Question

also is it possible that the generated number be between 30 and 90. or it is just generated between 0 and n?

Answer 1

// Returns a random integer between min and max
// Using Math.round() will give you a non-uniform distribution!
function getRandomInt(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

From Mozilla Developer Network