AppConfiguration.ts

import { config, DotenvConfigOutput, DotenvParseOutput } from "dotenv"
import { Err, Ok, Result } from "ts-results";

export class AppConfiguration {
  private static instance: AppConfiguration = new AppConfiguration()
  private static requiredEnvVars = [ "GATEWAY_NS", "GOOGLE_API_KEY", "MONGODB_URI", "SECRET_KEY"]
  private static envs = {}

  private constructor() {
    if(AppConfiguration.instance){
      throw new Error("Error: Instantiation failed: Use AppConfiguration.getInstance() instead of new.");
    }
    AppConfiguration.instance = this;
  }

  public static getInstance(): AppConfiguration {
    return AppConfiguration.instance;
  }

  private static confirmRequiredEnvsPresent(
    resultToCheck: DotenvConfigOutput,
  ): Result<DotenvParseOutput, Array<string>> {
    const { parsed: envs } = resultToCheck
    const errorMessages: Array<string> = []
    if (envs !== undefined) {  
      this.requiredEnvVars.forEach((envVar) => {
        if (!Object.prototype.hasOwnProperty.call(envs, envVar))
          errorMessages.push(`Missing ${envVar} in .env file`)
      })
      if (errorMessages.length === 0) return Ok(envs)
    } else errorMessages.push("Apparently no envs were loaded!")
    return Err(errorMessages)
  }

  public static setEnvVars(): void {
    const result = config()
    if (!result.error) {
      const checkEnvsResult = this.confirmRequiredEnvsPresent(result)
      if (checkEnvsResult.ok) {
        this.envs = checkEnvsResult.val
      } else {
        throw new Error(`Processing envs failed: ${checkEnvsResult.val}`)
      }
    } else throw new Error("Problem reading environment variables") 
  }
  
  public static getEnvVars(): Record<string, string | number> {
    return this.envs
  }
}