This question already has answers here:
Why are AngularJS $http success/error methods deprecated? Removed from v1.6?
(2 answers)
Is this a "Deferred Antipattern"?
(3 answers)
Closed 4 years ago.
I am currently using $q service from angular to make API calls like this:
var deferred = $q.defer();
$http.get(config.apiHost + details.url)
.success(function (data) {
deferred.resolve(data);
}).error(function (msg) {
deferred.reject(msg);
});
return deferred.promise;
but we can also use this approach also without using $q:
return $http.get(config.apiHost + details.url)
.success(function (data) {
return data;
}).error(function (msg) {
return msg;
});
and as $http itself returns the promise, I can also use more simplified approach:
$http.get(config.apiHost + 'posts')
.success(function (data) {
console.log(data)
}).error(function (msg) {
console.log(msg);
});
So what is the difference between all these specially between $q and $http, as both returns promise and both are async ? Does angular provides some additional functionality with $q ?
I am not able to find any good answer.
$http uses $q, the first example is redundant, and so is the second. You just need to return the promise that $http.get returns:
return $http.get(config.apiHost + details.url);
The above is the same as your first piece of code.
In addition, return msg is not the same as deferred.reject(msg). The equivalent would be throw msg or return $q.reject(msg)
Another thing to note is that success and error are non-standard, you want to use then and catch.
$q is mainly only used for compatibility with libraries that don't support promises by default and when you can't rely on a native implementation of Promise (for example - in older browsers like IE9). There's no reason (for you) to use it otherwise. An example would if you wanted to make a promise-based $timeout. $http itself uses $q under the hood for these exact reasons.
Unlike what other (since deleted) answers have suggested, you do not need to use $q in order to "store" the result of the $http promise. I would not recommend storing the promise at all (as this tends to lead to spaghetti code), but if you must absolutely do this, you can just store the resultant promise from $http; promises only ever execute once.
Any functions passed to then after a promise has resolved/rejected will be resolved on the next tick, without re-invoking the original action that created the promise in the first place - IOW, the result of the promise is memoized within that object.
Also note that promises chain, which is abit out of scope for this answer, but it essentially means that the following pieces of code are equivalent
function legacyGet() {
const deferred = $q.defer()
$http.get('http://www.google.com')
.then((response) => deferred.resolve(Object.assign(response, {foo: bar}))
.catch((error) => deferred.reject(error))
return deferred.defer
}
function modernGet() {
return $http.get('http://www.google.com')
.then((response) => Object.assign(response, {foo: bar}))
}
To summarise: Your title is wrong. We don't prefer using $q, we only use it sparingly. Prefer to use an ES6 Promise unless you need to support browsers that don't have it and you can't use a polyfill.
In angular mostly all the services returns promises only, but there are some instances where you would like to create your own deferred object using $q.
Case 1
When you are using a library which does not support promise or you have created your own function and want to return a promise.
Case 2
When you are using any construct which by default returns a promise but you want to return a separate promise based on some on some condition.
Example: In angular $http returns a promise only but now if you want that if the response of this promise contains a particular value then only you want to return resolved else return failure then you need to create your own deffered object and need to resolve or fail it based on the value returned by $http response.
Related
This question already has answers here:
AngularJS Promises, $q, defer
(2 answers)
Closed 5 years ago.
I am new to angularjs.I saw $q in restful api calls to check the promise.
$q.defer() was used to retain the promise object.
I read about the promises but I didn't get anything.
although I can make the api call without $q, however it is used somewhere in articles.
So I want to know the exact use of $q and difference in making api calls without $q.
Kindly help.
thanks
I think the article I wrote about $q might help you.
Introduction to $q
$q is an angular defined service. It’s the same as new Promise(). But $q takes things to the next level by enhancing additional feature that developers can use to perform complex tasks more simply.
This is a sample for creating a promise using $q
angular.module("app",[])
.controller("ctrl",function($scope,$q){
var work = "resolve";
var promise = $q(function(resolve, reject) {
if (work === "resolve") {
resolve('response 1!');
} else {
reject('Oops... something went wrong');
}
});
promise.then(function(data) {
alert(data)
})
})
$q.defer()
$q.defer() return the instance of the promise constructor. Once you create a defer object there are following methods and properties that you can access from that object
resolve(value) – resolves the derived promise with the value. If the value is a rejection constructed via $q.reject, the promise will be rejected instead.
reject(reason) – rejects the derived promise with the reason. This is equivalent to resolving it with a rejection constructed via $q.reject.
notify(value) - provides updates on the status of the promise's execution. This may be called multiple times before the promise is either resolved or rejected.
promise – {Promise} – promise object associated with this deferred
See the example
angular.module("app",[])
.controller("ctrl",function($scope,$q){
var work = "resolve";
function getData(){
var obj = $q.defer();
if (work === "resolve") {
obj.resolve('response 1!');
} else {
obj.reject('Oops... something went wrong');
}
return obj.promise;
}
getData().then(function(data) {
alert(data)
})
})
$q.all()
If a user need to send multiple request one shot,then the user can use $q.all() service.
$q.all([$http.get('data1.json'),$http.get('data2.json')])
.then(function(response){
console.log(response[0].data) // data1.json response
console.log(response[1].data) // data1.json response
})
In here,there are two http request sent simultaneously to two separate JSON files to get data. The response comes as an array and response order is same as the HTTP request order.
$q.race()
$q.race() is very similar to $q.all(). But instead of sending response of each request, it will only return the one request response. Specifically, only return the response of first request that been executed. That does not mean it’s not going to send other requests. All the requests are sending but it's only return the response of the first request that executed.
$q.race([$http.get('data1.json'),$http.get('data2.json')])
.then(function(response){
console.log(response[0].data) // return one response
})
In here response can be either data1.Json or data2.json. That's the downfall of using this method. Since its return the response of the first executed request, can’t be sure which request response will resolved by the promise. This method useful for bulk requests which you don’t want to see the response of all the requests
Conclusion
Use $q for constructing promises from non-promise Objects/callbacks, and utilize $q.all() and $q.race() to work with existing promises.
I like this question. Because, I too faced this.
This is a service that helps you run functions asynchronously, and use their return values when they are done processing.
Brief Description
Refer example
Promise with $q
Example :
app.service("githubService", function($http, $q) {
var deferred = $q.defer();
this.getAccount = function() {
return $http.get('https://api.github.com/users/haroldrv')
.then(function(response) {
// promise is fulfilled
deferred.resolve(response.data);
// promise is returned
return deferred.promise;
}, function(response) {
// the following line rejects the promise
deferred.reject(response);
// promise is returned
return deferred.promise;
});
};
});
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How do I return the response from an asynchronous call?
(41 answers)
Closed 3 years ago.
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I need to get the response.data out of the promise so it can be returned by the enclosing function. I know, I probably can't do it the way I've coded it because of normal JavaScript scope. Is there any way, it can be done?
The console.log at #1 produces the correct data. console.log #2 always produces 'a';
function addSiteParentId(nodeId) {
var theParentId = 'a';
var parentId = relationsManagerResource.GetParentId(nodeId)
.then(function(response){
theParentId = response.data;
console.log(theParentId); // #1
});
console.log(theParentId); // #2
return theParentId;
}
Any pointers would be appreciated.
One of the fundamental principles behind a promise is that it's handled asynchronously. This means that you cannot create a promise and then immediately use its result synchronously in your code (e.g. it's not possible to return the result of a promise from within the function that initiated the promise).
What you likely want to do instead is to return the entire promise itself. Then whatever function needs its result can call .then() on the promise, and the result will be there when the promise has been resolved.
Here is a resource from HTML5Rocks that goes over the lifecycle of a promise, and how its output is resolved asynchronously:
https://web.dev/promises/
I also don't like using a function to handle a property which has been resolved again and again in every controller and service. Seem I'm not alone :D
Don't tried to get result with a promise as a variable, of course no way. But I found and use a solution below to access to the result as a property.
Firstly, write result to a property of your service:
app.factory('your_factory',function(){
var theParentIdResult = null;
var factoryReturn = {
theParentId: theParentIdResult,
addSiteParentId : addSiteParentId
};
return factoryReturn;
function addSiteParentId(nodeId) {
var theParentId = 'a';
var parentId = relationsManagerResource.GetParentId(nodeId)
.then(function(response){
factoryReturn.theParentIdResult = response.data;
console.log(theParentId); // #1
});
}
})
Now, we just need to ensure that method addSiteParentId always be resolved before we accessed to property theParentId. We can achieve this by using some ways.
Use resolve in router method:
resolve: {
parentId: function (your_factory) {
your_factory.addSiteParentId();
}
}
then in controller and other services used in your router, just call your_factory.theParentId to get your property.
Referce here for more information:
http://odetocode.com/blogs/scott/archive/2014/05/20/using-resolve-in-angularjs-routes.aspx
Use run method of app to resolve your service.
app.run(function (your_factory) { your_factory.addSiteParentId(); })
Inject it in the first controller or services of the controller. In the controller we can call all required init services. Then all remain controllers as children of main controller can be accessed to this property normally as you want.
Chose your ways depend on your context depend on scope of your variable and reading frequency of your variable.
You have to return a promise instead of a variable.
So in your function just return:
return relationsManagerResource.GetParentId(nodeId)
And later resolve the returned promise.
Or you can make another deferred and resolve theParentId with it.
I don't know what is the exactly difference between AngularJS $q service and simply using .then() after async request.
Simple example with .then() :
function InboxService($http) {
this.getEmails = function getEmails() {
return $http.get('/emails');
};
}
And when using the service (just part of code):
InboxService.getEmails()
.then(function (response) {
// use response
});
What is the difference with $q service with resolve and reject ?
What is the difference with $q service with resolve and reject ?
I assume you are asking about the usage of var deferred = $q.defer() with subsequent deferred.resolve() or deferred.reject()? In this case, the answer is that you don't need it since you already have a promise object returned by $http service. In fact, manually constructing another new promise with $q is not recommended and considered an anti-pattern.
In cases where you work with asynchronous functions (timeouts, ajax-requests) that are not already wrapped into promise, then this is a case when you might want to use $q to create and return promise. But once again, in your case you don't need it as $http service constructs promise for you and one more is simply redundant.
The $q is superfluous and in most cases is not needed. http://www.codelord.net/2015/09/24/$q-dot-defer-youre-doing-it-wrong/
When I call $http.get from a factory it doesn't return the JSON object I need but a different one.
Here's my code:
var cart = angular.module('cart', []);
cart.factory('getItems',['$http', function($http){
return{
get:$http.get('index.php').
success(function(data){
return data;
})
}
}]);
cart.controller("TableCtrl", ['$scope', 'getItems', function($scope, getItems){
console.log(getItems.get);
}]);
now this is the JSON object that it should return:
[{"id":"1","name":"Cap"},{"id":"2","name":"Coat"},{"id":"3","name":"Shoes"}]
And this is what I get when I console.log
d {$$state: Object, success: function, error: function, then: function, catch: function…}
everything runs fine if I run the $http.get inside the controller, but if I place it in a factory it just doesn't.
It's returning a promise, which you need to resolve
In your controller..
getItems.get().then(function(response) {
console.log(response.data)
});
More information, see The Promise Api
Angular $q docs
$http.get returns a promise not the data itself. Your current use of the success function is not going to get data back.
In your factory:
return{ get: $http.get('myurl')};
Then consuming it should be:
myFactory.get().success(response){
var mydata = response.data;
}
Success is just an addition to the promise. I like to just use .then like standard, unadorned promises.
When I first heard about promises, I thought it meant you could use it without having to provide any sort of callback-like code on your consuming end. That is not true, but promises are still super awesome and very powerful. The coolest thing is that you can keep a promise around, and anybody can add a .then to the promise. Whoever does will get their code executed as soon as the asynchronous request finishes, or right away if it has already finished. So promises let you not have to worry about asynchronous behavior, but you still have to use what is basically its more powerful version of callbacks.
This question already has answers here:
Any way to know if a variable is an angularjs promise?
(4 answers)
Closed 8 years ago.
There are cases when I need to check if an object is a promise or not, e.g. to show a loading indicator when getting initial data from an API which might take a couple of seconds.
So far I'm using this code snippet:
if ($scope.data.$resolved === false){
// data is a promise as it is not resolved yet
}
else{
// data is not a promise as it either never was or has been resolved
}
This seems to work well but since I'm not an Angular expert (yet) I was wondering if there's a better / recommended way to check if an object is a promise. Thanks!
You don't need to know, if the promise is already resolved. If you pass your callback into the .then, this code will be executed, even if the promise is already resolved.
And it seems like a bad practice make your method return 2 different types.
A promise or the data directly.
Just always return a promise, even if the data is fetched from the cache.
Pass it into the .resolve, where ever the data did come from.
Something like this:
function getData(){
var data = getFromCache();
if( data ) {
var deferred = $q.defer(); //Make your own promise
deferred.resolve(data);
return deferred.promise;
}else{
return $http.get("someData.json") //Return the promise from the `$http.get`
.then(function(data){
return data;
});
}
}