// ...

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

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

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

export const environment = {
  production: false,
  apiKey: 'devKey'
};

export const environment = {
  production: true,
  apiKey: 'prodKey'
};

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

ng serve

ng serve --configuration=production

{
  // ...
  "projects": {
    "angular-environment-example": {
      // ...
      "prefix": "app",
        "build": {
          // ...
          "configurations": {
            "staging": {
              // ...
              "fileReplacements": [
                {
                  "replace": "src/environments/environment.ts",
                  "with": "src/environments/environment.stage.ts"
                }
              ],
              // ...
            },
            // ...
          },
        },
        "serve": {
          "builder": "@angular-devkit/build-angular:dev-server",
          "configurations": {
            "staging": {
              "browserTarget": "angular-environment-example:build:staging"
            },
          }
        },
      }
    }
  }
}

export const environment = {
  production: true,
  apiKey: 'stagingKey'
};

ng serve --configuration=staging