javascript-tutorials/async-await/event-loop.md

The Event Loop

JavaScript is single-threaded but it can still take advantage of asynchronous programming.

In order to do that, the runtime keeps a structure called The Event Loop which is responsible for holding operations to be executed asynchronously with respect to the main flow of execution. Whenever we meet a portion of code that may not be possible to execute now, it is put in the Event Loop. Let's look at some examples:

Example 1

setTimeout(() => console.log('test'), 1000);

console.log('test') will be put in the Event Loop.

Example 2

const promise = new Promise(function (resolve, reject) {
  console.log('test');
  resolve();
}).then(() => {
  console.log('test2');
});

console.log('test2') will be put in the Event Loop.