Javascript 当使用 Promises 时，每个 then() 都应该返回一个值或抛出
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Each then() should return a value or throw when using Promises
提问by Tometoyou
I have a few async methods that I need to wait for completion before I return from the request. I'm using Promises, but I keep getting the error:
我有一些异步方法需要在从请求返回之前等待完成。我正在使用 Promises，但我不断收到错误消息：
Each then() should return a value or throw // promise/always-return
Why is this happpening? This is my code:
为什么会这样？这是我的代码：
router.get('/account', function(req, res) {
var id = req.user.uid
var myProfile = {}
var profilePromise = new Promise(function(resolve, reject) {
var userRef = firebase.db.collection('users').doc(id)
userRef.get()
.then(doc => { // Error occurs on this line
if (doc.exists) {
var profile = doc.data()
profile.id = doc.id
myProfile = profile
resolve()
} else {
reject(Error("Profile doesn't exist"))
}
})
.catch(error => {
reject(error)
})
})
// More promises further on, which I wait for
})
采纳答案by Bergi
Just avoid the Promiseconstructor antipattern! If you don't call resolvebut return a value, you will have something to return. The thenmethod should be used for chaining, not just subscribing:
只要避免Promise构造函数反模式！如果您不调用resolve而是返回一个值，那么您将有一些东西要return. 该then方法应该用于链接，而不仅仅是订阅：
outer.get('/account', function(req, res) {
var id = req.user.uid
var userRef = firebase.db.collection('users').doc(id)
var profilePromise = userRef.get().then(doc => {
if (doc.exists) {
var profile = doc.data()
profile.id = doc.id
return profile // I assume you don't want to return undefined
// ^^^^^^
} else {
throw new Error("Profile doesn't exist")
// ^^^^^
}
})
// More promises further on, which I wait for:
// profilePromise.then(myProfile => { … });
})
回答by Marlhex
Add at the end of the then()
在末尾添加 then()
return null
That's it.
就是这样。
Each then() should return a value or throw Firebase cloud functions
回答by Santosh Shinde
In your case firebase.db.collection('users').doc(id)returning promiseitself, please check firebase snippet to herefor node-js.
在您firebase.db.collection('users').doc(id)返回promise自身的情况下，请在此处查看 node-js 的firebase 代码段。
If you have multiple promises and you need to call them one by one then use Promises chaining.
如果您有多个 Promise 并且需要一个一个地调用它们，则使用Promises chaining。
Please check this articlethis will help you.
请查看这篇文章，这将对您有所帮助。
Use following code in your case,
在您的情况下使用以下代码，
router.get('/account', function(req, res) {
var id = req.user.uid;
var myProfile = {};
var userRef = firebase.db.collection('users').doc(id)
userRef.get()
.then(doc => {
if (!doc || !doc.exists) {
throw new Error("Profile doesn't exist")
}
var profile = doc.data();
profile.id = doc.id;
myProfile = profile;
return myProfile;
})
.catch(error => {
console.log('error', error);
})
})
And use Promise.all if you have multiple promises and you want's to execute them in once.
如果您有多个 Promise 并且希望一次执行它们，请使用 Promise.all。
The Promise.all(iterable)method returns a single Promise that resolves when all of the promises in the iterable argument have resolved or when the iterable argument contains no promises. It rejects with the reason of the first promise that rejects.
该Promise.all(iterable)方法返回一个 Promise，当 iterable 参数中的所有承诺都已解决或当 iterable 参数不包含承诺时，该 Promise 会解决。它以第一个拒绝的承诺的原因拒绝。
For example:
例如：
var promise1 = new Promise((resolve, reject) => {
setTimeout(resolve, 100, 'foo1');
});
var promise2 = new Promise((resolve, reject) => {
setTimeout(resolve, 100, 'foo2');
});
var promise3 = new Promise((resolve, reject) => {
setTimeout(resolve, 100, 'foo3');
});
Promise.all([promise1, promise2, promise3])
.then(result => console.log(result))
//result [foo1, foo2, foo3]
Hopes this will help you !!
希望这会帮助你！！
回答by Abhin Krishna KA
If you can't fix this issue but still want to run your code...
如果您无法解决此问题但仍想运行您的代码...
open : eslintrc.json file (search from project root directory)
search : 'promise/always-return'
change : Case 1: if (existing value is 2) => change to 1
Case 2: else if(existing value is "error" => change to "warn")
It will make this error into warning, but be careful with it... Also use eslint plungin in your editor to remind of good practice. Otherwise you won't get any promise/always-return related warnings.
它会将这个错误变成警告，但要小心... 也在你的编辑器中使用 eslint plungin 来提醒良好的实践。否则，您将不会收到任何与承诺/始终返回相关的警告。
Also make sure you find the right eslintrc.json if more than 1 appears on your search
如果您的搜索中出现 1 个以上，请确保您找到了正确的 eslintrc.json
