JsonStore.ts

import {promises as fs} from 'fs';
import path from 'path';

const JsonStore = <T>(filename: string, initial: T) => {
  const filePath = path.resolve(__dirname, filename);

  const api = {
    read: async (): Promise<T> => {
      if((await fs.stat(filePath)).isFile()) {
        return JSON.parse(await fs.readFile(filePath, 'utf8'));
      }
      
      return initial;
    },
    write: async (value: T) => {
      return fs.writeFile(filePath, JSON.stringify(value))
    },
    update: async (callback: (value: T) => T) => {
      const prev = await api.read();
      await api.write(callback(prev))
    }
  }

  return api;
}