deps/v8/src/js/array.js
function SparseMove(array, start_i, del_count, len, num_additional_args) {
  // Bail out if no moving is necessary.
  if (num_additional_args === del_count) return;
  // Move data to new array.
  var new_array = new InternalArray(
      // Clamp array length to 2^32-1 to avoid early RangeError.
      MinSimple(len - del_count + num_additional_args, 0xffffffff));
  var big_indices;
  var indices = %GetArrayKeys(array, len);
  if (IS_NUMBER(indices)) {
    var limit = indices;
    for (var i = 0; i < start_i && i < limit; ++i) {
      var current = array[i];
      if (!IS_UNDEFINED(current) || i in array) {
        new_array[i] = current;
      }
    }
    for (var i = start_i + del_count; i < limit; ++i) {
      var current = array[i];
      if (!IS_UNDEFINED(current) || i in array) {
        new_array[i - del_count + num_additional_args] = current;
      }
    }
  } else {
    var length = indices.length;
    for (var k = 0; k < length; ++k) {
      var key = indices[k];
      if (key < start_i) {
        var current = array[key];
        if (!IS_UNDEFINED(current) || key in array) {
          new_array[key] = current;
        }
      } else if (key >= start_i + del_count) {
        var current = array[key];
        if (!IS_UNDEFINED(current) || key in array) {
          var new_key = key - del_count + num_additional_args;
          new_array[new_key] = current;
          if (new_key > 0xfffffffe) {
            big_indices = big_indices || new InternalArray();
            big_indices.push(new_key);
          }
        }
      }
    }
  }
  // Move contents of new_array into this array
  %MoveArrayContents(new_array, array);
  // Add any moved values that aren't elements anymore.
  if (!IS_UNDEFINED(big_indices)) {
    var length = big_indices.length;
    for (var i = 0; i < length; ++i) {
      var key = big_indices[i];
      array[key] = new_array[key];
    }
  }
}
function SparseMove(array, start_i, del_count, len, num_additional_args) {
  // Bail out if no moving is necessary.
  if (num_additional_args === del_count) return;
  // Move data to new array.
  var new_array = new InternalArray(
      // Clamp array length to 2^32-1 to avoid early RangeError.
      MinSimple(len - del_count + num_additional_args, 0xffffffff));
  var big_indices;
  var indices = %GetArrayKeys(array, len);
  if (IS_NUMBER(indices)) {
    var limit = indices;
    for (var i = 0; i < start_i && i < limit; ++i) {
      var current = array[i];
      if (!IS_UNDEFINED(current) || i in array) {
        new_array[i] = current;
      }
    }
    for (var i = start_i + del_count; i < limit; ++i) {
      var current = array[i];
      if (!IS_UNDEFINED(current) || i in array) {
        new_array[i - del_count + num_additional_args] = current;
      }
    }
  } else {
    var length = indices.length;
    for (var k = 0; k < length; ++k) {
      var key = indices[k];
      if (key < start_i) {
        var current = array[key];
        if (!IS_UNDEFINED(current) || key in array) {
          new_array[key] = current;
        }
      } else if (key >= start_i + del_count) {
        var current = array[key];
        if (!IS_UNDEFINED(current) || key in array) {
          var new_key = key - del_count + num_additional_args;
          new_array[new_key] = current;
          if (new_key > 0xfffffffe) {
            big_indices = big_indices || new InternalArray();
            big_indices.push(new_key);
          }
        }
      }
    }
  }
  // Move contents of new_array into this array
  %MoveArrayContents(new_array, array);
  // Add any moved values that aren't elements anymore.
  if (!IS_UNDEFINED(big_indices)) {
    var length = big_indices.length;
    for (var i = 0; i < length; ++i) {
      var key = big_indices[i];
      array[key] = new_array[key];
    }
  }
}
This function implements the optimized splice implementation that can use special array operations to handle sparse arrays in a sensible fashion.