My code:
let AuthUser = data => {
return google.login(data.username, data.password).then(token => { return token } )
}
And when i try to run something like this:
let userToken = AuthUser(data)
console.log(userToken)
I'm getting:
Promise { <pending> }
But why?
My main goal is to get token from google.login(data.username, data.password)
which returns a promise, into a variable. And only then preform some actions.
getFirstUser
function
The promise will always log pending as long as its results are not resolved yet. You must call .then
on the promise to capture the results regardless of the promise state (resolved or still pending):
let AuthUser = function(data) {
return google.login(data.username, data.password).then(token => { return token } )
}
let userToken = AuthUser(data)
console.log(userToken) // Promise { <pending> }
userToken.then(function(result) {
console.log(result) // "Some User token"
})
Why is that?
Promises are forward direction only; You can only resolve them once. The resolved value of a Promise
is passed to its .then
or .catch
methods.
Details
According to the Promises/A+ spec:
The promise resolution procedure is an abstract operation taking as input a promise and a value, which we denote as [[Resolve]](promise, x). If x is a thenable, it attempts to make promise adopt the state of x, under the assumption that x behaves at least somewhat like a promise. Otherwise, it fulfills promise with the value x. This treatment of thenables allows promise implementations to interoperate, as long as they expose a Promises/A+-compliant then method. It also allows Promises/A+ implementations to “assimilate” nonconformant implementations with reasonable then methods.
This spec is a little hard to parse, so let's break it down. The rule is:
If the function in the .then
handler returns a value, then the Promise
resolves with that value. If the handler returns another Promise
, then the original Promise
resolves with the resolved value of the chained Promise
. The next .then
handler will always contain the resolved value of the chained promise returned in the preceding .then
.
The way it actually works is described below in more detail:
1. The return of the .then
function will be the resolved value of the promise.
function initPromise() {
return new Promise(function(res, rej) {
res("initResolve");
})
}
initPromise()
.then(function(result) {
console.log(result); // "initResolve"
return "normalReturn";
})
.then(function(result) {
console.log(result); // "normalReturn"
});
2. If the .then
function returns a Promise
, then the resolved value of that chained promise is passed to the following .then
.
function initPromise() {
return new Promise(function(res, rej) {
res("initResolve");
})
}
initPromise()
.then(function(result) {
console.log(result); // "initResolve"
return new Promise(function(resolve, reject) {
setTimeout(function() {
resolve("secondPromise");
}, 1000)
})
})
.then(function(result) {
console.log(result); // "secondPromise"
});
I know this question was asked 2 years ago, but I run into the same issue and the answer for the problem is since ES2017, that you can simply await
the functions return value (as of now, only works in async
functions), like:
let AuthUser = function(data) {
return google.login(data.username, data.password)
}
let userToken = await AuthUser(data)
console.log(userToken) // your data
.then(token => return token)
, that's just an unnecessary passthrough. Simply return the the google login call.
await
outside of an async function. Perhaps the better example here would be to make the AuthUser
function async
, that then ends with return await google.login(...);
The then
method returns a pending promise which can be resolved asynchronously by the return value of a result handler registered in the call to then
, or rejected by throwing an error inside the handler called.
So calling AuthUser
will not suddenly log the user in synchronously, but returns a promise whose then registered handlers will be called after the login succeeds ( or fails). I would suggest triggering all login processing by a then
clause of the login promise. E.G. using named functions to highlight the sequence of flow:
let AuthUser = data => { // just the login promise
return google.login(data.username, data.password);
};
AuthUser(data).then( processLogin).catch(loginFail);
function processLogin( token) {
// do logged in stuff:
// enable, initiate, or do things after login
}
function loginFail( err) {
console.log("login failed: " + err);
}
If that situation happens for a multiple values like an array.
[
Promise { <pending> },
Promise { <pending> },
Promise { <pending> },
Promise { <pending> },
Promise { <pending> }
]
You can use Promise.all()
this will resolve all promises.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all
See the MDN section on Promises. In particular, look at the return type of then().
To log in, the user-agent has to submit a request to the server and wait to receive a response. Since making your application totally stop execution during a request round-trip usually makes for a bad user experience, practically every JS function that logs you in (or performs any other form of server interaction) will use a Promise, or something very much like it, to deliver results asynchronously.
Now, also notice that return
statements are always evaluated in the context of the function they appear in. So when you wrote:
let AuthUser = data => {
return google
.login(data.username, data.password)
.then( token => {
return token;
});
};
the statement return token;
meant that the anonymous function being passed into then()
should return the token, not that the AuthUser
function should. What AuthUser
returns is the result of calling google.login(username, password).then(callback);
, which happens to be a Promise.
Ultimately your callback token => { return token; }
does nothing; instead, your input to then()
needs to be a function that actually handles the token in some way.
return
is treated with the new(ish) closure syntax, in which case -- well, I strongly disapprove of that, but the error is still mine and I apologize for it.
token => { return token; }
does nothing as opposed to claiming it was counterproductive. You can say google.login(username, password).then(token=>{return token;}).then(token=>{return token;})
and so forth forever, but you'll only achieve returning a Promise
that resolves with a token—same as if you just left it as google.login(username, password);
. I'm not sure why you feel that this is "very wrong".
return token
doesn't work as the OP probably expected.
promise.then(result => { return result; })
is exactly equivalent to promise
, therefore the method call does nothing and should be dropped to simplify the code and enhance readability - a statement that is completely true.
Your Promise is pending, complete it by
userToken.then(function(result){
console.log(result)
})
after your remaining code. All this code does is that .then()
completes your promise & captures the end result in result variable & print result in console. Keep in mind, you cannot store the result in global variable. Hope that explanation might help you.
I had the same issue earlier, but my situation was a bit different in the front-end. I'll share my scenario anyway, maybe someone might find it useful.
I had an api call to /api/user/register
in the frontend with email, password and username as request body. On submitting the form(register form), a handler function is called which initiates the fetch call to /api/user/register
. I used the event.preventDefault()
in the beginning line of this handler function, all other lines,like forming the request body as well the fetch call was written after the event.preventDefault()
. This returned a pending promise
.
But when I put the request body formation code above the event.preventDefault()
, it returned the real promise. Like this:
event.preventDefault();
const data = {
'email': email,
'password': password
}
fetch(...)
...
instead of :
const data = {
'email': email,
'password': password
}
event.preventDefault();
fetch(...)
...
Try this
var number1 = document.getElementById("number1");
var number2 = document.getElementById("number2");
startAsync.addEventListener("click", function() {
if (number1.value > 0 && number2.value > 0) {
asyncTest(parseInt(number1.value), parseInt(number2.value)).then(function(result) {
document.getElementById("promiseResolved").textContent = "promiseResolved: " + result
});
} else {
asyncTest(1, 2).then(function(result) {
document.getElementById("promiseResolved").textContent = "promiseResolved: " + result
});
}
});
async function asyncTest(a, b) {
return await (a + b);
};
Success story sharing
Uncaught SyntaxError: Unexpected token .
. The second one needs a return forPromise
.then
on a uninvoked function. updated the answer