how to round to the smallest whole number in javascript code example

Example 1: round down the number javascript

+3.5 => +3.0
-3.5 => -4.0

+3.5 => +3.0 using Math.floor()
-3.5 => -3.0 using Math.ceil()

Example 2: calculate surface of a circle round to the nearest integer javascript

Math.round(Math.PI * radius * radius)