Understanding AngularJS Upgrade (ngUpgrade)

ngUpgrade is a powerful tool provided by the Angular team to help developers migrate from AngularJS (1.x) to Angular (2+) in a smooth and incremental manner. Migrating an entire application from AngularJS to Angular in one go can be a daunting and time-consuming task, especially for large-scale applications. ngUpgrade makes this process easier by allowing AngularJS and Angular code to coexist in the same application, enabling you to migrate the application module by module over time.

In this guide, we will explore what ngUpgrade is, how it works, and how you can use it to perform an incremental migration from AngularJS to Angular.

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Why Use ngUpgrade?

When migrating from AngularJS to Angular, several challenges arise, such as:

• Large Codebases: Migrating a large AngularJS application all at once can be difficult, error-prone, and time-consuming.
• Coexistence: During migration, you may need to run both AngularJS and Angular components together, and ngUpgrade allows you to do that seamlessly.
• Incremental Migration: You can migrate your AngularJS code to Angular gradually, module by module, instead of having to rewrite everything at once.

ngUpgrade solves these challenges by offering a way to run both frameworks side by side, while progressively upgrading your application.

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Key Concepts of ngUpgrade

Before diving into the details of how to use ngUpgrade, it’s important to understand some of the core concepts:

1. Hybrid Application: A hybrid app is one that uses both AngularJS and Angular code simultaneously. The idea behind ngUpgrade is to enable this hybrid approach where parts of the application are written in AngularJS, and other parts are written in Angular.
2. Upgrading AngularJS Components: You can upgrade AngularJS components to Angular components, allowing you to use Angular features while still leveraging your existing AngularJS code.
3. Downgrading Angular Components: Similarly, you can downgrade Angular components to AngularJS directives to allow Angular components to be used in AngularJS templates.
4. Common Dependency Injection (DI): ngUpgrade facilitates the sharing of services between AngularJS and Angular through a shared dependency injection system. This allows you to inject AngularJS services into Angular components and vice versa.
5. ngUpgrade Module: The ngUpgrade module helps in bootstrapping AngularJS and Angular together and provides an API for upgrading and downgrading components and services.

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How ngUpgrade Works

To understand how ngUpgrade works, let’s break it down into a series of steps you need to follow when migrating from AngularJS to Angular:

1. Bootstrapping the Application

When setting up a hybrid application, you need to bootstrap both AngularJS and Angular. You do this using the upgradeModule and downgradeModule functions provided by ngUpgrade.

• Upgrade Module: You create an Angular module that will upgrade your AngularJS components and make them available for use in your Angular code.
• Downgrade Module: Similarly, you can downgrade Angular components to be used in your AngularJS templates.

2. Creating Hybrid Components

With ngUpgrade, you can:

• Upgrade AngularJS components to Angular: You can use UpgradeComponent to upgrade an AngularJS component into an Angular component.
• Downgrade Angular components to AngularJS: You can use downgradeComponent to downgrade Angular components to AngularJS directives so that they can be used in AngularJS templates.

3. Sharing Services Between AngularJS and Angular

ngUpgrade makes it possible to share services between AngularJS and Angular code. By creating shared services, you can inject them into both AngularJS and Angular components.

• Shared services: You define services in Angular and AngularJS, and ngUpgrade will allow you to inject them into both frameworks seamlessly.

4. Using Angular Router in a Hybrid Application

You can use Angular’s Router in a hybrid app, but there are specific techniques for integrating AngularJS routes and Angular Router. ngUpgrade provides support to combine both routers, allowing you to move routes from AngularJS to Angular gradually.

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Steps to Implement ngUpgrade

Here’s a step-by-step guide on how to set up and use ngUpgrade for your migration:

1. Setting Up the Hybrid Application

1. Install Angular and ngUpgrade: To use ngUpgrade, you must install both Angular and the ngUpgrade library: bashCopyEditnpm install @angular/core @angular/common @angular/upgrade
2. Import Angular and AngularJS Modules: You need to import both the AngularJS module and the Angular module and use upgradeModule to initialize them together.
   import { NgModule } from '@angular/core'; import { BrowserModule } from '@angular/platform-browser'; import { UpgradeModule } from '@angular/upgrade/static'; import angular from 'angular'; // Angular Module @NgModule({ imports: [ BrowserModule, UpgradeModule ], providers: [], bootstrap: [] }) export class AppModule { constructor(private upgrade: UpgradeModule) {} } // AngularJS Module angular.module('myApp', []);

2. Upgrading AngularJS Components to Angular

To upgrade an AngularJS component to an Angular component, you can use the UpgradeComponent function.

1. Create an AngularJS Component: javascriptCopyEditangular.module('myApp').component('myComponent', { template: `<h1>{{ $ctrl.message }}</h1>`, controller: function() { this.message = 'Hello from AngularJS!'; } });
2. Upgrade the AngularJS Component: In the Angular application, use UpgradeComponent to upgrade the AngularJS component:
   import { UpgradeComponent } from '@angular/upgrade/static'; @Component({ selector: 'my-angular-component', template: '<my-component></my-component>' }) export class MyAngularComponent extends UpgradeComponent { constructor(elementRef: ElementRef, injector: Injector) { super('myComponent', elementRef, injector); } }

3. Downgrading Angular Components to AngularJS

If you want to use Angular components in AngularJS templates, you can downgrade them using the downgradeComponent function.

1. Create an Angular Component:
   @Component({ selector: 'app-hello', template: '<h1>Hello from Angular!</h1>' }) export class HelloComponent {}
2. Downgrade the Angular Component: Downgrade the component and register it as an AngularJS directive:
   import { downgradeComponent } from '@angular/upgrade/static'; angular.module('myApp').directive('appHello', downgradeComponent({ component: HelloComponent }));

4. Sharing Services Between AngularJS and Angular

To share services between AngularJS and Angular, you simply define a service in either framework and inject it into components of the other framework. Use UpgradeModule to share Angular services with AngularJS components.