Losing the logic! Random unique numbers in javaScript

I needed to write a function that'd generate 6 random numbers between 0 and 60 and then store them in a given array. In addition, this function could not store duplicates in the returned array. Well, I found a very helpful answer here on StackOverflow(Thanks Shouvik) which I'm gonna attach bellow. But here's the thing guys I had most of this logic but missed two core elements

1 - I was trying to do this without a second temporary Array 2 - I had no clue how'd I find duplicates inside the arrey

I knew I had to .indexOF the array I knew I had to .push a new value to the array

Here is the code I had prior to finding the solution:

    function gerarDezenas(){
  let jogo = [];
  for (var i = 0;i < 6;i++){
    jogo.push(Math.round(Math.random()*(1 , 60)+1));
    if (jogo.indexOf(jogo) == //clueless){
   .push() // ?? clueless either
    }
  }
return jogo
}
console.log(gerarDezenas())

So I found that I needed another Array and the if condition to compare it to

    function gerarDezenas(){
  let jogo = [];
  for (var i = 0;i < 6;i++){
    var temp = Math.round(Math.random()*(1 , 60)+1);
    if (jogo.indexOf(temp) == -1){
    jogo.push(temp)
    }
    else {
      i--;
    }
  }
return jogo
}

The code is now working as it was intended to but I don't really understand it and that the issue here! I don't know what these lines are doing:

if (jogo.indexOf(temp) == -1){
    jogo.push(temp)
    }
    else {
      i--;
    }
  }

Can somebody clarify to me what the if and the else are doing? If you read through the post thank you very much!

attached image please don't hate me