Double ternary in JavaScript

Question

I was going through some stuff in the jQuery source, specifically the inArray method and I found this line of code:

i = i ? i < 0 ? Math.max( 0, len + i ) : i : 0;

What I am seeing is two ternary operators, but I have no idea how this is used. I understand how the ternary operator works, but I have never seen it used like this before. How does this piece of code work??

Answer 1

Just break it down like you would 1 + 2 + 3:

if (i) {
    if (i < 0) {
        i = Math.max(0, len + i);
    } else {
       i = i; // no-op
    }
} else {
    i = 0; // also no-op, since if `i` were anything else it would be truthy.
}

In fact, that whole line seems inefficient to me. Personally I'd just use:

if (i < 0) { 
    i = Math.max(0, len + i);
}

Answer 2

i = i ? i < 0 ? Math.max( 0, len + i ) : i : 0;

Breaks down to:

var i;

if(i){
  if(i<0){
   i = Math.max(0, len + i);
  }else{
    i = i;
  }
}else{
  i = 0;
}