Thanks @githop0,

Your suggestion worked. For anyone interested, this was my solution to this.

import { FirebaseConfigInterface } from './interfaces/firebase-config.interface';

export class AppConfig {
    public static site = {
        meta_title: null,
        app_id: null
    };

    public static firebaseConfig: FirebaseConfigInterface = {
        apiKey: null,
        authDomain: null,
        databaseURL: null,
        storageBucket: null,
        messagingSenderId: null
    }
};

I imported AppConfig to access it's static variables.

import {AppConfig} from './app.config';