Event loops

⬅️ Execute Program JavaScript Concurrency

Event loops

Custom event loop:

/* Each event in the events array is an object {fn, runAt}.
 *   fn: the function to call.
 *   runAt: the timestamp when we should call the function. */
const events = [];
 
function runLoop() {
  while (events.length > 0) {
    const now = new Date().getTime();
    
    // Find the next event and run it if it's ready.
    for (let i=0; i<events.length; i++) {
      const event = events[i];
      if (now >= event.runAt) {
        events.splice(i, 1); // Remove the event at index i.
        event.fn();
        /* We changed the array that we're looping over, so abort the
         * loop. The outer loop will restart it anyway. */
        break;
      }
    }
  }
}
 
function queueEvent(fn, ms) {
  const now = new Date().getTime();
  const runAt = now + ms;
  events.push({fn, runAt});
}

• “Event loop” refers to the code that actually processes events, calling their callbacks. An “event queue” is the list of events that are waiting to run, like the events array in our examples here. You may see event loops called “message loops” or “message pumps”, but these mean roughly the same thing.