Execution of asynchronous code like Promises under the hood

Question

How does Node.js (and JavaScript in general) implement the execution of asynchronous code like Promises under the hood? Does it run a new thread? Does it use Worker for that or there is some OS or libuv functionality?

var promise1 = new Promise(function(resolve, reject) { 
    let now = new Date(); 
    while((now - 6000) < start){
        now = new Date();
    }
    resolve(now - start)
});

var promise2 = new Promise(function(resolve, reject) { 
    let now = new Date(); 
    while((now - 6000) < start){
        now = new Date();
    }
    resolve(now - start)
});
let start = new Date(); 
promise1.then(
    function(value) {
      console.log("Promise 1: ", value);
    }
)

promise2.then(
    function(value) {
      let finish = new Date();
      console.log("Promise 2: ", value);
      console.log(finish - start);
    }
)

Answer 1

Promises are not asynchronous in Node.js. But there is a new feature - Worker that is asynchronous. It runs in a separate proccess. So you can use that, here is a bit more info: https://nodejs.org/api/worker_threads.html