What is the event loop in Node.js?

It is tricky to understand how Node.js is single-threaded yet can perform non-blocking I/OInput/Output operations. While the privilege of not having to deal with thread management and the complexities it brings with it is fantastic, it comes at the cost of hiding this mess behind a well-written API. That is where event loop comes in, the soul of Node.js.

The event loop

Event loop, as the name suggests, is a single-thread, loop that is almost infinite. We say almost infinite as it has some hard limits if it does find itself in an infinite loop. The reason this single-threaded loop can perform otherwise blocking operations in a non-blocking manner is due to offloading. Node.js offloads operations to the system kernel whenever possible. The loop in event loop refers to Node.js cycling through operations in order. We call these operations phases. Let’s take a look at the event loop.

Phases

Let’s breakdown each phase to see what is happening.

Timers

This phase executes callbacks scheduled by setTimeout() and setInterval(). It should be noted that a timer denotes the minimum time Node.js will wait before executing the callback. The actual delay might be slightly longer depending on the poll phase.

Pending callbacks

This phase executes I/O callbacks that were deferred to the next loop iteration. Some system operations want to wait before executing a callback, this phase is used for that.

Idle, prepare

This phase is used internally by Node.js. There isn’t a lot of documentation on this phase, but it is mentioned in the official documentation.

Poll

This phase performs two operations. First, it calculates how long it should block and poll for I/O. Then, it processes the events in the poll queue.

Check

This phase is used to invoke setImmediate() callbacks. If the poll phase is empty or callbacks using setImmediate() have been queued, the event loop might skip the poll phase and continue to the check phase.

Close callback

Methods such as socket.destroycauses a socket or a handle to close abruptly are handled in this phase.