在幕后执行异步代码，如 Promises

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