AngularJS routing conflicts when upgrading to Angular

When upgrading from AngularJS to Angular (also known as ngUpgrade), one of the most common challenges developers face is routing conflicts between the two frameworks. Both AngularJS and Angular have their own routing mechanisms, and they need to coexist within the same application during the migration process.

In this guide, we’ll walk through the routing conflicts, how they arise, and how to resolve them while upgrading an AngularJS app to Angular, focusing on proper configuration of both routing systems.

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Step 1: Understanding AngularJS Routing and Angular Routing

AngularJS Routing (ngRoute or ui-router):

• AngularJS routing is typically managed by either ngRoute or ui-router.
• Both are client-side routing solutions, but ui-router is more feature-rich, supporting nested views and more complex routes.
• Routes are declared using $routeProvider (for ngRoute) or $stateProvider (for ui-router).

Example using $routeProvider:

angular.module('app', ['ngRoute'])
  .config(function($routeProvider) {
    $routeProvider.when('/home', {
      templateUrl: 'home.html',
      controller: 'HomeCtrl'
    }).otherwise({
      redirectTo: '/home'
    });
  });

Angular Routing:

• Angular’s routing is more flexible and powerful, based on @angular/router.
• Routes are defined using RouterModule.forRoot() in the AppModule and are used with PathMatch options, lazy loading, and child routes.

Example using Angular routing:

import { RouterModule, Routes } from '@angular/router';
import { HomeComponent } from './home/home.component';

const routes: Routes = [
  { path: 'home', component: HomeComponent },
  { path: '', redirectTo: '/home', pathMatch: 'full' }
];

@NgModule({
  imports: [RouterModule.forRoot(routes)],
  exports: [RouterModule]
})
export class AppModule {}

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Step 2: Identifying the Routing Conflict

When upgrading to Angular, there are two routing systems running simultaneously:

1. AngularJS Routing: Manages routes for the AngularJS part of the application.
2. Angular Router: Manages routes for the Angular part of the application.

Common Issues:

1. Route Conflict: Both routing systems may attempt to control the same URL paths, causing a conflict in navigation.
2. View Redraws: Angular’s router may trigger unwanted view redraws or router outlet changes when AngularJS routing is already in control.
3. Dependency Conflicts: Both AngularJS and Angular may need different configuration for shared dependencies (e.g., $location and Angular Router).

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Step 3: Configuring Dual Routing Systems

The key to resolving routing conflicts when upgrading from AngularJS to Angular is using ngUpgrade to make both routers work in harmony.

1. Use the ngUpgrade Library:

The ngUpgrade library helps you run AngularJS and Angular together in a hybrid app. One of its main jobs is to allow Angular to take over routing while still maintaining AngularJS routing.

In this setup, Angular’s Router will typically control the routing for the Angular parts of the app, and AngularJS’s routing system will control AngularJS views.

2. Load AngularJS Components Using ngUpgrade:

When bootstrapping your app, you need to wrap AngularJS components and configure routing accordingly.

import { UpgradeModule } from '@angular/upgrade/static';
import { AppComponent } from './app.component';
import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';

@NgModule({
  declarations: [AppComponent],
  imports: [UpgradeModule],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule {}

platformBrowserDynamic().bootstrapModule(AppModule)
  .then(platformRef => {
    const upgrade = platformRef.injector.get(UpgradeModule) as UpgradeModule;
    upgrade.bootstrap(document.body, ['angularJSApp'], { strictDi: true });
  });

3. Configuring Routing with ngUpgrade:

When configuring the routes, make sure to separate AngularJS routing from Angular routing. This is achieved by using Angular’s router for Angular parts of the app and AngularJS routing for AngularJS parts.

Example setup:

• Use Angular Router for Angular components.
• Use AngularJS $routeProvider or $stateProvider for AngularJS components.

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Step 4: Handling URL Synchronization Between AngularJS and Angular

You need to make sure that URL changes are properly synchronized between the two routing systems. This is important, as one router might update the URL without informing the other.

1. Use the $location Service in AngularJS:

The AngularJS $location service should be integrated with the Angular Router to ensure synchronization between Angular and AngularJS routes.

import { Location } from '@angular/common';

@Component({
  selector: 'app-root',
  template: `...`
})
export class AppComponent {
  constructor(private location: Location) {}

  ngOnInit() {
    // Sync URL changes between AngularJS and Angular routing
    window.addEventListener('popstate', () => {
      this.location.replaceState(window.location.pathname);
    });
  }
}

• The above code ensures that any URL changes by AngularJS are properly reflected in the Angular Router and vice versa.

2. Handle $state and RouterLink Coordination:

Ensure that AngularJS $state.go() and Angular Router’s RouterLink (or navigate()) coordinate correctly.

• Use $state (AngularJS’s router) for AngularJS navigation and Router.navigate() for Angular.
• Alternatively, you can use hybrid routing with ngUpgrade, where both can be used in parallel but require careful management of URLs.

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Step 5: Handling Redirection and Route Transitions

Routing conflicts can also occur when both routers try to handle redirection or route transitions simultaneously. To handle this, make sure to disable default behavior for one of the routers during a particular stage of the application.

For example:

• Disable AngularJS routing after the Angular Router has taken control of the application, or vice versa.
• Set default redirects to avoid unexpected behavior when both routers are active.

Example:

$routeProvider.when('/home', {
  templateUrl: 'home.html',
  controller: 'HomeCtrl'
}).otherwise({
  redirectTo: '/home'
});

In Angular, ensure that redirection logic (if any) is handled through Angular’s router, not AngularJS routing.

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Step 6: Handling Lazy Loading and Module Loading

If you are using lazy loading in Angular, make sure that the Angular router is able to load modules lazily without affecting AngularJS’s routing. This is especially true if AngularJS components are tightly coupled with Angular’s module system.

For hybrid apps, lazy-loaded modules should be isolated from AngularJS views to prevent cross-communication issues.

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Step 7: Testing and Debugging

Once you’ve configured your routing, it’s important to test and debug the routing flow:

1. Check for URL synchronization: Ensure that navigation between AngularJS and Angular views works smoothly, and URL paths are consistent.
2. Check browser dev tools: Use Chrome DevTools to inspect route changes, request headers, and check for potential issues in route transitions.
3. Test AngularJS views: Ensure that AngularJS views are loading and functioning as expected.
4. Test Angular components: Verify that Angular components are rendering and navigating properly with Angular’s router.

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