Rounding up and down

Number ceil() floor() round()

2.55 3 2 3

2.30 3 2 2

-2.5 -2 -3 -2

-2.3 -2 -3 -2