在ES6和ES5中promise的執行也有不同點(上述提到，ES6中promise屬microtask；在ES5中，暫未接觸到有api直接操作microtask的，所以.then的異步是用setTimeout代替，屬macrotask，導致輸出有差異)；關於promise也可參考上文 分步理解 Promise 的實現
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以前沒有直接操作 microtask的api
這段代碼，實現的功能 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 是一個操作符， await 后面接 expression
var a = await 3,
async 加在函數前面，自動返回的是一個 Promise
在函數里面，可以使用 await 調用前面的async定義的函數
全局環境，直接await 就可以， “局部”函數 里面，函數前面要加 async關鍵字
局部函數
參考： 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?
下面這種方式返回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));