Insert an Item at a Specific Index in Array with JavaScript

// The original array
var array = ["one", "two", "four"];
// splice(position, numberOfItemsToRemove, item)
array.splice(2, 0, "three");

array;  // ["one", "two", "three", "four"]

Array.prototype.insert = function (index, item) {
  this.splice(index, 0, item);
};

References
https://davidwalsh.name/array-insert-index