The async function returns a Promise object, and callbacks can be added using the then method. When a function executes, it returns as soon as it encounters await, waits for the asynchronous operation to complete, and then executes the following statement in the function body.

Method of use

• Async means that there are asynchronous operations in a function,

• Await means that the following expression needs to wait for the result.

Syntax examples

async function name() { 
 await  new Promise((resolve) = > {
       setTimeout(function(){
          console.log("testAwait");
          resolve();
       }, 1000);
   });
  
  console.log('helloAsync')}Copy the code

I. The result of async function execution is Promise

async function HiAsync() {
 return "hi";
}
async function HelloAsync() {
 return Promise.resolve('hello')}console.log(HiAsync())
console.log(HelloAsync())

HiAsync().then(res= > {
    console.log(res) // 'hi'
})
HelloAsync().then(res= > {
    console.log(res)  // 'hello'
})
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2. Await results (even if nested async)

function getSomething() {
    return "a";
}

async function testAsync() {
    return new Promise((resolve,reject) = > {
        setTimeout(() = > {resolve('b')},3000)})}async function deepAsync() {
    let p2 = new Promise((resolve, reject) = > {
        setTimeout(() = > { resolve('c')},1000)})return new Promise((resolve, reject) = > {
        setTimeout(() = > { resolve(p2) }, 3000)})}async function test() {
    const v1 = await getSomething();
    console.log('v1',v1)
    const v2 = await testAsync();
    console.log('v2',v2)
    const v3 = await deepAsync();
    console.log('v3',v3);
}
test();
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Pay attention to the point

Await = ‘await’; await = ‘await’; await = ‘await’; In the catch block.

async function myFunction() {
  try {
    await somethingThatReturnsAPromise();
  } catch (err) {
    console.log(err); }}// Another way to write it

async function myFunction() {
  await somethingThatReturnsAPromise().catch(function (err){
    console.log(err);
  });
}
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2. Await must be used in async functions

If used in normal functions, an error will be reported. There is also a problem with changing the arguments of forEach to async.

The correct way to write this is to use the for loop

async function dbFuc() {
  let docs = [1.2.3];

  for (let doc of docs) {
    console.log(doc)
  }
}
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