Increment a value up and down.

bounce.js

var up = true;
var value = 0;
var increment = 1;
var ceiling = 100;
var floor = -100;

if (up == true && value <= ceiling) {
  value += increment
  if (value == ceiling)
  {up = false;}
} else {
  up = false
  value -= increment;
  if (value == floor)
  {up = true;}
}