JavaScript has in built Math.random() method which generates the numbers randomly. Post that, using Math.floor() method you can round down the nearest integer. You can also generate random numbers within specific range.
Math.floor(Math.random() * 15);
// Returns number 0 -> 15
Math.floor(Math.random() * 15) + 1;
// Returns number 1 -> 15
Math.floor(Math.random() * 9) - 10;
// Returns number -10 -> -2