async、await.md

async/await

async is a keyword related to asynchronous operations in ES7, it returns a Promise object. await operator is used to wait for a Promise object, and it can only be used inside an asynchronous function async function. The purpose of async/await is to simplify synchronous behavior when using multiple promises and perform certain operations on a group of Promises. Just as Promises are similar to structured callbacks, async/await is more like a combination of generators and promises.

async

Syntax

async function name([param[, param[, ... param]]]) { statements }

• name: The function name.
• param: The name of the parameters to be passed to the function.
• statements: The body statements of the function.

The async function returns a Promise object. You can use the then method to add a callback function, and the returned Promise object will execute the result of the resolve asynchronous function. If an exception is thrown, it will execute the reject.

Example

async function asyncPromise(v){
    return v;
}

asyncPromise(1).then((v) => {
    console.log(v);
}).catch((e) => {
    console.log(e);
})

await

async is generally used in conjunction with the await instruction, which pauses the execution of the asynchronous function, waits for the Promise to be executed, and then continues to execute the asynchronous function and return the result. If the Promise encounters an exception rejected, the await expression will throw the reason for the Promise exception. In addition, if the value of the expression after the await operator is not a Promise, it will return the value itself.

Example

function promise(){
    return new Promise(function(resolve,reject){
        var rand = Math.random() * 2;
        setTimeout(function(){
            resolve(`solve ${rand}`);
        },1000)
    })
}

(async function asyncPromise(){
    var result = await promise();
    console.log(result);
})();

// Catching exceptions
function promise(){
    return new Promise(function(resolve,reject){
        throw new Error(`reject`);
    })
}

(async function asyncPromise(){
    var result = await promise();
    console.log(result);
})().catch((e) => {
    console.log(e);
});

// Return the value itself if it's not a promise object
function notPromise(){
    return 1;
}

(async function asyncPromise(){
    var result = await notPromise();
    console.log(result);
})();

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