We are considering Angular 8+ apps for this article.

Angular CLI projects already use a production environment variable to enable production mode when in the production environment at main.ts:

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

And you'll also notice that by default in the src/environment folder you have an environment file for development and one for production. Let's use this feature to allow us to use different API host URL depending if we're in development or production mode:

environment.ts:

export const environment = {
  production: false,
  apiHost: https://api.local.com
}

environment.prod.ts:

export const environment = {
  production: true,
  apiHost: https://api.production-url.com
};

And in our app.component.ts all we have to do in order to access the variable is the following:

import { Component } from '@angular/core';
import { environment } from '../environments/environment';

@Component({ ... })
export class AppComponent {
  apiHost: string = environment.apiHost;
}

Now in development mode the apiHost variable resolves to https://api.local.com and in production resolves to https://api.production-url.com. You may run ng build --prod and check.

Detecting Development Mode

Angular also provides us with an utility function called isDevMode that makes it easy to check if the app in running in dev mode:

import { Component, OnInit, isDevMode } from '@angular/core';

@Component({ ... })
export class AppComponent implements OnInit {
  ngOnInit() {
    if (isDevMode()) {
      console.log('Development!');
    } else {
      console.log('Cool. Production!');
    }
  }
}

Adding a Staging Environment

To add a new environment in Angular projects a new entry to configuration property should be added at angular.json file. Let's add a staging environment for example. Note that production property already exists.

"configurations": {
  "production": {
    "optimization": true,
    "outputHashing": "all",
    "sourceMap": false,
    "extractCss": true,
    "namedChunks": false,
    "aot": true,
    "extractLicenses": true,
    "vendorChunk": false,
    "buildOptimizer": true,
    "fileReplacements": [
      {
        "replace": "src/environments/environment.ts",
        "with": "src/environments/environment.prod.ts"
      }
    ]
  },
  "staging": {
    "optimization": true,
    "outputHashing": "all",
    "sourceMap": false,
    "extractCss": true,
    "namedChunks": false,
    "aot": true,
    "extractLicenses": true,
    "vendorChunk": false,
    "buildOptimizer": true,
    "fileReplacements": [
      {
        "replace": "src/environments/environment.ts",
        "with": "src/environments/environment.stating.ts"
      }
    ]
  }

And now we can add a staging environment file and suddenly be and build the project with ng build --configuration=staging on our CI (or deploy process) to deploy on staging environment:

environment.staging.ts

export const environment = {
  production: false,
  apiHost: https://staging.host.com
};