Skip to main content

What is async and await?

Both async and await are syntaxes provided in es7. Compared with the promise of es6, this method has higher code readability. Literally understand that async means asynchronous, await means waiting, then their role is very obvious: async: declare that a function is asynchronous; await: wait for an asynchronous function to complete execution.

async function:

  • The return value of the function is a promise object.

  • The result of the promise object is determined by the return value of the async function execution.

await expression:

  • The expression on the right side of await is usually a promise object, but it can be other values.

  • If the expression is a promise object, await returns the value of the promise's success.

  • If the expression is any other value, directly use this value as the return value of await.

  • await must be written in the async function, but there can be no await in the async function.

  • If the promise of await fails, an exception will be thrown, which needs to be caught and handled by try...catch.

See the code below:

    <script>
        async function fun(){
            let p=new Promise((resolve,reject)=>{
                // resolve("OK");
                reject("err66")
            })
            try{
                let a=await p;
                // console.log(a)
            }
            catch(e){
                console.log(e)
            }
        }
        fun()
    </script>

The async syntax has advantages when dealing with then chains. If we use the then nesting of promises, we may write multiple then chains. The combination of the two can solve the problem of callback hell. The following is a multi-interface call using async and await. Code:

     async function fun1() {
        try{
            console.log("fun1...start")
        let l1 = await fun2();
        let l2 = await fun3();
        console.log(l1,l2);
        }catch(e){
            console.log(e);
        }
    }
    function fun2() {
         return "222";
    }  
    function fun3() {
        return Promise.resolve("333");
    }
    fun1();
        console.log("main---1");
        console.log("main---2");
        console.log("main---3");
        console.log("main---4");
        console.log("main---5");
        //fun1...start
        // main---1
        // main---2
        // main---3
        // main---4
        // main---5
        // 222 333

The await expression will block the code behind await, execute the synchronous code outside async first, wait for the synchronous code to finish executing, and then return to the inside of async to continue execution.





Labels:

Comments

Post a Comment

Popular posts from this blog

Using ref to bind elements in Vue

 Vue provides us with a property to manipulate dom, ref. When bound to the dom element, it is used similar to the id. It is called through this.$refs, and it can also be bound to the component. It is very troublesome to obtain DOM elements in the previous native way. It needs to be obtained with such a long AP as: documentgetElementXXX/querySXXX.... Then jQuery was born to solve this pain point. The use of $("id") makes Our operation has become easier. But there are other problems, jQuery is just a library (plug-in) and cannot be used to build a framework. Now Vue can help us solve this problem. Vue also has its own method to obtain DOM, which is ref. It can get not only DOM elements but also components. If you add ref to the dom element, you can get the dom element, and if you add it to the component, you can get the component instance, and you can use the component method.
Post a Comment
Read more

Use of custom events in Vue components.

 The parent component uses props to pass data (such as a method) to the child component, and the child component needs to use custom events to pass data back to the parent component! We can use v-on to bind custom events, and then pass them to subcomponents, which will call after receiving the method. Each Vue instance implements the Events interface, that is: use $on(eventName) to listen for events and $emit(eventName) to trigger events.
Post a Comment
Read more

mapState, mapGetters, mapMutations, mapActions in vuex

 The above four objects are all in the vuex plug-in store, which is often used to store some data shared globally. But I still feel a little uncomfortable when using it, that is, I wrote a lot of repeated prefixes. At this time, these four objects appeared, which solved this pain point very well. mapState is mainly used to manipulate and obtain data, mapGetters is equivalent to computed properties, and mapMutations and mapActions are simplifications of mutation and action. They can be written as both objects and arrays.
Post a Comment
Read more