Async/Await 中的文件写入问题
file write issue in Async/Await
我在这里尝试检索对象并将它们推入数组。由于某种原因,当它应该包含更多对象时,只有一个记录被推入文件。你能帮我解决这个问题还是让我知道我哪里出错了?这是我的代码:
exports.createjson = (req, res, next) => {
try {
var myPromise = () => {
// ...
};
var callMyPromise = async () => {
const responsearray = [];
var result = await myPromise();
return new Promise((resolve, reject) => {
result.forEach(element => {
NewsModel.findOne({ _id: element.newsId }).exec(
async (err, result) => {
if (err) {
throw err;
}
reportsModel
.findOne({
$and: [
{ userId: req.query.userId },
{ newsId: element.newsId }
]
})
.exec((err, newsResult) => {
if (err) {
throw err;
}
// console.log(newsResult);
var response = {
newsId: element.newsId,
title: result.title,
collection: result.group,
belivibalityIndex: element.belivibalityIndex,
priorknowledge: element.priorknowledge,
readingTime: element.readingTime,
userId: element.userId,
comment: element.comment,
report: newsResult !== null ? newsResult.feedback : null
};
// #all object pushed and displayed in console
responsearray.push(response);
console.log(response);
console.log(responsearray.length);
// let data = JSON.stringify(responsearray);
// #here is the issue // fs.writeFileSync("abc.json", data, null, null, flag = 'a');
return responsearray;
});
}
);
});
});
};
callMyPromise().then(function(responsearray) {
res.json(responsearray);
});
} catch (error) {
next(error);
}
};
您没有正确使用 Promises。例如,您创建了一个 Promise
对象但从未调用 resolve
/reject
函数。在 forEach
循环中,您正在调用使用回调的函数,当该工作完成后,您可以 resolve
将其包装在其中的承诺。
您还在 forEach 循环中调用 res.json
并写入文件(尽管它被注释掉了)。这意味着 res.json
将被多次调用,这是不允许的。一个 http 请求只能有一个响应。
我重组了代码,以便它收集 Promise 数组中的每个 Promise,然后等待所有 Promise 解决。只有当所有的工作都完成后,我们才能写入文件并调用res.json
完成http请求。
exports.createjson = async (req, res, next) => {
const responsearray = [];
var elements = await myPromise();
var promises = []; // collect a bunch of promises to wait on
elements.forEach(element => {
// one promise per element that resolves when response is on the array
var promise = new Promise(function(resolve, reject) {
NewsModel.findOne({ _id: element.newsId }).exec((err, result) => {
if (err) { return reject(err); }
reportsModel
.findOne({
$and: [{ userId: req.query.userId }, { newsId: element.newsId }]
})
.exec((err, newsResult) => {
if (err) { return reject(err); }
var response = { /* response body */ };
responsearray.push(response);
console.log(response);
console.log(responsearray.length);
// complete the promise now that the response is on the array
return resolve();
});
});
});
// collect each promise in an array so we can wait for them all
promises.push(promise);
});
// wait for all the work to complete
await Promise.all(promises).catch(err => next(err));
// write the responsearray to a file as json
let data = JSON.stringify(responsearray);
fs.writeFileSync("abc.json", data);
return res.json(responsearray);
};
我还删除了 try/catch
块,因为 Promise
允许您以更简洁的方式使用 .catch
。它简化了嵌套,使其更易于阅读。
这里的关键是一般结构:
// get your array to work with
var array = await someFunction()
var manyPromises = []
var manyResults = []
// for each thing in the array create a promise
array.forEach( thing => {
manyPromises.push( new Promise((resolve,reject) => {
doSomething(thing, (err, result) => {
if (err) return reject(err);
// store the results in the array and resolve the promise
manyResults.push(result)
return resolve();
});
});
});
// wait for all promises in manyPromises to complete
await Promise.all(manyPromises).catch(err => return next(err));
// now the many promises are done and manyResponses are ready
saveResponsesToFile(JSON.stringify(manyResponses))
return res.json(manyReponses)
我在这里尝试检索对象并将它们推入数组。由于某种原因,当它应该包含更多对象时,只有一个记录被推入文件。你能帮我解决这个问题还是让我知道我哪里出错了?这是我的代码:
exports.createjson = (req, res, next) => {
try {
var myPromise = () => {
// ...
};
var callMyPromise = async () => {
const responsearray = [];
var result = await myPromise();
return new Promise((resolve, reject) => {
result.forEach(element => {
NewsModel.findOne({ _id: element.newsId }).exec(
async (err, result) => {
if (err) {
throw err;
}
reportsModel
.findOne({
$and: [
{ userId: req.query.userId },
{ newsId: element.newsId }
]
})
.exec((err, newsResult) => {
if (err) {
throw err;
}
// console.log(newsResult);
var response = {
newsId: element.newsId,
title: result.title,
collection: result.group,
belivibalityIndex: element.belivibalityIndex,
priorknowledge: element.priorknowledge,
readingTime: element.readingTime,
userId: element.userId,
comment: element.comment,
report: newsResult !== null ? newsResult.feedback : null
};
// #all object pushed and displayed in console
responsearray.push(response);
console.log(response);
console.log(responsearray.length);
// let data = JSON.stringify(responsearray);
// #here is the issue // fs.writeFileSync("abc.json", data, null, null, flag = 'a');
return responsearray;
});
}
);
});
});
};
callMyPromise().then(function(responsearray) {
res.json(responsearray);
});
} catch (error) {
next(error);
}
};
您没有正确使用 Promises。例如,您创建了一个 Promise
对象但从未调用 resolve
/reject
函数。在 forEach
循环中,您正在调用使用回调的函数,当该工作完成后,您可以 resolve
将其包装在其中的承诺。
您还在 forEach 循环中调用 res.json
并写入文件(尽管它被注释掉了)。这意味着 res.json
将被多次调用,这是不允许的。一个 http 请求只能有一个响应。
我重组了代码,以便它收集 Promise 数组中的每个 Promise,然后等待所有 Promise 解决。只有当所有的工作都完成后,我们才能写入文件并调用res.json
完成http请求。
exports.createjson = async (req, res, next) => {
const responsearray = [];
var elements = await myPromise();
var promises = []; // collect a bunch of promises to wait on
elements.forEach(element => {
// one promise per element that resolves when response is on the array
var promise = new Promise(function(resolve, reject) {
NewsModel.findOne({ _id: element.newsId }).exec((err, result) => {
if (err) { return reject(err); }
reportsModel
.findOne({
$and: [{ userId: req.query.userId }, { newsId: element.newsId }]
})
.exec((err, newsResult) => {
if (err) { return reject(err); }
var response = { /* response body */ };
responsearray.push(response);
console.log(response);
console.log(responsearray.length);
// complete the promise now that the response is on the array
return resolve();
});
});
});
// collect each promise in an array so we can wait for them all
promises.push(promise);
});
// wait for all the work to complete
await Promise.all(promises).catch(err => next(err));
// write the responsearray to a file as json
let data = JSON.stringify(responsearray);
fs.writeFileSync("abc.json", data);
return res.json(responsearray);
};
我还删除了 try/catch
块,因为 Promise
允许您以更简洁的方式使用 .catch
。它简化了嵌套,使其更易于阅读。
这里的关键是一般结构:
// get your array to work with
var array = await someFunction()
var manyPromises = []
var manyResults = []
// for each thing in the array create a promise
array.forEach( thing => {
manyPromises.push( new Promise((resolve,reject) => {
doSomething(thing, (err, result) => {
if (err) return reject(err);
// store the results in the array and resolve the promise
manyResults.push(result)
return resolve();
});
});
});
// wait for all promises in manyPromises to complete
await Promise.all(manyPromises).catch(err => return next(err));
// now the many promises are done and manyResponses are ready
saveResponsesToFile(JSON.stringify(manyResponses))
return res.json(manyReponses)