generating a random number without repeat the previous one

I am trying to generate a series of random numbers without repeating the same number twice, I came up with a simple function in javascript but it doesn't work, and I fail to understand why, can anyone help me please

function random(){
  var randNum;
  var prevnumb;
  do{
    randNum = Math.floor(Math.random() * quotes.length);
  } while(prevnumb == randNum);
  prevnumb = randNum;
  return randNum;
}