var arr = [1, 2, 4, 3];

arr = splice(arr, 2, 1);
console.log(arr); // [1, 2, 3]

arr = splice(arr, 2, 0, 100, 200, 300);
console.log(arr); // [1, 2, 100, 200, 300, 3]

function splice(arr, index, del) {
 if (del > 0) {
  var arr2 = [];
  
  for ( i = 0; i < index; i++) {
  	arr2[i] = arr[i]; 
  }
  for (i = (index + del); i < arr.length; i++) {
    arr2[(i-del)] = arr[i];
    return arr2;
  }
 } else if (arguments.length > 3) {
  for (var i = 3; i < arguments.length; i++) {
   arr[arr.length] = arguments[i];
  } 
  return arr;
 } 
 else { 
   return arr;
 };
}

//[1, 2, 100, 200, 300, 3]