Strategies for Upgrading from AngularJS to Angular

Anton Ioffe - November 28th 2023 - 9 minutes read

Embracing the evolution of JavaScript frameworks can be a monumental shift for any tech stack, but moving from AngularJS to Angular is a journey worth undertaking. In the forthcoming sections, we delve deep into the strategic considerations and technical manoeuvers required to streamline this transition. From laying the groundwork with a pre-migration strategy to orchestrating an incremental upgrade using ngUpgrade, and navigating complex hybrid routing techniques—the insights shared here will equip you with the finesse to tackle service layer integration and data management with precision. But the path is fraught with potential pitfalls; that's why we'll also unravel common mistakes and offer advanced tips for optimizing performance post-migration. Prepare to elevate your development prowess as we dissect, discuss, and decode the art and science behind upgrading your AngularJS applications to Angular, ensuring you emerge on the cutting edge of modern web development.

Assessing Readiness and Pre-Migration Strategy

Before undertaking the voyage from AngularJS to Angular, an essential first step is to assess the readiness of the existing application. Begin with a comprehensive analysis of your AngularJS codebase, focusing not only on the volume but also on the quality and structure of the code. Look for aspects that might complicate the migration, such as deeply nested scopes, heavy reliance on $scope, or outdated libraries that may not have direct equivalents in Angular. Such scrutiny enables developers to spotlight technical debt and other areas requiring attention during the upgrade.

Understanding the architectural differences between AngularJS and Angular is paramount. Recognize that AngularJS employs a more straightforward, controller-based architecture, while Angular utilizes a component-based architecture that aligns with modern web development practices. Grasping these differences will play a vital role in mapping out the new architecture of the application and will guide developers in reconceptualizing solutions when re-implementing features.

Setting up a TypeScript environment is central to the pre-migration strategy. While Angular can operate with languages like JavaScript and Dart, TypeScript has become the de facto language for Angular development because it provides strong typing and object-oriented features that are advantageous for larger codebases. To set up TypeScript, begin by configuring the TypeScript compiler and integrating it with your current build process. Proceed by incrementally introducing TypeScript to your AngularJS application, ensuring interoperability with the use of the @types/angular package. Here’s how you might configure TypeScript in your tsconfig.json:

  "compilerOptions": {
    "outDir": "./dist/",
    "sourceMap": true,
    "strictNullChecks": true,
    "module": "es2015",
    "target": "es5",
    "allowJs": true
  "include": [
  "exclude": [

This incremental introduction to TypeScript lays a robust foundation for seamlessly leveraging Angular's full capabilities post-migration.

For clarity within this discussion, AngularJS refers to the initial version of the Angular framework developed by Google, often associated with version 1.x. It is well-known for its two-way data binding and directives. Conversely, Angular (version 2 and above) is the revamped platform and framework for building dynamic client-side applications, encompassing a TypeScript-based ecosystem and a component-based architecture that promotes improved code management and scalability.

To effectively prepare for migration, it's important to not only understand the current state of the AngularJS application but also to be well-versed in the architectural advancements introduced by Angular. Making strategic decisions regarding the introduction of TypeScript and establishing a development environment for it are fundamental steps in this preparation. Performing these actions will ensure that the subsequent migration process is streamlined, ultimately yielding a codebase that is maintainable and scalable.

Implementing Incremental Upgrades with ngUpgrade

Leveraging the ngUpgrade library to incorporate Angular within an existing AngularJS application paves the way for a smooth and manageable upgrade process. To initiate a hybrid application, it's imperative that both frameworks are bootstrapped within the bootstrap process. This simultaneous activation is nontrivial, given the innately disparate bootstrapping mechanisms between AngularJS, which is more dynamic, and Angular, which is statically compiled. Here's a practical example of bootstrapping a hybrid app, providing precise control over when each framework is initialized:

import { UpgradeModule } from '@angular/upgrade/static';

  imports: [
class AppModule {
  constructor(private upgrade: UpgradeModule) { }
  ngDoBootstrap() {
    this.upgrade.bootstrap(document.body, ['angularJsApp'], { strictDi: true });

In this scenario, the AngularJS module, here referred to as 'angularJsApp', is initialized within the Angular environment through the UpgradeModule. By invoking bootstrap on UpgradeModule instead of relying on automatic bootstrap, you gain explicit control, which is critical for complex migrations.

Addressing performance considerations during this incremental migration is pivotal. While running both AngularJS and Angular in tandem, it's easy to encounter performance bottlenecks due to the duplication of change detection loops, $digest cycles in AngularJS, and Angular's change detection mechanism. To mitigate this, developers should avoid intermixing AngularJS and Angular components more than necessary and apply cautious reliance on one-way data flow to prevent redundant checks.

Memory leaks stand as a substantial challenge during the upgrade process. With two frameworks in memory, the application's footprint expands and careful attention is required to prevent and address memory leak issues. This entails detaching AngularJS components properly once they're no longer in use, responsible deregistration of event listeners, and ensuring services under ngUpgrade are singleton in nature to prevent multiple instances that could result in memory leaks.

The application stability throughout the gradual upgrade is maintained when it is structured with clear boundaries between Angular and AngularJS components. Here's an example demonstrating a sound AngularJS-to-Angular component strategy using the downgradeModule:

import { downgradeComponent } from '@angular/upgrade/static';
import { Component } from '@angular/core';

  selector: 'my-angular-component',
  template: '<h1>Angular Component within AngularJS</h1>'
export class MyAngularComponent {}

    downgradeComponent({component: MyAngularComponent})

As the application progresses through the upgrade cycle, applying these practices of compartmentalization, memory management, and performance monitoring ensures that the hybrid state remains stable and efficient, ultimately guiding a seamless transition to a fully Angular platform.

Hybrid Routing Techniques for AngularJS-to-Angular Transition

When transitioning from AngularJS to Angular, managing routing is critical to ensure a smooth and cohesive user experience. A hybrid application may have parts of its UI handled by Angular while other parts are still under AngularJS's control. The Angular Router offers a feature-rich, powerful solution for managing application states and navigation in Angular, but integrating it with the legacy AngularJS routing can be complicated. In a hybrid application setup, both Angular and AngularJS define their own routes, potentially leading to clashes where both frameworks compete to handle the same route.

To handle routing in a hybrid environment effectively, we may begin by housing the roots of both Angular and AngularJS applications side by side within the same HTML template:

    <div ng-view></div> <!-- AngularJS routing outlet -->
    <app-root></app-root> <!-- Angular routing outlet -->

This setup implies a clear pathway for migration where the AngularJS ng-view directive still controls legacy routes while new routes are managed by Angular's <app-root> component.

One familiar but effective strategy is to employ UI-Router in a "hybrid" mode. UI-Router's compatibility with both AngularJS and Angular allows for a synchronized routing ecosystem. To prevent the recursive routing loops that occasionally occur due to the differences in URL parsing between AngularJS and Angular, a custom UrlSerializer may be employed within Angular to harmonize the URL serialization process.

import { UrlSerializer, DefaultUrlSerializer, UrlTree } from '@angular/router';

export class CustomUrlSerializer implements UrlSerializer {
    private defaultSerializer: DefaultUrlSerializer = new DefaultUrlSerializer();

    parse(url: string): UrlTree {
        // Custom parsing logic here
        return this.defaultSerializer.parse(url);

    serialize(tree: UrlTree): string {
        // Custom serialization logic here
        return this.defaultSerializer.serialize(tree);

Injecting this serializer into the routing configuration ensures Angular understands URLs coming from AngularJS routes. This becomes essential when navigating between screens controlled by different frameworks.

Over time, with UI-Router’s ‘hybrid’ setup or a custom UrlSerializer at work, a clear transition from old to new can be tracked. For example, documenting the reduction of AngularJS controllers routed through ng-view and the increase of Angular components handled by <app-root> provides a visual metric of the migration’s progress.

Through careful planning and the implementation of these hybrid routing techniques, developers can tackle the complex task of migrating from AngularJS to Angular with confidence. The end goal is always to achieve a sharp, clear transition while maintaining the best user experience, even through the chaos of change. By focusing on these strategies, we minimize disruption and pave the way for a future where Angular's powerful features take the application's performance and development experience to the next level.

Service Layer Integration and Data Management

Integrating AngularJS services with Angular's dependency injection system presents a unique set of challenges, particularly when it comes to managing and sharing state across the two frameworks. When upgrading services, developers have the option of creating a new Angular service and maintaining a duplicate AngularJS service for compatibility. As an example, an existing AngularJS service can be made available to Angular by using the upgradeStatic method from @angular/upgrade/static.

import { upgradeStatic } from '@angular/upgrade/static';
import { MyAngularService } from './my-angular-service';

angular.module('myModule').factory('myService', upgradeStatic(MyAngularService));

  providers: [
      provide: 'myService',
      useFactory: (i: Injector) => i.get(MyAngularService),
      deps: ['$injector']
export class AppModule {}

When downgrading an Angular service to be used in AngularJS, you create an AngularJS-compatible service by utilizing the downgradeInjectable function.

import { Injectable } from '@angular/core';
import { downgradeInjectable } from '@angular/upgrade/static';

  providedIn: 'root'
export class MyService {
  // Service code here

angular.module('myModule').factory('myService', downgradeInjectable(MyService));

In terms of memory management, maintaining two versions of the service can lead to increased memory footprint and potential bottlenecks. This is especially true if the services maintain significant internal state or have complex initialization processes. Developers must weigh the short-term benefits of temporary duplication against the added complexity of modifying services to function uniformly in both frameworks.

From a reusability perspective, downgrading Angular services allows the authoring of services with the modern, modular, and maintainable architecture of Angular. This supports future development but can lead to increased overhead in a scenario where AngularJS components heavily depend on shared mutable states, conflicting with Angular's change detection.

When establishing communication protocols between AngularJS and Angular, proponents of interoperability advise for services to be stateless whenever possible. This strategy decouples business logic from state management and reduces the need for synchronization between the frameworks. Rather than direct state manipulation, one should use observables or store patterns, encouraging a reactive programming paradigm compatible with Angular's ecosystem.

import { Injectable } from '@angular/core';
import { BehaviorSubject } from 'rxjs';
import { take } from 'rxjs/operators';

  providedIn: 'root'
export class SharedStateService {
  private state = new BehaviorSubject(initialState);
  public state$ = this.state.asObservable();

  updateState(newState): void {;

angular.module('myModule').factory('sharedState', ['sharedStateService', (sharedStateService) => {
  return {
    getState: () => sharedStateService.state$.pipe(take(1)).toPromise(),
    updateState: (newState) => sharedStateService.updateState(newState)

By focusing on these strategies—upgrading and downgrading services judiciously, managing memory effectively and establishing unambiguous communication protocols—one can ensure that the integration of services across AngularJS and Angular does not compromise performance, modularity, or future maintenance.

Addressing Common Pitfalls and Finalizing the Migration

Despite the comprehensive guides and strategies available, migrations can still encounter a variety of common coding mistakes. One such mistake occurs when developers inadvertently mix syntax when bringing AngularJS code into the Angular context. For instance, using $scope in services or components that should now utilize Angular's Dependency Injection can introduce bugs and confusion.

// Mistake: Using `$scope` in Angular component
  selector: 'my-component',
  template: `<h1>{{title}}</h1>`
export class MyComponent {
  constructor($scope) {
    // Incorrect use of `$scope` in Angular
    $scope.title = 'Welcome';

The correct approach is to leverage Angular's constructor for dependency injection and class properties for data binding:

// Correct: Using Angular's constructor and properties for data binding
  selector: 'my-component',
  template: `<h1>{{title}}</h1>`
export class MyComponent {
  title = 'Welcome';

  constructor() {
    // Code related to component initialization

Post-migration, performance optimization is crucial. After ensuring functional parity, focus on leveraging Angular's change detection strategies to minimize unnecessary checks. Prioritize trackBy functions in ngFor directives and use pure pipes to limit the number of recalculations.

Once the application runs smoothly on Angular, developers must formulate a strategy for decommissioning AngularJS code parts. Gradually diminishing the old codebase can be achieved by mapping and removing AngularJS components no longer in use, effectively reducing the application's complexity and potential performance bottlenecks.

As the cleanup proceeds and the Angular application becomes the primary codebase, reflect on the improvements in terms of maintaining and scaling your application. Pose questions such as: "How much has the development experience improved post-migration?" or "What Angular features can we now leverage to further optimize our application?"

Finally, evaluate the team's approach to the upgrade process. Consider whether the chosen migration strategy was effective in minimizing downtime and whether the transition could have been managed differently, appreciating the complex dance between maintaining business continuity and technical renewal. This introspection is invaluable, as it enhances the team's capabilities for future technology shifts.


In this article on "Strategies for Upgrading from AngularJS to Angular," the author explores the process of transitioning from AngularJS to Angular and provides valuable insights and strategies for developers. The article covers essential steps such as assessing readiness, setting up a pre-migration strategy, implementing incremental upgrades using ngUpgrade, and adopting hybrid routing techniques. It also emphasizes the importance of service layer integration and data management, and addresses common pitfalls and tips for finalizing the migration. The article challenges readers to reflect on the improvements in terms of maintaining and scaling their applications post-migration and to evaluate their chosen migration strategy. A challenging technical task related to the topic could be to create a migration roadmap tailored to their specific AngularJS application, identifying potential challenges, mapping out the necessary steps, and setting timelines for each phase of the upgrade process.

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