When you develop a bigger application, chances are quite high that you need some kind of configuration. That can range from simply visualizing the app's version number to injecting custom themes etc. In Angular you have different kind of approaches: compile-time and runtime configurations. Let's take a look at both of them.

Compile-time configuration

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What is compile-time configuration? It basically means that you compile your configuration into your app, at the time when you compile and bundle it. If you're using the Angular CLI there's already a preconfigured setup for having such compile-time configuration options.

Inside the environments folder you have a environment.ts and environment.prod.ts file.

// environment.ts
export const environment = {
  production: false
};

// environment.prod.ts
export const environment = {
  production: true
};

Since these are just plain JavaScript objects, you can add your environment specific properties on them.

The default main.ts file that is responsible for bootstrapping our Angular application uses those environment files to determine whether the app is running production mode, in order to apply some runtime optimizations such as calling enableProdMode().

import { enableProdMode } from '@angular/core';
import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';

import { AppModule } from './app/app.module';
import { environment } from './environments/environment';

if (environment.production) {
  enableProdMode();
}

platformBrowserDynamic().bootstrapModule(AppModule)
  .catch(err => console.log(err));

You can arbitrarily get a reference to that environment object by simply importing environment.ts into your file.

import { environment } from '../environment/environment';

// do something meaningful with `environment`
console.log(environment);

Note, we're always importing environment.ts and never an environment specific file such as environment.prod.ts. The reason is that at compile time, the Angular CLI will take care of renaming the environment specific configuration file into environment.ts and to compile it into your app accordingly.

You can also create new files, say for your "staging" environment. Just create a new environment.staging.ts and make sure to configure it in your angular.json file:

{
  // ...
  "projects": {
    "demo": {
      //...
      "architect": {
        "build": {
          "builder": "@angular-devkit/build-angular:browser",
          // ...
          "configurations": {
            "staging": {
              "fileReplacements": [
                {
                  "replace": "src/environments/environment.ts",
                  "with": "src/environments/environment.staging.ts"
                }
              ],
              //...
            },
            "production": {
              "fileReplacements": [
                {
                  "replace": "src/environments/environment.ts",
                  "with": "src/environments/environment.prod.ts"
                }
              ],
              //...
            }
          }
        },
        //...
      }
    }
  }
}

Finally, we need to tell the CLI which environment we're building for, s.t. it is able to pick up the proper configuration file. That happens in the scripts section of our package.json:

{
  ...
  "scripts": {
    "ng": "ng",
    "build": "ng build --prod",
    "build:staging": "ng build --prod --env=staging"
  },
  ...
}

Note, we're passing in the --env=<your-environment> flag. the --prod is a built-in command which automatically uses the production configuration already, if not specified otherwise. Moreover it also enables the AOT compilation.

Runtime configuration

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If however you need to be able to change your app's configuration settings or maybe you even expose them via an API, then you need to use a runtime configuration approach. Normally you simply have some JSON file - say appConfig.json that contains the necessary configuration options which you then deploy with your app on your server. When your app runs, you execute an HTTP request to that JSON file and read the settings.

What you want however, is to start your app only after your settings have been loaded and applied. A naive approach could be as follows. In your app.component.ts you simply add an *ngIf guard:

@Component({
    selector: 'app-root',
    template: `
        <div *ngIf="configLoaded">

        </div>
    `
})
export class AppComponent implements OnInit {
    configLoaded = false;

    ngOnInit() {
        this.http.get('./assets/appConfig.json')
            .subscribe(config => {
                // do something with your configuration
                ...

                this.configLoaded = true;
            });
    }
}

That way your other components wouldn't start unless the configLoaded is true and consequently the <div> is being shown.

While this would work, there's a more elegant way of doing things: we can hook into Angular's bootstrapping phase by using the APP_INITIALIZER token. We first create an Angular service that handles the fetching of our remote configuration...

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