Angular 5 - preload config file

In Angular 5, you can preload a configuration file before the application starts by using a combination of TypeScript and Angular's APP_INITIALIZER token. The idea is to read and load the configuration file during the application bootstrap process and make it available for other parts of the application to use.

Here's a step-by-step guide on how to preload a config file in Angular 5:

1. Create the Configuration File: Start by creating a configuration file that holds the settings you want to preload. For example, you can create a JSON file named config.json in the assets folder of your Angular project. The assets folder is typically used to store static files like images, fonts, and configuration files.

   Example config.json:

   jsonCopy code
   {
     "apiBaseUrl": "https://api.example.com",
     "apiKey": "your_api_key_here"
   }
   

2. Create a Configuration Service: Next, create a service in Angular that will be responsible for reading the config.json file and providing the configuration settings to other parts of the application. Generate a new Angular service using the Angular CLI:

   bashCopy code
   ng generate service config
   

3. Read the Configuration File in the Service: In the config.service.ts file, use Angular's HttpClient to read the config.json file during the initialization of the application.

   config.service.ts:

   typescriptCopy code
   import { Injectable } from '@angular/core';
   import { HttpClient } from '@angular/common/http';
   
   @Injectable({
     providedIn: 'root'
   })
   export class ConfigService {
     private config: any;
   
     constructor(private http: HttpClient) { }
   
     loadConfig(): Promise<any> {
       return this.http.get('/assets/config.json')
         .toPromise()
         .then(data => {
           this.config = data;
         })
         .catch(err => {
           console.error('Error loading configuration:', err);
         });
     }
   
     getConfig(): any {
       return this.config;
     }
   }
   

4. Use APP_INITIALIZER to Load the Config Before Bootstrap: Now, you can use the APP_INITIALIZER token to call the loadConfig method of the ConfigService before the application starts. This ensures that the configuration is loaded and available before any other parts of the application need it.

   In your app.module.ts, import the ConfigService and provide the APP_INITIALIZER token:

   typescriptCopy code
   import { NgModule, APP_INITIALIZER } from '@angular/core';
   import { HttpClientModule } from '@angular/common/http';
   import { BrowserModule } from '@angular/platform-browser';
   
   import { AppComponent } from './app.component';
   import { ConfigService } from './config.service';
   
   function loadConfig(configService: ConfigService) {
     return () => configService.loadConfig();
   }
   
   @NgModule({
     imports: [BrowserModule, HttpClientModule],
     declarations: [AppComponent],
     bootstrap: [AppComponent],
     providers: [
       ConfigService,
       {
         provide: APP_INITIALIZER,
         useFactory: loadConfig,
         deps: [ConfigService],
         multi: true
       }
     ]
   })
   export class AppModule { }
   

5. Access the Configuration Throughout the Application: Now that the configuration is loaded during the application bootstrap, you can access it anywhere in the application by injecting the ConfigService and using the getConfig method.

   For example, in your components or services, you can do the following:

   typescriptCopy code
   import { Component } from '@angular/core';
   import { ConfigService } from './config.service';
   
   @Component({
     selector: 'app-root',
     template: `
       <p>API Base URL: {{ config.apiBaseUrl }}</p>
       <p>API Key: {{ config.apiKey }}</p>
     `
   })
   export class AppComponent {
     config: any;
   
     constructor(private configService: ConfigService) {
       this.config = this.configService.getConfig();
     }
   }
   

With the above setup, your Angular 5 application will preload the configuration file and make it available throughout the application. This allows you to easily manage and use configuration settings from a centralized location.