Javascript Promises returning strange object - javascript

(function() {
"use strict";
var storage = chrome.storage.sync;
var localStorage = null;
function getOutsideScope(property) {
if (localStorage.hasOwnProperty(property)) {
return localStorage[property];
}
}
function fulfill(data) {
return data;
}
function rejected(err) {
log(err);
}
window.app.storage = {
get: function(storageKey, storageProp) {
var promise = new Promise(function(fullfill, reject) {
chrome.storage.sync.get(storageKey, function(data) {
if (data.hasOwnProperty(storageKey)) {
fullfill(data[storageKey]);
} else {
reject(new Error(storageKey + " does not exist in the storage values."));
}
});
});
return promise.then(fulfill, rejected);
},
set: function(storageKey, storageItem) {
},
onChanged: function(fn) {
}
};
})();
So the above is my IIFE wrapper for chrome storage, and well the return is being a pain in the bleep. So I decided to give Promises a try, this is my first time so don't be too rough on me on this. Basically this is what I want to do
var property = app.storage.get("properties");
//property should equal "value"
//except it returns undefined
So adding the promises it does get the value except it returns this
Promise {[[PromiseStatus]]: "resolved", [[PromiseValue]]: "value"}
Am I doing something wrong with Promises I've tried reading HTML5Rocks, the MDN, and video tutorials except it doesn't really mention much of how to return a value. This does NOT work
get:function(storageKey,storageProp) {
chrome.storage.sync.get(storageKey,function(data) {
//for simplicity and no error checking -_-
return data[storageKey];
});
}

The function returns exactly what it is supposed to return: A promise. To get the value once the promise is fulfilled, you add a callback via .then:
app.storage.get("properties").then(function(property) {
// property will be "value"
});
From MDN:
A Promise represents a proxy for a value not necessarily known when the promise is created. It allows you to associate handlers to an asynchronous action's eventual success value or failure reason. This lets asynchronous methods return values like synchronous methods: instead of the final value, the asynchronous method returns a promise of having a value at some point in the future.
[...]
A pending promise can become either fulfilled with a value, or rejected with a reason (error). When either of these happens, the associated handlers queued up by a promise's then method are called.

Related

Strange/unexpected jQuery promise reject behaviour

I am experimenting some strange behaviours with jQuery promises on reject.
I have an array of promises, and I need to work with them when they all have been resolved/rejected.
To do so, I am using this:
var array_res = [];
array_res.push(promiseResolve('a'));
array_res.push(promiseReject('b'));
$.when.apply(null,array_res).always( function ( ) {
console.log(arguments);
//Work to do
} );
function promiseResolve (c) {
var promise = $.Deferred();
promise.resolve({a:c});
return promise;
}
function promiseReject (c) {
var promise = $.Deferred();
promise.reject({b:c});
return promise;
}
The issue is:
If I resolve both promises, everything works fine.
If I reject one of the promises, then the arguments come incomplete.
If I reject both of them, then the arguments come incomplete.
Here are 3 fiddles where you can check the behaviour:
https://jsfiddle.net/daepqzv1/1/
https://jsfiddle.net/daepqzv1/2/
https://jsfiddle.net/daepqzv1/3/
What I need is a way to get the arguments for both, rejected and resolved.
This is normal behavior for $.when(). If any promises you pass to $.when() reject, then $.when() will reject with the first reject reason that it finds. This is how it is coded.
This is similar to the way the ES6 Promise.all() works.
If you want all results, even if some promises reject, then you can use something like $.settle() or $.settleVal() that are defined in this code:
(function() {
function isPromise(p) {
return p && (typeof p === "object" || typeof p === "function") && typeof p.then === "function";
}
function wrapInPromise(p) {
if (!isPromise(p)) {
p = $.Deferred().resolve(p);
}
return p;
}
function PromiseInspection(fulfilled, val) {
return {
isFulfilled: function() {
return fulfilled;
}, isRejected: function() {
return !fulfilled;
}, isPending: function() {
// PromiseInspection objects created here are never pending
return false;
}, value: function() {
if (!fulfilled) {
throw new Error("Can't call .value() on a promise that is not fulfilled");
}
return val;
}, reason: function() {
if (fulfilled) {
throw new Error("Can't call .reason() on a promise that is fulfilled");
}
return val;
}
};
}
// pass either multiple promises as separate arguments or an array of promises
$.settle = function(p1) {
var args;
if (Array.isArray(p1)) {
args = p1;
} else {
args = Array.prototype.slice.call(arguments);
}
return $.when.apply($, args.map(function(p) {
// make sure p is a promise (it could be just a value)
p = wrapInPromise(p);
// Now we know for sure that p is a promise
// Make sure that the returned promise here is always resolved with a PromiseInspection object, never rejected
return p.then(function(val) {
return new PromiseInspection(true, val);
}, function(reason) {
// convert rejected promise into resolved promise by returning a resolved promised
// One could just return the promiseInspection object directly if jQuery was
// Promise spec compliant, but jQuery 1.x and 2.x are not so we have to take this extra step
return wrapInPromise(new PromiseInspection(false, reason));
});
})).then(function() {
// return an array of results which is just more convenient to work with
// than the separate arguments that $.when() would normally return
return Array.prototype.slice.call(arguments);
});
}
// simpler version that just converts any failed promises
// to a resolved value of what is passed in, so the caller can just skip
// any of those values in the returned values array
// Typically, the caller would pass in null or 0 or an empty object
$.settleVal = function(errorVal, p1) {
var args;
if (Array.isArray(p1)) {
args = p1;
} else {
args = Array.prototype.slice.call(arguments, 1);
}
return $.when.apply($, args.map(function(p) {
p = wrapInPromise(p);
return p.then(null, function(err) {
return wrapInPromise(errorVal);
});
}));
}
})();
$.settle() always resolves and it resolves with an array of PromiseInspection objects that you can then iterate over to see which promises resolved and which rejected and what the value or reason was.
$.settleVal() is a little simpler to iterate, but a little less generic because it doesn't give you the reject reason. It always resolves with an array where a reject will have a default value that you pass to it in the array in place of the resolved value.
FYI, both $.settle() and $.settleVal() can both be passed an array of promises like $.settle(arrayOfPromises) or multiple promise arguments as $.settle(p1, p2, p3) (as $.when() works). This saves having to use .apply() when you have an array of promises.

Possible Async Call in a Conditional

I'm refactoring some legacy code that I didn't originally write and I've come upon an issue with asynchronous data loading. The first time a particular modal is opened, a bunch of data representing a form object gets loaded. A function then cycles through the inputs of the form and fleshes them out as needed. It looks something like this (extremely simplified):
component.inputs.forEach(function(input) {
if (input.field == 'foo') {
input.cols = 5;
//etc.
}
if (input.field == 'bar') {
DataService.getBars().then(function(data){
data.forEach(function(e){
input.options.push(e.description);
});
};
}
if (input.field == 'baz') {
input.pattern = /regex/;
//etc.
}
});
return component;
The problem, of course, is that if my input.field is 'bar', the code continues running and hits the final return before the async call to DataService is resolved, so the first time the modal is opened, the input.options have not been filled out for 'bar' input.
Is it possible to make the code wait for the promise from the DataService to be resolved before continuing, or is there another way to handle the situation where in most cases the function is synchronous, but has to make an async call in only one case? Or have I shot myself in the foot by including an async call in this big chain of ifs?
One approach is to create a promise and attach it as a property to your returned object.
function getComponent() {
component.inputs.forEach(function(input) {
//create initial promise
var $promise = $q.when(input);
if (input.field == 'foo') {
input.cols = 5;
//etc.
}
if (input.field == 'bar') {
//chain from initial promise
$promise = $promise.then(function () {
//return promise for chaining
return getBarPromise(input);
});
}
//attach promise to input object
input.$promise = $promise;
});
var promises = [];
angular.forEach(inputs, function(input) {
promises.push(input.$promise);
});
//create composite promise
var $promise = $q.all(promises);
//final chain
$promise = $promise.then( function() {
//return component for chaining
return component;
});
//attach promise to component
component.$promise = $promise;
return component;
};
The returned component object will eventually be filled in with the results of the service calls. Functions that need to wait for completion of all the service calls can chain from the attached $promise property.
$scope.component = getComponent();
$scope.component.$promise.then( function (resolvedComponent) {
//open modal
}).catch( function(errorResponse) {
//log error response
});
Because calling the then method of a promise returns a new derived promise, it is easily possible to create a chain of promises. It is possible to create chains of any length and since a promise can be resolved with another promise (which will defer its resolution further), it is possible to pause/defer resolution of the promises at any point in the chain. This makes it possible to implement powerful APIs.1
If you want to stay with your existing code structure and make this work, you probably will need to use promises. You can also use javascript's map function. Note: you would need to inject $q into wherever you want to call this function.
function getComponent() {
var deferred = $q.defer(),
deferred2 = $q.defer(),
promises = component.inputs.map(function(input)) {
if (input.field == 'foo') {
input.cols = 5;
deferred2.resolve();
}
else if (input.field == 'bar') {
DataService.getBars().then(function(data) {
data.forEach(function(e){
input.options.push(e.description);
});
deferred2.resolve();
}).catch(function(err)) {
deferred2.reject(err);
});
}
else if (input.field == 'baz') {
input.pattern = /regex/;
deferred2.resolve();
}
return deferred2.promise;
});
$q.all(promises)
.then(function() {
deferred.resolve(component);
}).catch(function(err) {
deferred.reject(err);
});
return deferred.promise;
}
Once each input in component.inputs has been parsed appropriately, then the $q.all block will trigger and you can return your new component object.
Finally, to set your component object, simply do the following:
getComponent().then(function(result)) {
//Set component object here with result
$scope.component = result;
}).catch(function(err) {
// Handle error here
});

getting ".then() is not a function" error when using AngularJS

This is my JS:
self.obj = {}
self.obj.accessErrors = function(data) {
var cerrorMessages = [];
for (prop in data) {
if (data.hasOwnProperty(prop)){
if (data[prop] != null && data[prop].constructor == Object) {
self.obj.fetch[accessErrors](data[prop]);
}
else {
cerrorMessages.push(data[prop]);
}
}
}
return cerrorMessages;
};
self.obj.fetch = {
X: function() {
// do stuff.
},
Y: function(callback) {
$http.get('/yposts/').then(
function(response) {
self.posts = response.data;
callback(self.posts);
},
function(response) {
self.posts = {};
self.obj.accessErrors(response.data).then(function(cerrrorMessages) {
callback(posts, cerrorMessages);
});
}
);
}
};
And I am getting an error pointing to this line:
self.obj.accessErrors(response.data).then(function(cerrrorMessages) {
The error says:
TypeError: self.obj.accessErrors(...).then is not a function
Any idea how to solve this?
self.obj.accessErrors(response.data) does not return a promise so therefore, you can't use promise methods on it.
If you want it to return a promise and you want that promise to reflect when all the fetch() operations are done and those operations are actually async, then you will have to make all your async code into using promises and you will have to combine them all using Promise.all() or the angular equivalent and convert from using callbacks in fetch to just using a promise. Right now, you have a mix which is difficult to program with.
The .then() construction is only needed when using Promise objects - essentially, instead of returning a value, the function returns an object that resolves to a value at some point in the future (which is then passed into the function that you pass to .then().
But you are right in that you need an asynchronous pattern to do this correctly, since fetch.Y is an asynchronous method. A good thing to do would be to create an array of promises at the beginning of your accessErrors function, like so:
var fetchPromises = [];
and then replace self.obj.fetch[accessErrors](data[prop]); with something that calls push on that array to add the promise that fetch returns to it.
Then, instead of returning accessErrors, return Promise.all(fetchPromises).
This will require some fairly significant modification to your code, however - namely, you will need to rewrite it so that it uses the Promise API instead of this callback by itself (which shouldn't be too difficult to do).

deferred anti pattern of promises unclear example

from
https://github.com/petkaantonov/bluebird/wiki/Promise-anti-patterns
what promise is he talking about?
myApp.factory('Configurations', function (Restangular, MotorRestangular, $q) {
var getConfigurations = function () {
//Just return the promise we already have!
return MotorRestangular.all('Motors').getList().then(function (Motors) {
//Group by Cofig
var g = _.groupBy(Motors, 'configuration');
//Return the mapped array as the value of this promise
return _.map(g, function (m) {
return {
id: m[0].configuration,
configuration: m[0].configuration,
sizes: _.map(m, function (a) {
return a.sizeMm
})
}
});
});
};
return {
config: getConfigurations()
}
});
where is a promise? for me it looks more like it's an anti pattern to use his pattern. I cannot see any promise in this code apart from the word then nothing makes me think of a promise.
So what does return MotorRestangular... actually return?
One thing to remember is that, both the resolve and reject function return a promise. In other word, it's already promisified for you. Even if you don't explicitly return something from the resolve, what you get is a promise that has been resolved with a value of undefined.
In your example, MotorRestangular.all('Motors').getList() returns a promise and in the resolve function, the first param in then, another resolved promise is returned which will get the result of _.map function as input. It's like:
function YourCtrl(Configurations) {
Configurations.getConfiguration().then(function(resultOfMap) {
// use resultOfMap here
})
}
Note:
Don't get confused by resolved and onFulfilled. They are kinda the same but the former is for deferred object and the latter is from Promise specification.
MotorRestangular.all('Motors').getList()
is a promise returned by MotorRestangular. You can easily chain promises between methods whenever you just make the called function return a promise.

Cancelling promises

A user is able to make asynchronous calls by entering a value in a UI.
When the user changes the value in the UI, another async call is made - possibly before the callback supplied to the promise returned from the async call has been invoked.
The async API being used returns a Q based promise.
How can I cancel the original call gracefully, ensuring that even if the system returns with a value for the first call, the .then part of the promise is not invoked with that value (but that it is eventually invoked when the second async call completes)?
Back in the day I had a case like this (node.js) where I was either doing a web or a db request. I had a timeout object, which I think was cancellable (sry, don't remember the details) and had promises on both of them. So if the webpage returned first, I'd cancel the timeout and return the html. And if the timeout happened first, I updated a "timedout" field in some JSON to yes, so that if the web call ever returned, it would know just to die. It was a little mind-blowing, because with these promises, I could enter the function once, and actually return twice!
Hope that helps
-Tom
Absent some cancellation API, each async call will run to completion, but if all you're looking for is canceling the .then() functions, then that's doable. The most straight-forward way is a counter:
var counter = 0;
function onClick() {
counter++;
doAsyncCall().then(function(result) {
if (!--counter) {
myFunc(result);
}
});
}
But you asked for graceful, so maybe a more reusable construct would be something like this:
var idle;
function onClick(){
idle = Overtake(idle, doAsyncCall());
idle.then(myFunc).then(myOtherFunc);
}
which could be implemented like this:
function Overtaker(prior, callp) {
if (prior) {
prior.cancel();
}
var p;
p = new Promise(function(resolve, reject) {
callp.then(function(value) { return p && resolve(value); },
function(reason) { return p && reject(reason); });
});
this.cancel = function() { p = null; }
this.then = function(onFulfilled, onRejected) {
return p.then(onFulfilled, onRejected);
}
}
function Overtake(prior, p) {
return new Overtaker(prior, p);
}
I'm using ES6 promises, but Q promises seem similar, so hopefully this works there as well.

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