Non Blocking

So how does JavaScript manage to be so efficient with asynchronous code? The answer is the event loop.

The event loop is a single-threaded, non-blocking, event-driven, asynchronous execution model.

We already covered the single-threaded part, now let's grok non-blocking. Let's use this Python code as an example:

import time

print("Start")
time.sleep(2)
print("Middle")
time.sleep(2)
print("End")

This code prints "Start", then waits for 2 seconds, prints "Middle", waits another 2 seconds, and finally prints "End". The time.sleep(2) function calls are blocking: they stop the program's execution until 2 seconds have passed.

Let's write a similar example in JavaScript:

console.log("Start");
setTimeout(() => {
  console.log("End");
}, 4000);
console.log("Middle");

This code prints "Start", then "Middle" immediately, waits 4 seconds, then prints "End". The main thread in JavaScript cannot be blocked. That's why setTimeout takes a callback function as an argument, it basically says:

Hey, I know I can't block the program, but please Mr. JavaScript engine, can you take this function and run it for me in 4 seconds?

So, the main thread should always be available to do work, and blocking (read: waiting) is delegated "for later".