Promise原理示例代码

console.log('start method synchronously calling test');

setTimeout(() => {
  console.log('setTimeout');
}, 0);

Promise.resolve().then(() => {
  console.log('promise');
});

console.log('end method synchronously calling test');

console.log("before asyncFunc");

async function asyncFunc(_pos)
{
  let myPromise = new Promise(function(myResolve) {
    console.log("before setTimeout promise in asyncFunc with _pos:"+_pos);
    
    setTimeout(function() {
      myResolve("4000毫秒时间已到 with _pos:"+_pos);
      console.log("4000毫秒已到");
    },4000);
      
    console.log("after setTimeout promise in asyncFunc with _pos:"+_pos);
  });
    
  return myPromise;
}

console.log("after asyncFunc");

console.log("before testAsyncFunc");

async function testAsyncFunc()
{
  setTimeout(() => {
    console.log('setTimeout2');
  }, 0);
  
  Promise.resolve().then(() => {
    console.log('promise2');
  });

  console.log("before asyncFunc")

  let afPromise=asyncFunc("first");
  //asyncFunc("first");

  console.log("after asyncFunc");

  console.log("before asyncFunc.then");

  asyncFunc("second").then(function(value) {
    console.log(`ayncFunc.then's return value:${value}`); 
  })

  console.log("after asyncFunc.then");

  console.log("before await asyncFunc");

  let funcVal=await asyncFunc("third");

  console.log("await-asyncFunc's return value:"+funcVal);

  console.log("after await asyncFunc"); 

  console.log("before afPromise.then");
  afPromise.then(function(value) {
    console.log(`afPromise.then's return value:${value}`); 
  })
  console.log("after afPromise.then");

  console.log("exit testAsyncFunc");
}

console.log("after testAsyncFunc");

console.log("beofore testAsyncFunc.then");

testAsyncFunc().then(console.log('testAsyncFunc.then执行结束'));
  
console.log("after testAsyncFunc.then");

console.log("-----------分隔符-----------");

代码输出结果如下：