gecko/dom/downloads/tests/serve_file.sjs

// Serves a file with a given mime type and size at an optionally given rate.

function getQuery(request) {
  var query = {};
  request.queryString.split('&').forEach(function (val) {
    var [name, value] = val.split('=');
    query[name] = unescape(value);
  });
  return query;
}

// Timer used to handle the request response.
var timer = null;

function handleResponse() {
  // Is this a rate limited response?
  if (this.state.rate > 0) {
    // Calculate how many bytes we have left to send.
    var bytesToWrite = this.state.totalBytes - this.state.sentBytes;

    // Do we have any bytes left to send? If not we'll just fall thru and
    // cancel our repeating timer and finalize the response.
    if (bytesToWrite > 0) {
      // Figure out how many bytes to send, based on the rate limit.
      bytesToWrite =
        (bytesToWrite > this.state.rate) ? this.state.rate : bytesToWrite;

      for (let i = 0; i < bytesToWrite; i++) {
        this.response.write("0");
      }

      // Update the number of bytes we've sent to the client.
      this.state.sentBytes += bytesToWrite;

      // Wait until the next call to do anything else.
      return;
    }
  }
  else {
    // Not rate limited, write it all out.
    for (let i = 0; i < this.state.totalBytes; i++) {
      this.response.write("0");
    }
  }

  // Finalize the response.
  this.response.finish();

  // All done sending, go ahead and cancel our repeating timer.
  timer.cancel();
}

function handleRequest(request, response) {
  var query = getQuery(request);

  // Default values for content type, size and rate.
  var contentType = "text/plain";
  var size = 1024;
  var rate = 0;

  // optional content type to be used by our response.
  if ("contentType" in query) {
    contentType = query["contentType"];
  }

  // optional size (in bytes) for generated file.
  if ("size" in query) {
    size = parseInt(query["size"]);
  }

  // optional rate (in bytes/s) at which to send the file.
  if ("rate" in query) {
    rate = parseInt(query["rate"]);
  }

  // The context for the responseHandler.
  var context = {
    response: response,
    state: {
      contentType: contentType,
      totalBytes: size,
      sentBytes: 0,
      rate: rate
    }
  };

  // The notify implementation for the timer.
  context.notify = handleResponse.bind(context);

  timer =
    Components.classes["@mozilla.org/timer;1"]
              .createInstance(Components.interfaces.nsITimer);

  // sending at a specific rate requires our response to be asynchronous so
  // we handle all requests asynchronously. See handleResponse().
  response.processAsync();

  // generate the content.
  response.setHeader("Content-Type", contentType, false);
  response.setHeader("Content-Length", size.toString(), false);

  // initialize the timer and start writing out the response.
  timer.initWithCallback(context,
                         1000,
                         Components.interfaces.nsITimer.TYPE_REPEATING_SLACK);

}