To run some initialization code before the Angular application starts

In Angular 8, you can execute code before the application boots up using the APP_INITIALIZER token and the APP_INITIALIZER provider. This allows you to run some initialization code before the Angular application starts.

Here’s an example of how you can call a function before the Angular application boots up:

Create a Function
Create a function that performs the initialization task you want to execute before the Angular app starts. For instance:

   import { Injectable } from '@angular/core';

   @Injectable({
     providedIn: 'root'
   })
   export class MyInitializationService {
     constructor() {}

     initializeApp(): Promise<any> {
       // Perform initialization tasks here
       return new Promise((resolve, reject) => {
         // Simulating asynchronous task, you can replace this with your logic
         setTimeout(() => {
           console.log('Initialization task completed');
           resolve();
         }, 2000); // Simulating a delay of 2 seconds
       });
     }
   }

Use APP_INITIALIZER in AppModule
Open your app.module.ts file and provide the function you want to execute before bootstrap using APP_INITIALIZER:

   import { NgModule, APP_INITIALIZER } from '@angular/core';
   import { BrowserModule } from '@angular/platform-browser';
   import { MyInitializationService } from './my-initialization.service';

   // ... other imports and declarations

   export function initializeApp(myInitializationService: MyInitializationService) {
     return () => myInitializationService.initializeApp();
   }

   @NgModule({
     declarations: [/* ... */],
     imports: [BrowserModule],
     providers: [
       MyInitializationService,
       {
         provide: APP_INITIALIZER,
         useFactory: initializeApp,
         deps: [MyInitializationService],
         multi: true
       }
     ],
     bootstrap: [/* ... */]
   })
   export class AppModule { }

In this example, MyInitializationService is an Injectable service that contains the initializeApp() method which performs the initialization tasks. Then, in the AppModule, APP_INITIALIZER is used to call this initialization function before the Angular app starts.

Remember to replace the initialization logic inside initializeApp() with your actual initialization tasks.

This setup ensures that your function (initializeApp() in this case) runs before the application bootstraps, allowing you to perform any required setup or tasks.

August 2, 2022

Arun Krishnan

Angular