async和await的返回值——NodeJS, get return value from async await

 

 

 在ES6和ES5中promise的执行也有不同点(上述提到，ES6中promise属microtask；在ES5中，暂未接触到有api直接操作microtask的，所以.then的异步是用setTimeout代替，属macrotask，导致输出有差异)；关于promise也可参考上文 分步理解 Promise 的实现

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以前没有直接操作 microtask的api 

https://stackoverflow.com/questions/41075724/javascript-api-to-explicitly-add-micro-tasks-or-macro-tasks

这段代码，实现的功能 就是操作micro tasks and macro tasks

let pushToMicroTask = f => Promise.resolve().then(f);
let pushToMacroTask = f => setTimeout(f);
let say = msg => console.log(msg);

pushToMacroTask(say.bind(null, 'Macro task runs last'));
pushToMicroTask(say.bind(null, 'Micro task runs first'));

　　

 

Is there is any W3C spec concerning micro/macro tasks?

W3C speaks of task queues:

 

 

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await 是一个操作符， await 后面接 expression

var a = await 3,

 

async 加在函数前面，自动返回的是一个 Promise

在函数里面，可以使用 await 调用前面的async定义的函数

全局环境，直接await 就可以， “局部”函数 里面，函数前面要加 async关键字

 

 局部函数

 

 

 

参考： https://stackoverflow.com/questions/48375499/nodejs-get-return-value-from-async-await

https://www.academind.com/learn/javascript/callbacks-vs-promises-vs-rxjs-vs-async-awaits/

I have an async await function that uses mongoose:

const createModelB = async (id) => {
    try {
        let user = await User.findOne({id: id});

        if (user) {
            let modelB = new ModelB({ user_id: user.id });
            modelB = await scrum.save();
            return modelB;
        }
        return null;
    } catch (err) {
        console.error(err);
    }

    return null;
};
Now then I'm calling this function from somewhere else:

let modelB = createModelB(123);
console.log(modelB);
Instead of outputting Models fields, console returns to me this:

Promise {<pending>}
What did I miss?

　　

下面这种方式返回promise的值。

function fetchUser() {
  return checkAuth()
            .then(auth => { return getUser(auth) })
            .then(user => { return user });
}
fetchUser().then((user) => console.log(user.name));

　　---------------------------------------

async function fetchUser() {
  const auth = await checkAuth(); // <- async operation
  const user = await getUser(auth); // <- async operation
  return user;
}
fetchUser().then((user) => console.log(user.name));