Generate one random index from an array but exclude one. javascript

Question

I need to generate one random number from an array but exclude one index and return the index of the random number.
I tried this:

function randNum(arr,excludeNum){
    var randNumber = Math.floor(Math.random()*arr.length);
    if(arr[randNumber]==excludeNum){
        randNum(arr,excludeNum);
    }else{
        return randNumber;
    }
}
alert(randNum([7, 10, 11],7));

But somtimes it returns the numbers I want it to return and sometimes it returns undefined.
What is the problem?

Answer 1

It is because your function doesn't return anything when the generated random number is the one you want to exclude. Add a return to the randNum function call in the if clause.

function randNum(arr,excludeNum){
    var randNumber = Math.floor(Math.random()*arr.length);
    if(arr[randNumber]==excludeNum){
        return randNum(arr,excludeNum);
    }else{
        return randNumber;
    }
}
alert(randNum([7, 10, 11],7));

Answer 2

You need a return keyword in front of randNum(arr,excludeNum); .

Answer 3

Because you are using floor and eventually the stack will fill up. (also I think that you missed return)

Why not just remove that number from the array and chose a random index into the rest of it?