|Initial release||October 20, 2010|
1.6.6 / August 18, 2017
|Platform||Cross-platform, see Legacy browser support|
|Size||144 KB production
1 MB development
AngularJS is built on the belief that declarative programming should be used to create user interfaces and connect software components, while imperative programming is better suited to defining an application's business logic. The framework adapts and extends traditional HTML to present dynamic content through two-way data-binding that allows for the automatic synchronization of models and views. As a result, AngularJS de-emphasizes explicit DOM manipulation with the goal of improving testability and performance.
AngularJS's design goals include:
Angular implements the MVC pattern to separate presentation, data, and logic components. Using dependency injection, Angular brings traditionally server-side services, such as view-dependent controllers, to client-side web applications. Consequently, much of the burden on the server can be reduced.
AngularJS uses the term "scope" in a manner akin to the fundamentals of computer science.
Scope in computer science describes when in the program a particular binding is valid. The ECMA-262 specification defines scope as: a lexical environment in which a Function object is executed in client-side web scripts; akin to how scope is defined in lambda calculus.
As a part of the "MVC" architecture, the scope forms the "Model", and all variables defined in the scope can be accessed by the "View" as well as the "Controller". The scope behaves as a glue and binds the "View" and the "Controller".
AngularJS directives allow the developer to specify custom and reusable HTML-like elements and attributes that define data bindings and the behavior of presentation components. Some of the most commonly used directives are:
ng-* attributes are not valid in HTML specifications,
data-ng-* can also be used as a prefix. For example, both
data-ng-app are valid in AngularJS.
<span ng-bind="name"></span>displays the value of 'name' inside the span element. Any change to the variable 'name' in the application's scope reflect instantly in the DOM.
ng-bind, but establishes a two-way data binding between the view and the scope.
AngularJS two-way data binding is its most notable feature, largely relieving the server backend of templating responsibilities. Instead, templates are rendered in plain HTML according to data contained in a scope defined in the model. The
$scope service in Angular detects changes to the model section and modifies HTML expressions in the view via a controller. Likewise, any alterations to the view are reflected in the model. This circumvents the need to actively manipulate the DOM and encourages bootstrapping and rapid prototyping of web applications. AngularJS detects changes in models by comparing the current values with values stored earlier in a process of dirty-checking, unlike Ember.js and Backbone.js that trigger listeners when the model values are changed.
$watch is angular method, for dirty checking. Any variable or expression assigned in $scope automatically sets up a $watchExpression in angular. So assigning a variable to $scope or using directives like ng-if, ng-show, ng-repeat etc all create watches in angular scope automatically. Angular maintains a simple array of $$watchers in the $scope objects
Different ways of defining a watcher in AngularJS.
<input ng-model="person.username" />
$digest is angular method, which is invoked internally by angularjs in frequent intervals. In $digest method, angular iterates overall $watches in its scope/child scoples.
$apply is a angular method, internally invokes $digest. This method is used when you want to tell angular manually start dirty checking (execute all $watches)
$destroy is both a method and event in angularjs. $destroy method, removes a scope and all its children from dirty checking. $destroy event is called by angular when ever a $scope or $controller is destroyed.
AngularJS was originally developed in 2009 by Mi?ko Hevery at Brat Tech LLC as the software behind an online JSON storage service, that would have been priced by the megabyte, for easy-to-make applications for the enterprise. This venture was located at the web domain "GetAngular.com", and had a few subscribers, before the two decided to abandon the business idea and release Angular as an open-source library.
This release added many of the concepts of Angular to AngularJS, including the concept of a component-based application architecture. This release among others removed the Sandbox, which many developers believed provided additional security, despite numerous vulnerabilities that had been discovered that bypassed the sandbox. 
The current stable release of AngularJS is 1.6.6.
Angular 2+ versions are simply called Angular. It is a TypeScript-based open-source front-end web application platform. Angular 4 was announced on 13 December 2016, skipping 3 to avoid a confusion due to the misalignment of the router package's version which was already distributed as v3.3.0.
In July 2012, the Angular team built an extension for the Google Chrome browser called Batarang, that improves the debugging experience for web applications built with Angular. The extension aims to allow for easy detection of performance bottlenecks and offers a GUI for debugging applications. For a time during late 2014 and early 2015, the extension was not compatible with recent releases (after v1.2.x) of Angular. It currently, as of late 2016, still works with all versions of Angular (although there have been no updates to the source code since March 2016).
AngularJS sets out the paradigm of a digest cycle. This cycle can be considered a loop, during which Angular checks if there is any change to all the variables watched by all the
$scopes. So, if
$scope.myVar is defined in a controller and this variable was marked for watching, Angular will monitor the changes on myVar in each loop iteration.
This approach potentially leads to slow rendering when AngularJS checks on too many variables in the
$scope every cycle. Hevery suggests keeping fewer than 2000 watchers on any page.
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