Add one or more elements at the specified index and returns the new length of the array.

Array.insertAt.js

// Syntax
// ------
// array.insertAt(index, element1, ..., elementN)

// Parameters
// ------
// index
// :  Index at which to insert elements. If negative, will begin that many elements from the end.
// element1, ..., elementN
// :  The elements to add to the array.

Array.prototype.insertAt = function (index) {
  var my = this,
      args = my.slice.call(arguments, 1);

  if (!index) {
    return my.unshift.apply(my, args);
  }

  if (index >= my.length) {
    return my.push.apply(my, args);
  }

  my.splice.apply(my, [index, 0].concat(args));
  return my.length; 
};