Promise_ Async and await

Small mistakes in thinking

There was such a question before , I'm working on functions 1 Use in await Waiting for asynchronous execution , In function 2 Calling function in 1, That function 2 The synchronization code in will wait for the function 1 Do you want to execute after execution ？

• Here's an example ： call listChenge function , Print first 111 Or print it first 222 Well ？

  •  async awaitMethod(){
            
          const res = await new Promise((resolve,reject)=>{
            
            setTimeout(()=>{
            
              console.log('111')
              resolve()
            },5000)
          })
          console.log('333')
        },
        listChenge(){
            
          this.awaitMethod()
          console.log('222')
        }
    

• The answer is to print first 222, wait for 5 Second printing 111,333

• The reason is simple —>await Will suspend At present async Execution of a function , wait for promise Execute the following code after execution ;

• If we want to print first 111, Re print 222

  •  async awaitMethod(){
            
          const res = await new Promise((resolve,reject)=>{
            
            setTimeout(()=>{
            
              console.log('111')
              resolve()
            },5000)
          })
          console.log('333')
        },
        async listChenge(){
            
          await this.awaitMethod()
          console.log('222')
        }
    

async And await

Due to the small mistake above , Review again async And await Knowledge point ;

effect

Use async/await You can write forms Sync Code to handle asynchronous technological process ;

Concept

await

• Be careful ：await Operators can only be used in async Use in a function , Otherwise, an exception will be thrown ;

  • ‘await’ is only allowed within async functions

• grammar ：

  • [ Return value ] = await  expression 
    

  • The expression can be a Promise Object or any value to wait for ;

    • Promise object —> The return value is Promise Processing results of ;

      • resolveAfter2Seconds(x) {
                    
          return new Promise(resolve => {
                    
             setTimeout(() => {
                    
                resolve(x);
             }, 2000);
           });
        },
        async listChenge(){
                    
            const res = await this.resolveAfter2Seconds(1)
            console.log(' result ', res) // 1
        }
        

    • Not Promise object —> The return value is the value itself

      • async listChenge(){
                    
            const res = await 1
            console.log(' result ', res) // 1
        }
        

• await Expression will Pause the current async Execution of a function , wait for Promise Processing is complete , And go on with it async function

  • promise-fulfilled—> Carry on async function;
  • promise-rejected—> take Prmoise The exception reason of is thrown ;

async function

• Definition ： Use async The function declared by the keyword is called async function ;

• grammar ：

  • async function  Function name (){}
    

• Return value

  • Return to one Promise

    • If the return value is not a promise,async The function will wrap the value as a Promise

      • async function test(){
                    
          return 222
        }
        
        //  Equivalent to 
        async function test(){
                    
          return new Promise(){
                     resolve(222)}
        }
        

    • If there is no return value , The default value is undefined

• async And await Use a combination of

  • async in 0 One or more await;
  • await The expression pauses the entire async The execution process of a function and cedes its control , Only if its waiting is based on promise The process will not resume until the asynchronous operation of is honored or rejected .promise The solution value of is treated as the solution value of await Return value of expression