工作案例:
当我们调用异步函数并且该函数返回 promise resolve() 时,异步 await 工作正常
不工作的情况:
异步等待不适用于 mongo 数据库查询
try then(), async/await
我有 2 个 JS 文件。
在一个.js文件中,我正在导入函数中的函数.js
工作案例:
当一个.js看起来像
var functiononestatus = transactions.functionone(req.session.email).then((came) => {
console.log(came); // getting `need to be done at first` message
console.log("exec next")
});
当函数.js看起来像
module.exports.functionone = functionone;
async function functionone(email) {
return await new Promise((resolve, reject) => {
resolve('need to be done at first')
});
});
不工作的情况(当需要执行 MONGO DB 查询时):
当一个.js看起来像
var functiononestatus = transactions.functionone(req.session.email).then((came) => {
console.log(came); // getting undefined
console.log("exec next")
});
当函数.js看起来像
module.exports.functionone = functionone;
async function functionone(email) {
//mongo starts
var collection = await connection.get().collection('allinonestores');
await collection.find({
"email": email
}).toArray(async function(err, wallcheck) {
return await new Promise((resolve, reject) => {
resolve(wallcheck[0])
});
});
快速澄清:
-
.collection('name')
返回一个Collection
实例,而不是一个承诺,因此无需await
它。 -
toArray()
以两种模式运行:一种在提供函数时使用回调,要么在未提供回调函数时返回 Promise。
在提供回调函数时,您基本上期望 Promise 结果超出toArray()
,从而导致undefined
,因为回调优先并且由于 toArray() 的双重操作模式,不返回任何承诺。
此外,toArray(callback)
不会将 async
函数作为回调。
下面是用于检索集合的代码的外观:
const client = await MongoClient.connect('your mongodb url');
const db = client.db('your database name'); // No await here, because it returns a Db instance.
const collection = db.collection('allinonestores'); // No await here, because it returns a Collection instance.
然后,用于获取结果的代码:
const db = <get db somehow>;
// You could even ditch the "async" keyword here,
// because you do not do/need any awaits inside the function.
// toArray() without a callback function argument already returns a promise.
async function functionOne(email) {
// Returns a Collection instance, not a Promise, so no need for await.
const collection = db.collection('allinonestore');
// Without a callback, toArray() returns a Promise.
// Because our functionOne is an "async" function, you do not need "await" for the return value.
return collection.find({"email": email}).toArray();
}
和代码替代,使用回调:
const db = <get db somehow>;
// You could even ditch the "async" keyword here,
// because you do not do/need any awaits inside the function.
// You're forced to return a new Promise, in order to wrap the callback
// handling inside it
async function functionOne(email) {
// Returns a Collection instance, not a Promise, so no need for await.
const collection = db.collection('allinonestore');
// We need to create the promise outside the callback here.
return new Promise((resolve, reject) => {
db.find({"email": email}).toArray(function toArrayCallback(err, documents) {
if (!err) {
// No error occurred, so we can solve the promise now.
resolve(documents);
} else {
// Failed to execute the find query OR fetching results as array someway failed.
// Reject the promise.
reject(err);
}
});
});
}
注意:首先,我真的需要感谢@mihai Potra的最佳答案。
来吧
案例1:
如果它是一个需要查找文档并从 MongoDb 返回的函数,如 mihai 提到的那样,下面的答案非常酷
const db = <get db somehow>;
async function functionOne(email) {
const collection = db.collection('allinonestore');
return collection.find({"email": email}).toArray();
}
案例2:
如果有嵌套函数需要每次返回值,那么据我所知
,这将是最好的- 不需要每个函数的 async/await 关键字,或者不需要 then()
function one(<parameter>) {
return new Promise(function(resolve, reject) {
const collection = connection.get().collection('<collection_name>');
const docs = collection.find({"Key": Value_fromFunction}).toArray( function (err, result) {
resolve(result[0]);
});
就是这样,在需要时使用解析回调。