Difference between AngularJS and Angular

Angular~4 min read

AngularJS (1.x) and Angular (2+) are two different frameworks that share a name but almost nothing else.

Theory

TL;DR

AngularJS (2010): JavaScript + controllers + $scope; Angular (2016): TypeScript + components + decorators
Main difference: $scope digest cycle vs Zone.js and a component tree
AngularJS official support ended December 2021; Angular is active, v18 shipped in 2024
Google rewrote Angular from scratch in 2016, it is not a version upgrade
Use AngularJS only for legacy maintenance; Angular for everything new

Quick Example

html

<!-- AngularJS 1.x: two-way binding via $scope -->
<div ng-app="app" ng-controller="MainCtrl">
  <input ng-model="name">
  <p>Hello {{ name }}!</p>
</div>

angular.module('app', []).controller('MainCtrl', function($scope) {
  $scope.name = 'World'; // $digest() watches every change
});

typescript

// Angular: component class, no $scope needed
@Component({
  selector: 'app-hello',
  template: `<p>Hello {{ name }}!</p>`
})
export class HelloComponent {
  name = 'World'; // TypeScript class property
}

In AngularJS, $scope is the shared object between controller and template. Angular replaces this with a class whose properties map directly to the template.

AngularJS binds a global $scope tree to templates and runs $digest() cycles to check every watcher for changes. It works for small apps but slows down once the watcher count grows. Angular compiles TypeScript ahead of time (AOT), builds a component tree, and uses Zone.js to know when async operations complete, then checks only the affected branch. With ChangeDetectionStrategy.OnPush, it skips branches where @Input has not changed. The result is 5-10x smaller bundles and noticeably faster rendering on large apps.

I've seen teams still running AngularJS in 2023 because migration kept getting deprioritized. The digest-loop overhead in those apps was consistently the top performance complaint, especially on dashboards with 500+ bound values.

When to Use

Old internal tool, no budget for migration: AngularJS (avoid adding new features)
New SPA, enterprise app, or mobile-first project: Angular
Team familiar with TypeScript: Angular (better IDE support and refactoring)
Already on AngularJS and need to scale: use ngUpgrade for incremental migration

Comparison Table

Characteristic	AngularJS (1.x)	Angular (2+)
Released	2010	2016 (full rewrite)
Language	JavaScript	TypeScript
Data binding	Two-way via `$scope`/digest	One-way `@Input`/`@Output`
Architecture	MVC, controllers, directives	Components, modules, services
CLI	None (manual bower/gulp)	Angular CLI (`ng generate`, `ng build`)
Mobile	Basic (PhoneGap)	Native-ready (Ionic, Cordova)
Performance	Digest loop slows on 1000+ watchers	AOT + lazy loading, OnPush detection
Support	Ended December 2021	Active, v18 in 2024
When to use	Legacy maintenance only	New projects, enterprise apps

How Angular Handles Change Detection

AngularJS parses HTML for ng-* directives, builds a $scope tree, then runs $digest() repeatedly until no watchers fire. Each iteration is O(n) over all watchers, and the loop can repeat up to 10 times per interaction. Angular takes a different path: the ngc compiler pre-builds component templates into JavaScript at build time. Zone.js patches browser APIs like setTimeout and addEventListener to signal when async work completes. Only then does Angular walk the affected part of the component tree.

Common Mistakes

Mutating $scope inside a native timer in AngularJS:

// Bad: no $digest triggered, UI stays stale
setTimeout(function() {
  $scope.items.push('new item');
});

// Fix: wrap in $apply
$scope.$apply(function() {
  $scope.items.push('new item');
});

Overusing [(ngModel)] in Angular forms:

html

<!-- Triggers full change detection on every keystroke -->
<input [(ngModel)]="user.name">

For forms with many fields, use reactive forms with FormControl and patchValue. Fewer checks, better performance under load.

Missing module imports in Angular:

typescript

@NgModule({
  declarations: [MyComponent]
  // missing: imports: [CommonModule, FormsModule]
})

*ngIf and *ngFor live in CommonModule. Forgetting the import gives a confusing template parse error that trips up many juniors.

Real-World Usage

Netflix migrated dashboard components from AngularJS to Angular for smaller bundle sizes
Google Workspace uses Angular modules for feature isolation across large teams
Delta Airlines uses Angular CLI for their booking SPA
IBM replaced AngularJS promise chains with Angular + RxJS for reactive forms

Follow-up Questions

Q: Why did Google rewrite Angular instead of updating AngularJS?
A: The $scope/digest design could not support AOT compilation or server-side rendering without a complete redesign. Starting fresh was the practical choice.

Q: What is the $digest cycle and why does it slow large apps?
A: It loops through all registered watchers, comparing current and previous values. If anything changed, it loops again, up to 10 times. With hundreds of watchers this becomes expensive on every user interaction.

Q: How does Zone.js replace $scope.$apply?
A: Zone.js patches setTimeout, Promise, and DOM events. Angular uses those patches to know when async work finishes and triggers change detection automatically. In AngularJS you had to call $apply manually when running code outside the framework.

Q: How do you migrate a large AngularJS app to Angular?
A: Use ngUpgrade to run both frameworks side by side. Migrate components to Angular one at a time. AngularJS and Angular components communicate during the transition via shared services.

Q: What are Angular signals and how do they change things?
A: Signals (stable in Angular 18) give fine-grained reactivity without Zone.js. A signal tracks which components depend on it and only re-renders those when the value changes, instead of diffing the full tree.

Examples

Hello World in Both Frameworks

html

<!-- AngularJS 1.x -->
<div ng-app="app" ng-controller="MainCtrl">
  <p>{{ message }}</p>
</div>
<script>
  angular.module('app', []).controller('MainCtrl', function($scope) {
    $scope.message = 'Hello from AngularJS'; // controller binds to $scope
  });
</script>

typescript

// Angular: class property replaces $scope
@Component({
  selector: 'app-root',
  template: `<p>{{ message }}</p>`
})
export class AppComponent {
  message = 'Hello from Angular';
}

Two syntaxes, two mental models. AngularJS connects controller to view through $scope. Angular uses a TypeScript class where properties become template data directly, with no shared mutable object in between.

Todo List: `ng-repeat` vs `*ngFor`

html

<!-- AngularJS: ng-repeat watched by $digest -->
<ul>
  <li ng-repeat="todo in todos">{{ todo.text }}</li>
</ul>

$scope.todos = [{text: 'Buy milk'}];
$scope.todos.push({text: 'Walk dog'}); // $digest detects array change, re-renders list

typescript

// Angular: trackBy prevents unnecessary DOM re-creation
@Component({
  template: `
    <ul>
      <li *ngFor="let todo of todos; trackBy: trackById">{{ todo.text }}</li>
    </ul>
    <input [(ngModel)]="newTodo" (keyup.enter)="add()">
  `
})
export class TodosComponent {
  todos = [{id: 1, text: 'Buy milk'}];
  newTodo = '';

  trackById(index: number, todo: {id: number}) { return todo.id; }

  add() {
    this.todos.push({id: Date.now(), text: this.newTodo});
    this.newTodo = '';
  }
}

trackBy tells Angular which <li> nodes to reuse when the list updates. Without it, Angular destroys and recreates every element on each change. In AngularJS, any array mutation triggers a full list re-render regardless.

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