Node.js Event Loop


Blocking Operation Example

let loop = (i, max) => {
  while (i < max) i++
  return i

// This operation will block Node.js
// Because, it's CPU-bound
// You should be careful about this kind of code
loop(0, 1e+12)

Non-Blocking IO Operation Example

let i = 0

const step = max => {
  while (i < max) i++
  console.log('i = %d', i)

const tick = max => process.nextTick(step, max)

// this will postpone tick run step's while-loop to event loop cycles
// any other IO-bound operation (like filesystem reading) can take place
// in parallel
console.log('this will output before all of tick operations. i = %d', i)
console.log('because tick operations will be postponed')

event loop diagram

In simpler terms, Event Loop is a single-threaded queue mechanism which executes your CPU-bound code until end of its execution and IO-bound code in a non-blocking fashion.

However, Node.js under the carpet uses multi-threading for some of its operations through libuv Library.

Performance Considerations

  • Non-blocking operations will not block the queue and will not effect the performance of the loop.
  • However, CPU-bound operations will block the queue, so you should be careful not to do CPU-bound operations in your Node.js code.

Node.js non-blocks IO because it offloads the work to the operating system kernel, and when the IO operation supplies data (as an event), it will notify your code with your supplied callbacks.