nodejs event loop

nodejs event loop

• javascript: synchronous - blocking - single thread
• libuv library helps handle asynchronous operations
• nodejs fakes multithreading by multitasking, not speads tasks across threads/processes but time
• synchronous callbacks (call stack) > asynchronous callbacks (microtask queue) > setTimeout (pushed into task queue)
• reactor pattern
• event loop
• libuv library uses native OS/ its thread pool to process asynchronous operations (keep main thread non blocking), then put callbacks to event queue
• call stack must be empty before tasks in event queue are processed (loaded into call stack)
• order of tasks in event queue (includes 6 queues)
  • microtask (nextTick, promise)
  • timer queue (setTimeout, setInterval)
  • microtask
  • I/O queue (fs/http/…)
  • microtask
  • check queue (setImmediate)
  • microtask
  • close queue (close event)
  • microtask

effective promise

• use Promise.all
  • add .catch() to promises

  const [user, product] = await Promise.all([
  	fetchUser().catch(onReject), // ⬅️
  	fetchProduct().catch(onReject) // ⬅️
  ]);

- use `Promise.allSettled`
	- returns `{status, value, reason}`, status is `fulfilled/rejected`