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
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));
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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));