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How to return a promise in a function from the last promise in a chain of "then"

I'm writing a test using Selenium and JavaScript. I'm new to both, and also new to functional programming and promises. I'm trying to create a function that needs to do 3 things:
Click on an input
Clear the input
SendKeys to input
My current function does not work:
var clearAndSendKeys = function(driver, elementIdentifier, sendKeys) {
var returnValue;
driver.findElement(elementIdentifier).then(function(inputField){
inputField.click().then(function() {
inputField.clear().then(function() {
returnValue = inputField.sendKeys(sendKeys);
});
});
});
return returnValue;
}
The function would then be called as for example:
clearAndSendKeys(driver, webdriver.By.id('date_field'), '14.09.2015').then(function(){
//Do stuff
});
I expected the variable returnValue to contain the promise from sendKeys. However the function clearAndSendKeys returns the undefined variable before sendKeys is ran. I assume this is because returnValue was never defined as a promise, and so the program does not know that it needs to wait for sendKeys.
How can I make my function clearAndSendKeys return the promise from sendKeys? I'd rather avoid having to add a callback to the clearAndSendKeys function.
Edit: Removed .then({return data}) from the code as this was a typo.

You have to return each promise from the .then callback:
var clearAndSendKeys = function(driver, elementIdentifier, sendKeys) {
return driver.findElement(elementIdentifier).then(function(inputField){
return inputField.click().then(function() {
return inputField.clear().then(function() {
return inputField.sendKeys(sendKeys);
});
});
});
}
The promise returned by .then will resolve to the same value as the value returned from the callback.
See Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference for why your current code does not work. Promises are asynchronous.

First of all its probably not the best idea to nest promises, completely defeating their main purpose of eliminating callback hell. then callback can return Thenable object that allows to create nice chains of async operations.
In this case you just need to store reference to input field available as a result of the first async operation in the scope of the main function and then create chain of async operations that can be returned from this function.
var clearAndSendKeys = function(driver, elementIdentifier, sendKeys) {
var inputFieldRef;
return driver.findElement(elementIdentifier)
.then(function(inputField){
inputFieldRef = inputField;
return inputField.click();
}).then(function() {
return inputFieldRef.clear();
}).then(function() {
return inputFieldRef.sendKeys(sendKeys);
});
}

How to implement dependency between asynchronous functions in JavaScript?

As a simplified case, I have two async functions, foo and bar. bar needs the result of foo, i.e. bar depends on foo. I have no idea about which function will be called first.
If bar is invoked first, bar will call foo and start itself right after foo is done.
If foo is invoked first and done, bar can use the result of foo.
If foo is invoked first and bar is invoked before foo is done, bar needs to wait for foo's result. (Don't invoke a new call to foo, just wait for the already-fired call to foo)
How can I achieve this?
Is it possible to register an async function dependency chain (something like the dependency in require.js define['foo'], function() { bar(); })?
Can I use $.deferred() to achieve it?
How?

In circumstances like this, the standard approach is to cache the lower level promise.
Typically you will establish, in some suitable outer scope, a js plain object as a promise cache, and always look there first before calling your async process.
var promiseCache = {};
function foo() {
if(!promiseCache.foo) {
promiseCache.foo = doSomethingAsync();
}
return promiseCache.foo;
}
function bar() {
return foo().then(doSomethingElseAsync);
}
Of course, there's nothing to prevent you also caching the higher level promise, if appropriate.
function bar() {
if(!promiseCache.bar) {
promiseCache.bar = foo().then(doSomethingElseAsync);
}
return promiseCache.bar;
}
EDIT: forceRefresh feature
You can force a function to refresh its cached promise by passing an (extra) parameter.
function foo(any, number, of, other, arguments, forceRefresh) {
if(forceRefresh || !promiseCache.foo) {
promiseCache.foo = doSomethingAsync();
}
return promiseCache.foo;
}
By making forceRefresh the last argument, leaving it out is the same as passing false and foo will use the cached promise if available. Alternatively, pass true to guarantee that doSomethingAsync() be called and the cached value be refreshed.
EDIT 2: setName()/getName()
With the forceRefresh mechanism in place in getName() :
setName(newName).then(getName.bind(null, true)); //set new name then read it back using forceRefresh.
Alternatively, omit the forceRefresh mechanism and, assuming the cache property to be promiseCache.name :
setName(newName).then(function() {
promiseCache.name = $.when(newName);//update the cache with a simulated `getName()` promise.
});
The first method is more elegant, the second more efficient.

You can simply think of both functions as independent. That way, you don't go daisy-chaining dependencies that operate asynchronously. You can then have one other module that uses them.
Since they do async stuff, consider using promises. You can use jQuery's deferreds for compatibility. Think of deferreds as read/write while promises are read-only.
// foo.js
define(function(){
return function(){
return new Promise(function(resolve, reject){
// Do async stuff. Call resolve/reject accordingly
});
};
});
// bar.js
define(function(){
return function(){
return new Promise(function(resolve, reject){
// Do async stuff. Call resolve/reject accordingly
});
};
});
// Your code (Excuse the CommonJS format. Personal preference)
define(function(require){
// Require both functions
var foo = require('foo');
var bar = require('bar');
// Use them
foo(...).then(function(response){
return bar();
}).then(function(){
// all done
});;
});

Try creating an object property with possible values undefined , "pending" , true ; call deferred.resolve() when obj.active is true , deferred.reject() when obj.active is "pending"
var res = {
active: void 0
};
var foo = function foo(state) {
var t;
var deferred = function(type) {
return $.Deferred(function(dfd) {
if (res.active === "pending" || state && state === "pending") {
res.active = "pending";
dfd.rejectWith(res, [res.active])
} else {
res.active = state || "pending";
t = setInterval(function() {
console.log(res.active)
}, 100);
setTimeout(function() {
clearInterval(t)
res.active = true;
dfd.resolveWith(res, [res.active])
}, 3000);
}
return dfd.promise()
})
.then(function(state) {
console.log("foo value", state);
return state
}, function(err) {
console.log("foo status", err)
return err
})
}
return deferred()
}
var bar = function bar(result) {
var deferred = function(type) {
return $.Deferred(function(dfd) {
if (result && result === true) {
setTimeout(function() {
dfd.resolveWith(result, [true])
}, 1500)
} else {
dfd.rejectWith(res, [res.active || "pending"])
};
return dfd.promise()
})
}
return deferred().then(function(data) {
console.log("bar value", data);
}, function(err) {
console.log("bar status", err);
})
}
$("button").click(function() {
$(this).is(":first")
? foo().then(bar, bar)
: bar(res.active === true ? res.active : "pending")
.then(foo, foo).then(bar, bar)
})
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js">
</script>
<button>foo</button>
<button>bar</button>

Not sure I understood correctly the question. But here is my take at it:
Put you function foo into a variable
var foo_fn = function foo(foo_args){// Your async code goes here}
foo is async and returns something at some point. In your definition of foo, I recommend that you use promises, the concept is designed to manage composition of asynchronous functions in a clean and scalable way. jQuery implementation of the concept is convenient in a lot of simple use cases but suffers from some drawbacks which make it interesting for you at some point to use one of the many promises library which follow the Promises/A specification. For more information, you can refer to :
Cf. https://thewayofcode.wordpress.com/2013/01/22/javascript-promises-and-why-jquery-implementation-is-broken/ and https://blog.domenic.me/youre-missing-the-point-of-promises
so, say foo takes args, and returns a promise which later resolves into some value.
var foo_fn = function foo(foo_args) {
return foo_fn.promise = new RSVP.Promise (resolve, reject) {
// Your async code goes here
}
}
Here I use the RSVP promise library but any promise library following the Promises/A specification could do the job.
When bar is called, you can just do:
function bar (bar_args) {
var foo_promise = foo_fn.promise;
// if foo was called, whether the computation is in progress or finished,
// the foo_fn.promise field will be non-empty, as foo returns immediately
// with a promise anytime it is called
``
if (!foo.promise) {
// foo has not yet been called so call it
foo_promise = foo(foo_args);
}
foo_promise.then (function (foo_result) {/*some async code here*/})
}
NOTE : That solution is quite similar to the one proposed by Roamer-1888. One difference is that in Roamer proposal, the foo function will always return the same value after performing once its asyncronous computation. Don't know if this is the intended behaviour. In my implementation, foo executes the async. computation every time it is called. bar will use the latest computed value that is stored in the field foo_fn.promise. Older computations are lost, possible computation in progress is not taken into account.

If you are going to have this pattern often used in your code, you can also create a function working on the model of the define
function in require.js.
You will need :
a registry to hold the dependencies functions (foo in your example)
the dependant function (bar in your example) will need to accept the dependencies functions computed value as part of their signature. For example, a hash of the dependencies could be passed as first parameter, so bar signature could be: {foo: foo_result}, other_bar_args...
the dependencies function must follow the model of my previous answer, i.e. register their promise value as a property on themselves when they execute.
Reminder : you need to name those dependencies functions to reference them inside their body, and then add that object to the registry.
In the define function body, you wrap the dependent function into another one which :
Get all dependencies from the registry
Get all dependencies values, executing the dependencies when necessary (similarly to my previous answer). This means you end up having a list of promises, whose results you then congregate together (RSVP.hash for example with RSVP promise library). I believe jQuery has a similar function with jQuery.when
you call the dependent function (bar) with this hash of results as a first argument, other arguments being the same as the wrapped function
that wrapped function is the new bar, so when bar is called, it will be the wrapped function which will be called.
A bit lengthy but it should work. If you want to see some code, let me know if this is what you were looking for. In any case, if you are going to have complex async. in your code, it could be interesting for you to use a compliant promise library. $.deferred is also to be used only when you have nothing better at sight as it makes it harder for you to track the behaviour of your functions : you need to keep track of all places where this deferred appears to be able to reason about your program.

Promises: q.all() with mixed sync and async functions

I'm still learning about Promises and I'm facing a problem. Maybe you can help me.
I have a task which does some checks before running the real stuff. Some of those checks are sync, others are async.
So I'd like to have something like this:
q.all([
jenkins.checkConfig,
gitlab.checkConfig,
sonar.checkConfig
])
.then(
function() {
doSomethingReallyCoolHere();
}
);
But what if sonar.checkConfig is not a promise ? How can I give it a promise behavior ?
Currently I'm doing this
var checkConfig = function() {
var qChecked = q.defer();
var isOK = awesomeSyncTestHere();
if (isOK) {
qChecked.resolve();
}
else {
qChecked.reject();
}
return qChecked.promise;
}
But it looks stupid.
I guess that's not the good way to do it, right ?
Thanks a lot

There is no problem with placing non-promise values in the input array to Q.all. They will just be treated like a promise that had been fulfilled with them.
However, you need to put actual values (or promises) in the array, not the functions that would return them. Just go for
Q.all([
jenkins.checkConfig(),
gitlab.checkConfig(),
sonar.checkConfig()
]).then(doSomethingReallyCoolHere);
If you do want your checks to throw (and prevent the then callback from running), you will need to return a rejected promise indeed. You shouldn't use a deferred for that, though:
function checkConfig() {
if (awesomeSyncTestHere())
return Q(true);
else
return Q.reject(new Error("awesome fail"));
}
or, if you have a test that really throws, just use Q.try
function checkConfig() {
return Q.try(awesomeSyncTestHere);
}

Javascript how to execute code after for loop completes

I'm trying to work through this js/async scenario and i'm trying to know how the rest of the js world handles this.
function doStuff(callback) {
cursor.each(function(err, blahblah) {
...doing stuff here takes some time
});
... Execute this code ONLY after the `cursor.each` loop is finished
callback();
EDIT
Here's a more concrete example updated using most of the suggestions below which still doesn't work.
function doStuff(callback) {
MongoClient.connect(constants.mongoUrl, function(err, db) {
var collection = db.collection('cases2');
var cursor = collection.find();
var promises = []; // array for storing promises
cursor.each(function(err, item) {
console.log('inside each'); // NEVER GETS LOGGED UNLESS I COMMENT OUT THIS LINE: return Q.all(promises).then(callback(null, items));
var def = Q.defer(); // Create deferred object and store
promises.push(def.promise); // Its promise in the array
if(item == null) {
return def.resolve();
}
def.resolve(); // resolve the promise
});
console.log('items'); // ALWAYS GETS CALLED
console.log(items);
// IF I COMMENT THIS LINE OUT COMPLETELY,
// THE LOG STATEMENT INSIDE CURSOR.EACH ACTUALLY GETS LOGGED
return Q.all(promises).then(callback(null, items));
});
}

without using promises or any other dependencies/libraries you can simply
function doStuff(callback) {
add a counter
var cursor = new Array(); // init with some array data
var cursorTasks = cursor.length;
function cursorTaskComplete()
{
cursorTasks--;
if ( cursorTasks <= 0 ) {
// this gets get called after each task reported to be complete
callback();
}
}
for ( var i = 0; i < cursor.length; i++ ) {
...doing stuff here takes some time and does some async stuff
check after each async request
...when async operation is complete call
cursorTaskComplete()
}
}

Without knowing the details of the async calls you're making within the cursor.each loop, I shall assume that you have the ability to invoke a callback each time the functions invoked therein have completed their async task:
function doStuff() {
var promises = []; // array for storing promises
cursor.each(function(err, blahblah) {
var def = Q.defer(); // create deferred object and store
promises.push(def.promise); // its promise in the array
call_async_function(..., def.resolve); // resolve the promise in the async function's callback
});
// pass the array to Q.all, only when all are resolved will "callback" be called
return Q.all(promises);
}
and the usage then becomes:
doStuff().then(callback)
Note how the invocation of the callback now never touches the doStuff function - that function now also returns a promise. You can now register multiple callbacks, failure callbacks, etc, all without modifying doStuff. This is called "separation of concerns".
[NB: all the above based on the Q promises library - https://github.com/kriskowal/q]
EDIT further discussion and experimentation has determined that the .each call is itself async, and gives no indication to the outside when the last row has been seen. I've created a Gist that demonstrates a resolution to this problem.

if you want to do it with the async module, you can make use of the async forEachSeries function
Code snippet:
function doStuff(callback) {
async.forEachSeries(cursor, function(cursorSingleObj,callbackFromForEach){
//...do stuff which takes time
//this callback is to tell when everything gets over execute the next function
callbackFromForEach();
},function(){
//over here the execution of forEach gets over and then the main callback is called
callback();
});
}

In my mind an elegant/ideal solution would be to have something like
cursor.each(........).then( function() { ....your stuff});
But without that you can do this....UPDATED
http://plnkr.co/edit/27l7t5VLszBIW9eFW4Ip?p=preview
The gist of this is as shown below...notice....when
var doStuff = function(callback) {
cursor.forEach(function(cursorStep) {
var deferred = $q.defer();
var promise = deferred.promise;
allMyAsyncPromises.push(promise);
cursorStep.execFn(cursorStep.stepMeta);
promise.resolve;
});
$q.when(allMyAsyncPromises).then(callback);
}
After hitting the start button wait for few seconds...the async tasks have been simulated to finish in 5 seconds so the status will update accordingly.
Not having access to a real cursor object..I had to resort of fake cursor like and array.

Node.js asynchronous control flow with 3 processes working on shared data

I have node.js app with 3 functions, each one has own interval (or can by fired by another event):
processOrders
clearExpiredOrders
clearCancelledOrders
Functions are working on shared data, so only one of them can access this data at the time.
When there is a collision, it should wait until the actual function ends and then run second function immediately, but only once.
For example, if there is processOrders running and clearCancelledOrders is triggered 100 times, it will run clearCancelledOrders only once after processOrders finishes its work.
How would you solve this in node.js? Or do you have another idea how to solve this task?

Here is an example of how to do this very simply with promises:
var p = Promise.resolve(); // assuming Bluebird promises or Node 0.11.13+ with Promises.
// create a new empty resolved promise.
function doFirst(act){
return p = p.then(function(){
return someFirstAction(act); // assumes someFirstAction returns a promise
})
}
function doSecond(act){
return p p.then(function(){
return someSecondAction(act); // assumes someFirstAction returns a promise
})
}
What this does is queue the operations on the single chain. When it resolves the chain resolves. It also returns the returned promise, so you can unwrap it and get the value.
For example:
doFirst(1);
doSecond(2);
// some point in the future
doFirst(3).then(function(value){
// both doFirst(1) and doSecond(2) are done here
// the value argument is the resolution value of doFirst(3)
});
If you're unsure on how to convert your API to promises - see this question.
Since, you also want to limit the number of times a particular action is run, you can create special methods for it:
doFirst.queued = false;
function doFirst(act){
if(doFirst.queued) return; // already queued
doFirst.queued = true;
return p = p.then(function(){
return someFirstAction(act).finally(function(){
doFirst.queued = false;
});
})
}
doSecond.queued = false;
function doSecond(act){
if(doSecond.queued) return; // already queued
doSecond.queued = true;
return p = p.then(function(){
return someSecondAction(act); // assumes someFirstAction returns a promise
}).finally(function(){
doSecond.queued = false;
});
}