var arr = [],
      i,
      obj = {};
  for(i = 0; i < 100; i += 1) {
      arr.push(i);
      obj[i] = i;
  }
  
  function getRandomIndex(obj) {
     var idx = Math.floor(Math.random()*99);
     if(false === obj.hasOwnProperty(idx)) {
        while (false === obj.hasOwnProperty(idx)) {
            idx = Math.floor(Math.random()*99);
        }
     }
     return idx;
  }

Teardown JS

Test cases

Test #1

Title *

Async

Code *

var idx = getRandomIndex(arr),
    a = arr[idx];

arr[idx] = Math.floor(Math.random()*99);
arr.splice(idx,0);
arr[length] = a;

Test #2

Title *

Async

Code *

var idx = getRandomIndex(obj),
    a = obj[idx];

obj[idx] = Math.floor(Math.random()*99);
delete obj[idx];
obj[idx] = a;

Test case details

Preparation Code

Test cases

Test #1

Test #2