Property 'firebase' does not exist on type { production: boolean; }

When you run ng build --prod angular-cli will use the environment.prod.ts file and your environment.prod.ts files environment variable doesn't have the firebase field hence you are getting the exception.

Add the field to environment.prod.ts

export const environment = {
  production: true,
  firebase: {
    apiKey: '...',
    authDomain: 'project.firebaseapp.com',
    databaseURL: 'https://project.firebaseio.com',
    projectId: 'project',
    storageBucket: 'project.appspot.com',
    messagingSenderId: '...',
  },
};

My approach is to merge common environment object with prod one. Here's my environment.prod.ts:

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

export const environment = {
  ...common,
  production: true
};

So common environment object acts as an overridable default for all other environments.

I hate duplicates in code
so let's create a separate file named environments/firebase.ts with content

export const firebase = {
    //...
};

the usage

import {firebase} from '../environments/firebase';
//...
AngularFireModule.initializeApp(firebase)

all is clear as for me

I got the same error because I had misspelled 'firebase' as 'firebas'

firebas: { apiKey: "...", authDomain: "project.firebaseapp.com", databaseURL: "https://project.firebaseio.com", projectId: "project", storageBucket: "project.appspot.com", messagingSenderId: "..." }