How do i generate a non-repeating random number?

How do i ensure that i don't get a repeat of a random number? Right now this isnt working. I'm using a local array to store previous results.

getUniqueRandomNumber(x){
     var index;
     var viewedIndices = [];
     index = Math.floor(Math.random() * (x));
     if(viewedIndices.includes(index)) 
     {
       viewedIndices.push(index);
       this.getUniqueRandomNumber(x);
     }
     else { 
       console.log(index);
       return index;
    }
 }