How to use ES6 async and await (basic)

First , These two words can be understood as modifiers . So what is their relationship ？ What's the role ？ How to use it ？

async and await Keywords allow us to write in a more concise way based on Promise Asynchronous behavior of , Without the need to deliberately chain call promise. It is suggested to read this article before eating promise！

async

async The modifier is to use async Functional Declaration modifier . That is, you add async keyword , You can make this The result of the function To become a promise function ：

async function fn1() {}
const a = fn1()
console.log(a);

 a The result is ：

await

await The operator is used to wait for a Promise  object . It can only be async Modified asynchronous functions use , however async There can be no await keyword . Generally speaking, it is the same as this word , wait for ！ When the function is executed to this place, it will be suspended , Until this gets the return value , Carry on .

When await The right side is

• promise when ： The return result is promise Results of processing ;
• Not promise when ： Take this value as the return value .

Application example

let p1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('ok')
  }, 1000);
})

let p2 = new Promise((resolve, reject) => {
  reject('fail')
})

async function main() {
  //1. Not promise
  let res1 = await 10
  console.log('res1:', res1);

  //2.promise object , success 
  let res2 = await p1
  console.log('res2:', res2);

  //3.promise object , Failure （ need try catch  Throw an exception ）
  try {
    let res3 = await p2
    console.log('res3:', res3);
  } catch (e) {
    console.log(' Exception thrown :', e);
  }

}

main()

Output results ：

  But if in front promise After the function handles the exception , You don't have to try...catch. Because you will get the return value at this time , without return The return value is undefined, If there is a return value, the return value shall prevail .

For example, in p2 Add exception handling at , And specify to return 1（ The rest remains the same ）：

let p2 = new Promise((resolve, reject) => {
  reject('fail')
}).catch(() => {
   // The return value is not specified undefined
   return '1'

})

The result is ：