angular js ng book preview book

8/13/2019 Angular JS Ng Book Preview book

1/13


2/13


3/13

Tweet This Book!

Please help Ari Lerner by spreading the word about this book onTwitter!

The suggested tweet for this book is:

I just bought #ng-book, the beginners guide to AngularJS! Im ready to build advanced, modern

webapps!

The suggested hashtag for this book is#ngbook.

Find out what other people are saying about the book by clicking on this link to search for this

hashtag on Twitter:

https://twitter.com/search?q=#ngbook
https://twitter.com/search?q=%23ngbookhttps://twitter.com/search?q=%23ngbookhttp://twitter.com/


4/13

Also ByAri LernerThe Rails 4 Way

Riding Rails with AngularJS
http://leanpub.com/angularjs-railshttp://leanpub.com/tr4whttp://leanpub.com/u/auser


5/13

Contents

Promises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Whats a promise? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Why promises? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Promises in Angular . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

chaining requests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7


6/13

PromisesAngulars event system (we talk about this in-depth in theunder the hood) provides a lot of power

to our Angular apps. One of the most powerful features that it enables is automatic resolution of

promises.

Whats a promise?

Promises are a method of resolving a value or not in an asynchronous manner. Promises are objects

that represent the return value or a thrown exception that a function may eventually provide.

Promises are incredibly useful in dealing with remote objects and we can think of them as a proxy

for them.

Traditionally, javascript uses closures, or callbacks to respond with meaningful data that is not

available in a synchronous time, such as XHR requests after a page has loaded. Rather than

depending upon a callback to be fired, we can interact with the data instead as though it has already

returned.

Callbacks have worked for a long time, but the developer suffers a lot when using them. They have

no consistency in callbacks, no guaranteed call, steal code flow when callbacks depend upon other

callbacks, and in general make debugging incredibly difficult. At every step of the way, we have to

deal withexplicitlyhandling errors.Instead of fire-and-hopefully-get-a-callback-run when executing asynchronous methods, promises

offer a different abstraction. They return a promise object.

For example, in traditional callback code we might have a method that sends a message from one

user to one of their friends.

1 // Sample callback code

2 User.get(fromId, {

3 success: function(err, user) {

4 if (err) return {error: err};

5 user.friends.find(toId, function(err, friend) {

6 if (err) return {error: err};

7 user.sendMessage(friend, message, callback);

8 });

9 },

10 failure: function(err) {

11 return {error: err}
http://-/?-


7/13

Promises 2

12 }

13 });

This callback-pyramid is already getting out of hand and we havent included any robust error

handling code either. Additionally, well need to know the order in which the arguments are called

from within our callback.

The promised-based version of the previous code might look something closer to:

1 User.get(fromId)

2 .then(function(user) {

3 return user.friends.find(toId);

4 }, function(err) {

5 // We couldn't find the user

6 })

7 .then(function(friend) {

8 return user.sendMessage(friend, message);


10 // The user's friend resulted in an error

11 })

12 .then(function(success) {

13 // user was sent the message


15 // An error occurred

16 });

Not only is this more readable, but it is also much easier to grok. We can also guarantee that the

callback will resolve to a single value, rather than dealing with the callback interface.

Notice, in the first example we have to handle errors differently to non-errors and well need to

make sure that using callbacks to handle errors will all need to implement the same API (usually

with(err, data)being the usual method signature).

In the second example, we handle the success and error in the same way. Our resultant object will

receive the error in the usual manner. Additionally, since the promise API is specific about resolving

or rejecting promises, we dont have to worry about our methods implementing a different method

signature.

Why promises?

Although a bi-product of promises is the fact that we can get away from callback hell, the point of

promises is to make asynchronous functions look more like synchronous ones. With synchronous

functions, we can capture both return values and exception values as expected.


8/13

Promises 3

We can capture errors at any point of the process and bypass future code that relies upon the error

of that process. We do all this without thinking about the benefits of this synchronous code as its

the nature of it.

Thus, the point of using promises is to regain the ability to do functional composition and errorbubbling while maintaining the ability of the code to run asynchronously.

Promises arefirst-classobjects and carry with them a few guarantees:

Only one resolve or reject will ever be called

resolve will be called with a single fulfillment value

reject will only be called with a single rejection reason

If the promise has been resolved or rejected, any handlers depending upon them will still be

called

Handlers will always be called asynchronously

Additionally, we can also chain promises and allow the code to process as it will normally would

run. Exceptions from one promise bubble up through the entire promise chain.

They are always asynchronous, so we can use them in the flow of our code without worry that they

will block the rest of the app.

Promises in Angular

Angulars event-loop gives angular the unique ability to resolve promises in its $rootScope.$evalAsync

stage (seeunder the hood for more detail on the run loop). The promises will sit inert until the$digestrun loop finishes.

This allows for Angular to turn the results of a promise into the view without any extra work. It

enables us to assign the result of an XHR call directly to a property on a $scopeobject and think

nothing of it.

Lets build an example that will return a list of open pull requests for Angular JS from github.

Play with it

http://jsbin.com/UfotanA/3/edit
http://jsbin.com/UfotanA/3/edithttp://jsbin.com/UfotanA/3/edithttp://-/?-


9/13

Promises 4

1 Open Pull Requests for Angular JS

2

3

4

5 {{ pr.title }}

6

7

If we have a service that returns a promise (covered in-depth in theserviceschapter), we can simply

place the promise in the view and expect that Angular will resolve it for us:

1 angular.module('myApp', [])

2 .controller('DashboardCtrl', [

3 '$scope', 'UserService',

4 function($scope, UserService) {

5 // UserService's getFriends() method

6 // returns a promise

7 $scope.friends = User.getFriends(123);

8 }]);

When the asynchronous call to getPullRequests returns, the $scope.pullRequests value will

automaticallyupdate the view.

How to create a promise

In order to create a promise in Angular, well use the built-in $q service. The $q service provides a

few methods in its deferred API.

First, well need to inject the $q service into our object where we want to use it.


2 .factory('GithubService', ['$q', function($q) {

3 // Now we have access to the $q library

4 }]);

To create a deferred object, well call the method defer():

1 var deferred = $q.defer();

The deferred object exposes three methods and the single promiseproperty that can be used in

dealing with the promise.

resolve(value)

Theresolvefunction will resolve the deferred promise with the value.
http://-/?-


10/13

Promises 5

1 deferred.resolve({name: "Ari", username: "@auser"});

reject(reason)

This willrejectthe deferred promise with a reason. This is equivalent to resolving a promise with arejection

1 deferred.reject("Can't update user");

2 // Equivalent to

3 deferred.resolve($q.reject("Can't update user"));

notify(value)

This will respond with the status of a promises execution.

TODO: Add notify example

We can get access to the promise as a property on the deferred object:

1 deferred.promise

A full example of creating a function that responds with a promise might look similar to the following

method on theGithubServiceas mentioned above.


2 .factory('GithubService', [

3 '$q', '$http',

4 function($q, $http) {5 var getPullRequests = function() {

6 var deferred = $q.defer();

7 // Get list of open angular js pull requests from github

8 $http.get('https://api.github.com/repos/angular/angular.js/pulls')

9 .success(function(data) {

10 deferred.resolve(data);

11 })

12 .error(function(reason) {

13 deferred.reject(reason);

14 })

15 return deferred.promise;

16 }

17

18 return { // return factory object

19 getPullRequests: getPullRequests

20 };

21 }]);


11/13


12/13

Promises 7

Angulars $q deferred objects are chainable in that even then returns a promise. As soon as the

promise is resolved, the promise returned by thenis resolved or rejected.

These promise chains are how Angular can support $https interceptors.

The$q service is similar to the original Kris Kowals Q library:

1. $q is integrated with the angular $rootScope model, so resolutions and rejections happen

quickly inside of angular

2. $q promises are integrated with angulars templating engine which means that any promises

that are found in the views will be resolved/rejected in the view

3. $q is tiny and doesnt contain the full functionality of the Q library

chaining requests

TODO:

The$q library comes with several different useful methods.

all(promises)

If we have multiple promises that we want to combine into a single promise, then we can use the

$q.all(promises)method to combine them all into a single promise. This method takes a singleargument:

promises (array or object of promises)

Promises as an array or hash of promises

The all()method returns a single promise that will be resolved with an array or hash of values.

Each value will correspond to the promises at the same index/key in the promises hash. Ifanyof

the promises are resolved with a rejection, then the resulting promise will be rejected as well.

defer()

Thedefer()method creates a deferred object. It takes no parameters. It returns a new instance of a

single deferred object.


13/13

Promises 8

reject(reason)

This will create a promise that is resolved as rejected with a specific reason. This is specifically

designed to give us access to forwarding rejection in a chain of promises. This is akin tothrowin

javascript. In the same sense that we cancatchan exception in javascript and we can forward therejection, well need to rethrow the error. We can do this with $q.reject(reason).

This method takes a single parameter:

reason (constant, string, exception, object)

The reasons for the rejection.

Thisreject()method returns a promise that has already been resolved as rejected with the reason.

when(value)

The when()function wraps an object that might be a value then-able promise into a $q promise.

This allows for us to deal with an object that may or may not be a promise.

Thewhen()function takes a single parameter:

value

This is the value or a promise

Thewhen()function returns a promise that can be then used like any other promise.

angular js ng book preview book

Documents