What is the fastest way to replace a section of an array with another array in Javascript?

Question

I'm working with Dygraph and have implemented a reloader which adds higher resolution data as the user zooms into a graph.

In order to keep the original file boundaries, I'm keeping the original data and insert the newly loaded data at the respective zoom interval like so:

function spliceAndInject(my_graph_gadget, my_hi_res_data_dict) {
  ...

  for (i = 0, len = current_data_set.length; i < len; i += 1) {
    point = current_data_set[i];
    // NOTE: a point = [time_in_milliseconds, value_for_graph]
    timer = point[0];

    if (timer < lower_bound || timer > higher_bound) {
      new_data_set.push(point);
    } else if (is_injected === undefined) {
      is_injected = true;
      new_data_set = new_data_set.concat(hi_res_data_set);
    }
  }
  ...
}

I'm wondering whether this could be done any faster, because the graph rendering gets notably slower the more data I'm carrying around.

Question :
What is the fastest way to replace a section of an array with another array?

Thanks!

Answer 1

If you know the indices beforehand and want to modify the array with the data, using the native splice method is probably the best - although for optimized performance you'd need to test against a hand-written solution that manually does the moving.

If you want to create a new array, I'd use this:

var len = current_data_set.length,
    new_data_set = new Array(len);
for (var i=0; i<len; i++)
  var point = current_data_set[i];
  if (point[0] < lower_bound)
    new_data_set[i] = point;
  else
    break;
for (var j=i, k=0, l=hi_res_data_set.length; k<l; k++)
  new_data_set[j++] = hi_res_data_set[k];
for (; i<len; i++)
  if (current_data_set[i][0] > higher_bound)
    break;
for (; i<len; i++)
  new_data_set[j++] = current_data_set[i];

I think this is faster than yours because

• it doesn't use concat which creates a second new array
• it doesn't test all points against lower_bound and higher_bound
• it doesn't use that odd is_injected variable (which should be statically typed as a boolean if at all), whose only purpose is to jump over a loop section