Regarding JavaScript for() loop voodoo

Question

I was for quite some time under the impression that a for loop could exist solely in the following format:

for (INITIALIZER; STOP CONDITION         


        

Answer 1

shuffle = function(o){
     for (
          var j,                // declare j
              x,                // declare x
              i = o.length;     // declare i and set to o.length
          i;                    // loop while i evaluates true
          j = parseInt(Math.random() * i), // j=random number up to i
            x = o[--i],         // decrement i, and look up this index of o
            o[i] =  o[j],       // copy the jth value into the ith position
            o[j] = x            // complete the swap by putting the old o[i] into jth position
          );
     return o;
     };

This is starting with i equal to the number of positions, and each time swapping the cards i and j, where j is some random number up to i each time, as per the algorithm.

It could be more simply written without the confusing comma-set, true.

By the way, this is not the only kind of for loop in javascript. There is also:

 for(var key in arr) {
      value = arr[key]);
 }

But be careful because this will also loop through the properties of an object, including if you pass in an Array object.