Async keyword

async keyword

Add... Before the definition of ordinary function async keyword Ordinary functions become asynchronous functions

Asynchronous functions return... By default promise object

Used inside asynchronous functions return Keyword for result return The result will be wrapped promise In the object return Keywords replace resolve Method

Used inside asynchronous functions throw Keyword throws a program exception

Call asynchronous function and chain call then Method to get the execution result of the asynchronous function

Call asynchronous function and chain call catch Method to get the error information of asynchronous function execution

// 1. Add... Before the ordinary function definition async keyword   Ordinary functions become asynchronous functions 
// 2. The default return value of an asynchronous function is promise object 
// 3. Used inside asynchronous functions throw Keyword to throw an error 
// 
// await keyword 
// 1. It can only appear in asynchronous functions 
// 2.await promise  It can pause the execution of asynchronous functions   wait for promise Object returns the result before executing the function down 

// async function fn () {
//     throw ' There have been some mistakes ';
//     return 123;
// }

// // console.log(fn ())
// fn ().then(function (data) {
//     console.log(data);
// }).catch(function (err){
//     console.log(err);
// })

async function p1 () {
    return 'p1';
}

async function p2 () {
    return 'p2';
}

async function p3 () {
    return 'p3';
}

async function run () {
    let r1 = await p1()
    let r2 = await p2()
    let r3 = await p3()
    console.log(r1)
    console.log(r2)
    console.log(r3)
}

run();

await keyword 

await Keywords can only appear in asynchronous functions 

await promise await You can only write promise object   Write about other types of API It's impossible 

await Keyword can pause asynchronous function execution   until promise Return results 

const fs = require('fs');
//  Transform existing asynchronous functions api  Let it go back to promise object   This supports asynchronous function syntax 
const promisify = require('util').promisify;
//  call promisify Methods to transform the existing asynchronous system API  Let it go back to promise object 
const readFile = promisify(fs.readFile);

async function run () {
    let r1 = await readFile('./1.txt', 'utf8')
    let r2 = await readFile('./2.txt', 'utf8')
    let r3 = await readFile('./3.txt', 'utf8')
    console.log(r1)
    console.log(r2)
    console.log(r3)
}

run();