AngularJS (1.x) and Angular (2+) share a name but are different frameworks. Angular (2+) is a rewrite with TypeScript, components, RxJS-first patterns, modern tooling, and a different rendering/change-detection model. Explain the practical differences and why migration is non-trivial.
What is the difference between Angular and AngularJS?
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Core distinction
AngularJS (1.x) = JavaScript + controllers/scopes + digest cycle (watchers).
Angular (2+) = TypeScript + components + dependency injection + RxJS + build-time tooling (CLI/compiler).
They are not backward-compatible; a real migration is usually a refactor/rewrite, not an upgrade.
Aspect | AngularJS (1.x) | Angular (2+) |
|---|---|---|
Language | JavaScript (ES5/ES6) | TypeScript-first (still outputs JS) |
UI model | Controllers + $scope | Components + Inputs/Outputs |
Change detection | Digest cycle + watchers per binding | Change detection over component tree (optimizable via OnPush, trackBy, etc.) |
Dependency injection | DI exists but older style | Modern DI system with providers, hierarchical injectors |
Templates | Directives + expressions like {{ }} | Declarative templates + structural directives like *ngIf/*ngFor |
Routing | Typically ngRoute/ui-router (external) | Built-in Angular Router |
Async patterns | Promises/callbacks (plus $q) | RxJS Observables are a core primitive |
Tooling/build | Manual/grunt/gulp-era setups | Angular CLI + compiler + bundlers/test integration |
Rendering/compilation | Runtime heavy; template compilation differs by mode | AOT/JIT options, modern compiler pipeline |
Status | Legacy / end-of-life ecosystem | Actively maintained framework |
Why Angular was a rewrite
AngularJS scaled poorly when apps grew: lots of watchers + digest cycles made performance and maintainability harder. Angular (2+) moved to a component model, stronger typing, and a modern compilation/tooling pipeline to make large apps more predictable.
What interviewers typically want to hear |
|---|
“Same name, different framework. Angular is a rewrite; not an upgrade.” |
“AngularJS: $scope + digest loop; Angular: component tree + change detection.” |
“Angular is TypeScript-first with strong DI + CLI + RxJS patterns.” |
“Migration is non-trivial: architecture changes (controllers→components, services, routing, forms, async model).” |
HTML
<!-- AngularJS: directives + controller + scope -->
<div ng-app="app" ng-controller="MainCtrl as vm">
<h1>{{ vm.message }}</h1>
</div>
TYPESCRIPT
// Angular (2+): component class + template
import { Component } from '@angular/core';
@Component({
selector: 'app-root',
template: `<h1>{{ message }}</h1>`,
})
export class AppComponent {
message = 'Hello';
}
Migration notes (real-world) |
|---|
Simple “upgrade” doesn’t exist; you either rewrite incrementally or build a hybrid app (ngUpgrade) for a period. |
Main shifts: controllers/scopes → components + services; digest watchers → change detection; promise-heavy code → observable-friendly APIs. |
Testing/tooling also changes: Karma/Jasmine setups may differ, CLI conventions become the default, and build pipelines change. |
Summary
AngularJS (1.x) is the legacy framework built around controllers/$scope and the digest cycle (watchers). Angular (2+) is a TypeScript-first, component-based rewrite with modern DI, RxJS-first async patterns, and CLI/compiler tooling. Same name, different architecture—migration is typically a refactor/rewrite rather than an “upgrade”.
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