Advanced application of ES6 modular and asynchronous programming

Basic use

1~ Default export syntax :export default Default export member
2~ Default import Syntax :import Receive name from ' Module name '
    
3~ matters needing attention
               1- Can only be used once per module export default, Otherwise, an error will be reported ***
    2- When importing a package, add js suffix , Otherwise, it will report a mistake
    3- The receiving name can be arbitrarily selected , But it should be legal , Cannot start with a number
4~ Export... On demand
     grammar :export  let xx='xxx'
           export function xx(){}
           xport default   You can add the default export
5~ Import on demand
     grammar :import {xx}from ' Module name '
    xx The name of must be consistent with the imported name   {xx,xx as Change the name }
    import info ,{xx,xx} from ' Module name '   Import is used with default import  
    info Contains the contents exported by default  log Print info that will do
6~ Directly import and execute the code in the module
     direct :import 'xxx.js'   Path to module name
Promise
    1- Back to hell
         Code coupling is too strong Pull a body , Difficult to maintain
         A lot of redundant code is nested with each other , Poor readability
    2-Promise The basic concept of
    
        ① Promise It's a constructor

         We can create Promise Example const p = new Promise()

        new Coming out Promise Instance object , Represents an asynchronous operation

        ② Promise.prototype Contains a .then() Method

         every time new Promise() The instance object obtained by the constructor ,

         Can be accessed through the prototype chain .then() Method , for example p.then()

        ③ .then() Method is used to specify success and failure callback functions in advance

        p.then( Successful callback function , Failed callback function )

        p.then(result => { }, error => { })

         call .then() When the method is used , Successful callback function is required 、 The failed callback function is optional
        
    7-then-fs Basic use of
         install then-fs
         call then-fs Provided readFile() Method , You can read the contents of the file asynchronously , Its return value is Promise          Instance object of . because This can call .then() Method for each Promise Asynchronous operation specifies the return after success and failure             Tonal function

   8-async/await --

                 async function getAllFile() {
                 console.log('B')
                 const r1 = await thenFs.readFile('./files/1.txt', 'utf8')
                 console.log(r1)
                 const r2 = await thenFs.readFile('./files/2.txt', 'utf8')
                 console.log(r2)
                 const r3 = await thenFs.readFile('./files/3.txt', 'utf8')
                 console.log(r3)
                 console.log('D')
            }

    9- Macro task
        setTimeout、setInterval、
         asynchronous Ajax request 、
         File operations
         Other macro tasks

   10- Micro task
        Promise.then、.catch and .finally
        process.nextTick
         Other micro tasks

11- priority
         After each macro task is executed , Will check whether there are micro tasks to be executed ,
         If there is , After all micro tasks are executed , Continue to the next macro task
         priority   Sync --》 Micro task ----》 Macro task