Async/await asynchronous function

1、async Use of asynchronous functions

// await/async How to write it 
async function foo1() {
    }

const foo2 = async () => {
    };

2、async The execution flow of asynchronous functions

//  It's all synchronous code , It's no different from ordinary function code 
async function foo() {
    
  console.log(" Internal code execution 1");
  console.log(" Internal code execution 2");
  console.log(" Internal code execution 3");
}

console.log("script start");
foo();
console.log("script end");

3、async The difference between asynchronous functions and ordinary functions 1 - Return value

//  The return value of an asynchronous function must be Promise
async function foo() {
    
  console.log("foo start...");
  // 1、 If you don't write by default , The default return is undefined

  // 2、 Returns a value 
  // return "aaa";

  // 3、 Return to one thenable
  // return {
    
  // then: function (resolve, reject) {
    
  // resolve("haaaaaa");
  // },
  // };

  // 4、 Return to one Promise
  return new Promise(function (resolve, reject) {
    
    setTimeout(function () {
    
      resolve("hehehehhehe");
    }, 2000);
  });
}

const promise = foo();
//  This then Is in foo() Yes return Only when it's executed , And it is added to the micro task queue 
promise.then((res) => {
    
  console.log(res);
});

4、async The difference between asynchronous functions and ordinary functions 2 - abnormal

async function foo() {
    
  console.log("foo function start~");
  console.log(" Intermediate code ");

  //  Exception in asynchronous function , Will be returned as an asynchronous function Promise Of reject It's worth it , Can be in catch Get the error message 
  throw new Error("error Message~");

  console.log("foo function end");
}

foo().catch((err) => {
    
  console.log('chen', err);
});
console.log(" There are codes to follow ~");

5、async Use in await

// async function foo() {
    
// await  expression (Promise)
// }

// 1、await Keep up with the expression 
// function requestData() {
    
// return new Promise((resolve, reject) => {
    
// setTimeout(() => {
    
// resolve(111);
// }, 1000);
// });
// }
// async function foo() {
    
// //  When to return res ->  Namely requestData() call resolve When 
// const res = await requestData();

// //  These later codes , amount to await requestData(); in Promise then Implemented in .
// console.log('res', res);
// console.log("=============");
// }
// foo();


function requestData() {
    
  return new Promise((resolve, reject) => {
    
    setTimeout(() => {
    
      reject(111);
    }, 1000);
  });
}
// 2、 Keep up with other values 
async function foo() {
    
  // 2.1、await  Keep up with the normal value , It will return immediately 
  // const res1 = await 123;

  // 2.2、await Keep up with the object 
  // const res1 = await {
    
  // then: function (resolve, reject) {
    
  // resolve("aaaaaaa");
  // },
  // };

  // 2.3、await keep pace with Promise
  // const res1 = await new Promise((resolve) => {
    
  // resolve("bbbb");
  // });
 

  // 2.4、reject value , When we are here reject When ,reject Value , As the whole asynchronous function foo() Of Promise Of reject Value , We need to be outside catch
  const res1 = await requestData();
   console.log("res", res1);
}
foo().catch((err) => {
    
  console.log('err---', err);
})