config_source.ts

/**
 * Create a configuration getter that reads from command line arguments, environment variables,
 * and a .env file in the priority.
 * @param args Result of deno std's [parseArgs](https://deno.land/std/cli/parse_args.ts)
 * @param getEnv Function to get environment variables
 * @param dotEnv Record from a .env file
 * @returns A function that returns the value for a configuration parameter
 */
export function configSource<T>(
  { args, getEnv, dotEnv }: {
    args: Readonly<T>;
    getEnv?: typeof Deno.env.get;
    dotEnv?: Readonly<Record<string, string>>;
  },
) {
  return function getConfig<R>(
    parameterName: R extends Exclude<keyof RemoveIndex<T>, "_"> ? R : never,
    environmentKey: string,
  ) {
    const name = parameterName;
    const key = environmentKey;
    return args[name] ?? getEnv?.(key) ?? dotEnv?.[key];
  };
}
  
type RemoveIndex<T> = {
  // https://stackoverflow.com/a/68261113/4197293
  // deno-lint-ignore ban-types
  [K in keyof T as {} extends Record<K, 1> ? never : K]: T[K];
};