environment.ts

declare var process: {
    env: {}
}

class EnvironmentConfig {
    name: string
    required_variable: string
    optional_variable: string | undefined

    constructor(env: any) {
        this.name = env.NODE_ENV || 'development'
        this.required_variable = env.REQUIRED_VARIABLE || undefined
        if (!this.required_variable) {
            throw new Error('REQUIRED_VARIABLE is not set')
        }
        this.optional_variable = env.OPTIONAL_VARIABLE
    }
}

export const ENV = new EnvironmentConfig(process.env)

example.ts

import { ENV } from './environment'

console.log(ENV.name);