深入了解JavaScript Promise

Promise.resolve('foo').then(function(s) {
  console.log(s);
}).then(function(s) {
  // Never executed
  console.log(s);
});

Promise.resolve('foo').then(function(s) {
  console.log(s);
  return s + 'bar';
}).then(function(s) {
  console.log(s);
});

// foo
// foobar

Promise.resolve('foo').then(function(s) {
  return new Promise((resolve, reject) => {
      console.log(s);
      setTimeout(() => {
          resolve(s + 'bar')
        }, 1000);
    });
}).then(function(s) {
  console.log(s);
});


// foo
// foobar (在 "foo" 显示了 1s 后显示)

// demo
const promise1 = Promise.resolve(3);
const promise2 = 42;
const promise3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 100, 'foo');
});
 
Promise.all([promise1, promise2, promise3]).then((values) => {
  console.log(values);
});
// expected output: Array [3, 42, "foo"]

//sync await   操作时间2秒
async function Index2() {
      console.time()
      const p1 =await new Promise((resolve, reject) => {
        console.log('这里是p1')
        setTimeout(() => {
          resolve('这里是p1的返回')
        }, 1000)
      })
      const p2 =await new Promise((resolve, reject) => {
        console.log('这里是p2')
        setTimeout(() => {
          resolve('这里是p2的返回')
        }, 1000)
      })
      console.log(p1)
      console.log(p2) 
      console.timeEnd()
   }
    Index2();

//  使用Promise.all()来实现调用。操作时间1秒
 function Index() {
      console.time()
      const p1 = new Promise((resolve, reject) => {
        console.log('这里是p1')
        setTimeout(() => {
          resolve('这里是p1的返回')
        }, 1000)
      })
      const p2 = new Promise((resolve, reject) => {
        console.log('这里是p2')
        setTimeout(() => {
          resolve('这里是p2的返回')
        }, 1000)
      })
      Promise.all([p1, p2]).then((val) => {
        console.log(val)
        console.timeEnd()
      })
}