严格来说，由于JavaScript是单线程，所以没有并发。有类似并发操作的，那就是协程。ES6中有async和await，用它们来实现异步操作，简直很爽。但是，并不是用了async就可以并发，或者接近并发了。一般有两种方式可以让他们“并发”。

有趣的是，Python 3.5以后也有了async和await这两个关键字的协程操作了。真的是天下语言一大抄！喂，Ruby，你睡着了么？

//方法1
let results = await Promise.all([testPromise(), testPromise()])

//方法2
let p1 = testPromise();
let p2 = testPromise();
let r1 = await p1;
let r2 = await p2;

下面是一个例子，源于MDN，自己加上了Promise.all。前一个测试同步操作，即9秒后打印结果。后两个会并发操作，即3秒打印结果。

function timeoutPromise(interval) {
    return new Promise((resolve, reject) => {
      setTimeout(function(){
        resolve("done");
      }, interval);
    });
  };

async function slowTimeTest() {
    await timeoutPromise(3000);
    await timeoutPromise(3000);
    await timeoutPromise(3000);
}


let startTime = Date.now();
slowTimeTest().then(() => {
  let finishTime = Date.now();
  let timeTaken = finishTime - startTime;
  console.log("slowTimeTest finished in milliseconds: " + timeTaken);
})

async function fastTimeTest1() {
    const timeoutPromise1 = timeoutPromise(3000);
    const timeoutPromise2 = timeoutPromise(3000);
    const timeoutPromise3 = timeoutPromise(3000);
  
    await timeoutPromise1;
    await timeoutPromise2;
    await timeoutPromise3;
}

startTime = Date.now();
fastTimeTest1().then(() => {
  let finishTime = Date.now();
  let timeTaken = finishTime - startTime;
  console.log("fastTimeTest1 finished in milliseconds: " + timeTaken);
})


async function fastTimeTest2() {
    let res = await Promise.all([timeoutPromise(3000), timeoutPromise(3000), timeoutPromise(3000)]);
    console.log('Data', res);
}

startTime = Date.now();
fastTimeTest2().then(() => {
  let finishTime = Date.now();
  let timeTaken = finishTime - startTime;
  console.log("fastTimeTest2 finished in milliseconds: " + timeTaken);
})