I have a function loadItems() that loads something async, and then fails or succeeds. Both fail and success callbacks must do something in the library, and then pass it on to the implementer, which can choose to implement fail, or success, or both, or none.
The problem I now have is if the library implements both the success and fail handler, it will always return a new promise that resolves to success.
This is what I have:
// The library:
function loadItems() {
return store.get('todo').then(function(rsp) {
// Save these locally
items = rsp.value || [];
}, function(rsp) {
// Do nothing, but let the user know
alert(rsp.error);
});
}
store.get('todo') (from yet another library) returns a promise. loadItems() has no control over it.
// The library implementer:
function init() {
loadItems().then(showItems);
}
What I want, and expected to happen:
loadItems() runs the async code from the library (store.get())
It implements success and fail because they have mandatory actions
It passes the success/failure on to the implementer
The implementer only implements success, because it doesn't care about errors, because the library handled it
So it uses .then(onSuccess) to 'add another success callback'
What happens instead (with an error):
The library's failure callback is executed
A new promise is passed on to the implementer
The new promise always resolves with success
The implementer's success callback is fired, with broken result, because the library's success callback didn't fire
Are Promises seriously too cool to have multiple, sync success handlers??
I can imagine even the source library (that defines store.get()) wants to handle the error (for logging sneakily), and then pass success/failure on.
My 'solutions':
Have loadItems()' failure callback throw an error. Unfortunately, that means init() has to catch it (or the browser whines about it). Not cool.
Use a simple callback to talk back to init(), instead of a promise, which only fires after a success. That's not good, because init() might choose to handle the error some day.
I'm sure I'm doing something wrong, but I don't see it.
If you want to have a reject handler to do something based on the error, but want the returned promise to still be rejected, you just rethrow the error (or return a rejected promise) after your reject handling code. This will allow the reject to propagate back through the returned promise.
// The library:
function loadItems() {
return store.get('todo').then(function(rsp) {
// Save these locally
items = rsp.value || [];
}, function(rsp) {
// Do nothing, but let the user know
alert(rsp.error);
// rethrow the error so the returned promise will still be rejected
throw(rsp);
});
}
Supplying a reject handler that does not throw or return a rejected promise tells the promise system that you have "handled" the error and the return value of your reject handler becomes the new fulfilled value of the promise. So, if you want the promise to "stay" rejected, but want to have a handler to do something based on the rejection (logging the rejection is very common), then you have to either rethrow the error in your reject handler or return a rejected promise.
While this may initially seem counter-intuitive, it gives you the most flexibility because you can either completely handle the error and let the returned promise be resolved "successfully" OR you can choose to tell the promise system that you want to propagate an error and you can even choose which error you want that to be (it does not have to be the same error).
The part of your question about multiple success handlers is a bit confusing to me. You can easily have multiple success handlers with any promise:
var p = someFuncThatReturnsSuccessfulPromise();
p.then(someSuccessHandler);
p.then(someOtherSuccessHandler);
If p is a successfully resolved promise, then these two success handlers will both be called in the order they are attached and what happens in someSuccessHandler will have no impact on whether someOtherSuccessHandler is called or not. If the original promise is resolved successfully, then both handlers will always be called.
If you chain your success handlers, then it is a completely different use case. This is completely different:
var p = someFuncThatReturnsSuccessfulPromise();
p.then(someSuccessHandler).then(someOtherSuccessHandler);
Because the second .then() handler is not attached to p, but is attached to p.then(someSuccessHandler) which is a different promise whose outcome is potentially influenced by what happens in someSuccessHandler.
The code in your question is chained. You are returning:
return store.get().then(...)
So, when the caller then chains onto that, the full chain is:
return store.get().then(yourhandler).then(theirhandler)
In this way, yourhandler can influence the outcome that is passed to theirhandler.
In addition to my first recommendation of just rethrowing the error, you could also have done this:
// The library:
function loadItems() {
var p = store.get('todo');
p.then(function(rsp) {
// Save these locally
items = rsp.value || [];
}, function(rsp) {
// Do nothing, but let the user know
alert(rsp.error);
});
return p;
}
Here you ware making sure that your handlers don't affect what is being returned. This can be done if you don't have any async operations in your handlers and you aren't trying to change the resolved value or rejected error of the original store.get() promise. I generally don't recommend this because it is a cause for problems if your handlers are doing other async things or want to influence the return values, but it can also be used in appropriate circumstances.
If we have a promise like the following, what are the answers to the questions in its comments?
p.then(function ok () {
// Can we get err() to trigger from inside here?
}, function err () {
// Can we get ok() to trigger from inside here?
});
I know that one can attach a new then which can wait for the results or reverse the results of p, but I'm wondering, assuming that p is constant, whether the conditions can call each other recursively also (and without assigning the functions to variables and invoking them by name)...
UPDATE
My use case is as follows.
In IndexedDB, when opening a database, you can listen for the following:
db.onsuccess = ...
db.onerror = ...
db.onblocked = ...
db.js, a library I'm expanding to meet my needs, adds these events with a Promise API, such that a success will resolve the promise, and errors or blocking will reject it.
A common use case for listening for blocking would be to close the database connection which is causing the blocking and IndexedDB will thereupon automatically call onsuccess. The problem is that if we treat onblocked as a rejection, it apparently has no way to retrigger the resolve condition (i.e., the onsuccess). In order to get around this, I can have the blocking be supplied instead as a callback, but I was wondering whether there were any way to do it exclusively using the Promises approach since technically, the error would no longer be an error and would like to give a chance for the original resolve callback to resume with its handling of success.
Can we get err() to trigger from ok?
Can we get ok() to trigger from err?
No. The promises spec mandates that at most one of the two then handlers gets called. If the promise fulfills the first gets called, if the promise rejects the second will, and a promise can't do both things.
So while you could easily create two named functions and call each other manually, it is not programmatically possible to trigger the promise into calling the other.
My use case is as follows […]
What you actually seem to be looking for is
db.open().catch(function(err) {
if (err.isBlocking)
return db.closeAndWaitForNextSuccess(err.blocker); // get another promise
else
throw err;
}).then(function ok(res) {
…
});
Because you're specifying the error handler as a second parameter to .then, what it semantically means is that it will only trigger if the promise "p" got rejected. So the two paths are mutually exclusive. It also means that if inside your "ok" handler you throw an error, it will NOT be caught by the "err" function.
By contrast, if your code used a .catch, it would catch both an error bubbling up from the promise's rejection, and from within the success handler. So if you had this:
p.then(function () {
throw new Error("Within the .then");
}).catch(function (error) {
console.log(error);
});
It would always log the error to the console, regardless of whether p had resolved or rejected.
So for your first question: the ok handler could trigger err function, IF instead of doing .then with two arguments you do .then(successHandler).catch(errorHandler). But for the second question it's "no" no matter what -- there is no logical path from either an error handler or a "catch" to a path that was explicitly skipped due to the rejection.
Update based on updated question description:
Assuming that you get some sort of indicator in the "catch" on why the error occurred (and whether you should really reject or whether you should continue as if it's a success), there's no reason why you couldn't call the same function as you would for success. It's just that it would have two entry points:
p.then(successHandler, failureHandler)
.then( /* other stuff if you want to chain */ )
.catch(function(e) {
// catch-all handler just in case
})
function successHandler(data) {
// Do something. Note that to avoid
// "breaking the promise chain", the code here
// should either be all-synchronous or return another promise.
}
function failureHandler(error) {
if (error.wasDueToBlocking()) {
return successHandler(error.partialData);
} else {
// handle the true error. Again, to avoid
// "breaking the promise chain", the code here
// should either be all-synchronous or return another promise.
}
}
Update 2 based on requirement to not create standalone named function/variables
I'm not entirely sure why you wouldn't want named functions, and the approach I'm about to show is perhaps a little strange. But conceivably you could do this, instead, with the success handler not doing anything other than chaining through to an actual common handler.
p.then(function(data) {
return {goOn: true, data: data};
}, function(error) {
if (error.wasDueToBlocking()) {
return {goOn: true, data: error.partialData};
} else {
// Do some errorHandling logic
return {goOn: false};
}
}
.then(function(passedThroughData) {
if (passedThroughData.goOn) {
// Do something with it.
// Otherwise ignore, since error handler
// already took care of whatever it needed above
}
})
.then(function() {
// whatever you wanted to happen next
})
.catch(function(e) {
// catch-all handler just in case
})
I have a chain of promises, where I use catch to catch errors.
this.load()
.then(self.initialize)
.then(self.close)
.catch(function(error){
//error-handling
})
What function gets called if the chain has finished without rejection?
I was using finally, but this gets called also if an error occurs.
I'd like to call a function after the catch-function which gets only called if no promise was rejected.
I'm using node.js with the q - module.
Adding another then will do.
this.load()
.then(self.initialize)
.then(self.close)
.then(function() {
//Will be called if nothing is rejected
//for sending response or so
})
.catch(function(error){
//error-handling
})
I would change your .catch() to be a .then() and supply both an onFullfill and an onRejected handler. Then you can determine exactly which one happened and your code is very clear that one or the other will execute.
this.load()
.then(self.initialize)
.then(self.close)
.then(function() {
// success handling
}, function(error){
//error-handling
});
FYI, this isn't the only way to do things. You could also use .then(fn1).catch(fn2) which would similarly call fn1 or fn2 based on what the promise state was prior except that both could get called if fn1 returned a rejected promise or threw an exception because that would also be handled by fn2.
In angular code, I have a chained promise like this:
// a function in LoaderService module.
var ensureTypesLoaded= function(){
return loadContainerTypes($scope).then(loadSampleTypes($scope)).then(loadProjectList($scope)).then(loadSubjectTypes($scope));
}
Each of these functions return a promise, that loads things from a resource and additionally modifies $scope on error and success, e.g.:
var loadProjectList = function ($scope) {
// getAll calls inside a resource method and returns promise.
return ProjectService.getAll().then(
function (items) {
// succesfull load
console.log("Promise 1 resolved");
$scope.projectList = items;
}, function () {
// Error happened
CommonService.setStatus($scope, 'Error!');
});
};
I intent to use in in a code in a controller initialziation as such:
// Part of page's controller initialization code
LoaderService.ensureTypesLoaded($scope).then(function () {
// do something to scope when successes
console.log("I will do something here after all promises resolve");
}, function () {
// do something when error
});
However, this does not work as I'd like to. Ideally message "I will do something here after all promises resolve" must appear after all promises resolve. Instead, I can see that it appears earlier than messages from resolved promises within functions that are listed ensureTypesLoaded.
I would like to create a function ensureTypesLoaded such, that:
it returns a promise that is resolved when all chained loads are resolved;
if any of "internal" promises fail, the function should not proceed to next call, but return rejected promise.
obviously, if I call ensureTypesLoaded().then(...), the things in then() must be called after everything inside ensureTypesLoaded is resolved.
Please help me with correct building of chained promises.
I thinks that problem is in your loadProjectList function. Because .then() should receive function which is called at resultion time. Usually is function returning chain promise.
But in your case you call all load immediately in parallel. It's little complicated with $scope passing. But I think your code should appear like this
//this fn is called immediatly on chain creation
var loadProjectList = function ($scope) {
//this fn is called when previous promise resolves
return function(data) {
//don't add error handling here
ProjectService.getAll().then(...)
}
}
This cause serial loading as you probably want. (just notice: for parallel excution in correct way $q.all is used)
Finally you should have error handler only in ensureTypesLoaded, not in each promise.
I’ve been seeing code that looks like:
myObj.doSome("task").then(function(env) {
// logic
});
Where does then() come from?
The traditional way to deal with asynchronous calls in JavaScript has been with callbacks.
Say we had to make three calls to the server, one after the other, to set up our
application. With callbacks, the code might look something like the following (assuming
a xhrGET function to make the server call):
// Fetch some server configuration
xhrGET('/api/server-config', function(config) {
// Fetch the user information, if he's logged in
xhrGET('/api/' + config.USER_END_POINT, function(user) {
// Fetch the items for the user
xhrGET('/api/' + user.id + '/items', function(items) {
// Actually display the items here
});
});
});
In this example, we first fetch the server configuration. Then based on that, we fetch
information about the current user, and then finally get the list of items for the current
user. Each xhrGET call takes a callback function that is executed when the server
responds.
Now of course the more levels of nesting we have, the harder the code is to read, debug,
maintain, upgrade, and basically work with. This is generally known as callback hell.
Also, if we needed to handle errors, we need to possibly pass in another function to each
xhrGET call to tell it what it needs to do in case of an error. If we wanted to have just one
common error handler, that is not possible.
The Promise API was designed to solve this nesting problem and the
problem of error handling.
The Promise API proposes the following:
Each asynchronous task will return a promise object.
Each promise object will have a then function that can take two arguments, a success
handler and an error handler.
The success or the error handler in the then function will be called only once, after
the asynchronous task finishes.
The then function will also return a promise, to allow chaining multiple calls.
Each handler (success or error) can return a value, which will be passed to the next
function as an argument, in the chain of promises.
If a handler returns a promise (makes another asynchronous request), then the next
handler (success or error) will be called only after that request is finished.
So the previous example code might translate to something like the following, using
promises and the $http service(in AngularJs):
$http.get('/api/server-config').then(
function(configResponse) {
return $http.get('/api/' + configResponse.data.USER_END_POINT);
}
).then(
function(userResponse) {
return $http.get('/api/' + userResponse.data.id + '/items');
}
).then(
function(itemResponse) {
// Display items here
},
function(error) {
// Common error handling
}
);
Propagating Success and Error
Chaining promises is a very powerful technique that allows us to accomplish a lot of
functionality, like having a service make a server call, do some postprocessing of the
data, and then return the processed data to the controller. But when we work with
promise chains, there are a few things we need to keep in mind.
Consider the following hypothetical promise chain with three promises, P1, P2, and P3.
Each promise has a success handler and an error handler, so S1 and E1 for P1, S2 and
E2 for P2, and S3 and E3 for P3:
xhrCall()
.then(S1, E1) //P1
.then(S2, E2) //P2
.then(S3, E3) //P3
In the normal flow of things, where there are no errors, the application would flow
through S1, S2, and finally, S3. But in real life, things are never that smooth. P1 might
encounter an error, or P2 might encounter an error, triggering E1 or E2.
Consider the following cases:
• We receive a successful response from the server in P1, but the data returned is not
correct, or there is no data available on the server (think empty array). In such a
case, for the next promise P2, it should trigger the error handler E2.
• We receive an error for promise P2, triggering E2. But inside the handler, we have
data from the cache, ensuring that the application can load as normal. In that case,
we might want to ensure that after E2, S3 is called.
So each time we write a success or an error handler, we need to make a call—given our
current function, is this promise a success or a failure for the next handler in the promise
chain?
If we want to trigger the success handler for the next promise in the chain, we can just
return a value from the success or the error handler
If, on the other hand, we want to trigger the error handler for the next promise in the
chain, we can do that using a deferred object and calling its reject() method
Now What is deferred object?
Deferred objects in jQuery represents a unit of work that will be
completed later, typically asynchronously. Once the unit of work
completes, the deferred object can be set to resolved or failed.
A deferred object contains a promise object. Via the promise object
you can specify what is to happen when the unit of work completes. You
do so by setting callback functions on the promise object.
Deferred objects in Jquery : https://api.jquery.com/jquery.deferred/
Deferred objects in AngularJs : https://docs.angularjs.org/api/ng/service/$q
then() function is related to "Javascript promises" that are used in some libraries or frameworks like jQuery or AngularJS.
A promise is a pattern for handling asynchronous operations. The promise allows you to call a method called "then" that lets you specify the function(s) to use as the callbacks.
For more information see: http://wildermuth.com/2013/8/3/JavaScript_Promises
And for Angular promises: http://liamkaufman.com/blog/2013/09/09/using-angularjs-promises/
As of ECMAScript6
The .then() method has been included with pure JavaScript with Promises.
From the Mozilla documentation:
The then() method returns a Promise. It takes two arguments: callback
functions for the success and failure cases of the Promise.
The Promise object, in turn, is defined as
The Promise object is used for deferred and asynchronous
computations. A Promise represents an operation that hasn't completed
yet, but is expected in the future.
That is, the Promise acts as a placeholder for a value that is not yet computed, but shall be resolved in the future. And the .then() function is used to associate the functions to be invoked on the Promise when it is resolved - either as a success or a failure.
Before ECMAScript6
To my knowledge, there isn't a built-in then() method in javascript (at the time of this writing).
It appears that whatever it is that doSome("task") is returning has a method called then.
If you log the return result of doSome() to the console, you should be able to see the properties of what was returned.
console.log( myObj.doSome("task") ); // Expand the returned object in the
// console to see its properties.
Here is a thing I made for myself to clear out how things work. I guess others too can find this concrete example useful:
doit().then(function() { log('Now finally done!') });
log('---- But notice where this ends up!');
// For pedagogical reasons I originally wrote the following doit()-function so that
// it was clear that it is a promise. That way wasn't really a normal way to do
// it though, and therefore Slikts edited my answer. I therefore now want to remind
// you here that the return value of the following function is a promise, because
// it is an async function (every async function returns a promise).
async function doit() {
log('Calling someTimeConsumingThing');
await someTimeConsumingThing();
log('Ready with someTimeConsumingThing');
}
function someTimeConsumingThing() {
return new Promise(function(resolve,reject) {
setTimeout(resolve, 2000);
})
}
function log(txt) {
document.getElementById('msg').innerHTML += txt + '<br>'
}
<div id='msg'></div>
Here is a small JS_Fiddle.
then is a method callback stack which is available after a promise is resolved it is part of library like jQuery but now it is available in native JavaScript and below is the detail explanation how it works
You can do a Promise in native JavaScript : just like there are promises in jQuery, Every promise can be stacked and then can be called with Resolve and Reject callbacks, This is how you can chain asynchronous calls.
I forked and Edited from MSDN Docs on Battery charging status..
What this does is try to find out if user laptop or device is charging battery. then is called and you can do your work post success.
navigator
.getBattery()
.then(function(battery) {
var charging = battery.charging;
alert(charging);
})
.then(function(){alert("YeoMan : SINGH is King !!");});
Another es6 Example
function fetchAsync (url, timeout, onData, onError) {
…
}
let fetchPromised = (url, timeout) => {
return new Promise((resolve, reject) => {
fetchAsync(url, timeout, resolve, reject)
})
}
Promise.all([
fetchPromised("http://backend/foo.txt", 500),
fetchPromised("http://backend/bar.txt", 500),
fetchPromised("http://backend/baz.txt", 500)
]).then((data) => {
let [ foo, bar, baz ] = data
console.log(`success: foo=${foo} bar=${bar} baz=${baz}`)
}, (err) => {
console.log(`error: ${err}`)
})
Definition :: then is a method used to solve Asynchronous callbacks
this is introduced in ES6
Please find the proper documentation here Es6 Promises
.then returns a promise in async function.
Good Example would be:
var doSome = new Promise(function(resolve, reject){
resolve('I am doing something');
});
doSome.then(function(value){
console.log(value);
});
To add another logic to it, you can also add the reject('I am the rejected param') call the function and console.log it.
It's about the use of curly braces {} in our arrow functions:
Those 3 examples are doing the same thing (nothing, but have valid grammar, and are a valid Promise chain!)
new Promise(function(ok) {
ok(
/* myFunc1(param1, param2, ..) */
)
}).then(function(){
/* myFunc1 succeed */
/* Launch something else */
/* console.log(whateverparam1) */
/* myFunc2(whateverparam1, otherparam, ..) */
}).then(function(){
/* myFunc2 succeed */
/* Launch something else */
/* myFunc3(whatever38, ..) */
})
console.log("This code has no errors GG!")
The same logic using arrow functions shorthand without {}
new Promise((ok) =>
ok(
/* myFunc1(param1, param2, ..) */
).then(() =>
0 // HEY DID YOU NOTICE! A number that does nothing,
// but otherwise the parsing will fail!
// The code is pretty clean but have a major downside
// As arrow functions without {} can contains only one declaration
// console.log("something") will FAIL here
).then(() =>
"" // HEY DID YOU NOTICE! An empty string that does nothing,
// but otherwise the parsing will fail!
// As arrow functions without {} can contains only one declaration
// We can't add more code here, hence:
// console.log("something")
// Will break the whole promise
// This is likely the error in y(our) code ;)
))
console.log("This code has no errors GG!")
Arrow function with {}
new Promise( (ok) => {
ok(
/* myFunc1(param1, param2, ..) */
)
}).then( () => {
/* myFunc1 succeed */
/* Launch something else */
}).then( () => {
/* myFunc2 succeed */
/* Launch something else */
/* myFunc3(whatever38, ..) */
console.log("something")
/* More console logs! */
console.log("something else")
})
console.log("This code has no errors GG!")
I suspect doSome returns this, which is myObj, which also has a then method. Standard method chaining...
if doSome is not returning this, being the object on which doSome was executed, rest assured it is returning some object with a then method...
as #patrick points out, there is no then() for standard js
doSome("task")must be returning a promise object , and that promise always have a then function .So your code is just like this
promise.then(function(env) {
// logic
});
and you know this is just an ordinary call to member function .
In this case then() is a class method of the object returned by doSome() method.
The ".then()" function is wideley used for promised objects in Asynchoronus programming For Windows 8 Store Apps.
As far as i understood it works some way like a callback.
Find Details in this Documentantion
http://msdn.microsoft.com/en-us/library/windows/apps/hh700330.aspx
Of Cause it could also be the name for any other defined function.
I am about 8 years late, well...anyways, I don't really know what then() does but maybe MDN might have an answer. Actually, I might actually understand it a little more.
This will show you all the information (hopefully), you need. Unless someone already posted this link.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then
The format is promise.prototype.then()
The promise and prototype are kind of like variables but not like variables in javascript, I mean like other things go there like navigator.getBattery().then() where this one actually exists but is barely used on the web, this one shows statuses about the battery of the device, more information and more on MDN if you are curious.