Detailed explanation of the usage of async and await

This time I will bring you a detailed explanation of the usage of async and await. What are the precautions when using async and await. The following is a practical case, let's take a look.

Koa is a very famous Node server-side framework, with versions 1.x and 2.x. The former uses a generator to perform asynchronous operations, and the latter uses the latest async/await solution

When I first started using this writing method, I encountered a problem, the code is as follows:

const Koa = require('koa');
const app = new Koa();
const doSomething = time => {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('task done!')
    }, time)
  })
}
// 用来打印请求信息
app.use((ctx, next) => {
  console.log(`${ctx.method}:::${ctx.url}`)
  next()
})
app.use(async ctx => {
  const result = await doSomething(3000)
  console.log(result);
  ctx.body = result
})
app.listen(3000);

Let us test it: curl http://localhost:3000

Expected result:

(After 3 seconds...)task done!

However, the reality is :

(immediately)
Not Found

What the hell? Why isn't it performing as expected? This requires us to understand how the middleware in Koa is connected in series. Looking through the source code, the code for concatenating middlewares is as follows:

function compose (middleware) {
 return function (context, next) {
  // 这个index用来计数，防止next被多次调用
  let index = -1
  // 执行入口
  return dispatch(0)
  
  function dispatch (i) {
   // 如果next被多次调用，报异常
   if (i <p style="text-align: left;">With the above foundation, let’s take a look at the previous question. Why does the response return immediately without waiting for the second middleware to be executed? </p><p style="text-align: left;">Because the first middleware is not an asynchronous function. </p><p style="text-align: left;">Since each execution of the next method actually returns a Promise object, so if we perform an asynchronous operation in a middleware, if we want to wait for it to complete, we must do so before executing the middleware. Add await</p><p style="text-align: left;"> Then let’s rewrite the previous code</p><pre class="brush:php;toolbar:false">app.use(async (ctx, next) => {
  console.log(`${ctx.method}:::${ctx.url}`)
  await next()
})
app.use(async ctx => {
  const result = await doSomething(3000)
  console.log(result);
  ctx.body = result
})

Okay, there is no problem, everything is executed as expected:clap:

##Error handling

With the help of the powerful power of Promise and the async/await syntax, we only need to write the try/catch operation in the outermost middleware to capture all subsequent middleware Something unusual!

app.use(async (ctx, next) => {
  try{
    await next()
  }catch(err){
    console.log(err)
  }
})
app.use(async (ctx)=>{
  throw new Error('something wrong!')
  ctx.body = 'Hello'
})

Full control based on the middleware chain, and the fact that it is based on Promise makes everything easy to operate. There is no longer if (err) return next(err) everywhere, but only promise

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the PHP Chinese website!

Recommended reading:

How to use datepicker

Detailed explanation of the use of high-level components of mixin

Usage of ejsExcel template in Vue.js

The above is the detailed content of Detailed explanation of the usage of async and await. For more information, please follow other related articles on the PHP Chinese website!