Create a promise from a node async function.

promisify.ts

export interface Promisify {
  <T>(func: (callback: (err: any, result: T) => void) => void): () => Promise<T>;
  <T, A1>(func: (arg1: A1, callback: (err: any, result: T) => void) => void): (arg1: A1) => Promise<T>;
  <T, A1, A2>(func: (arg1: A1, arg2: A2, callback: (err: any, result: T) => void) => void): (arg1: A1, arg2: A2) => Promise<T>;
  <T, A1, A2, A3>(func: (arg1: A1, arg2: A2, arg3: A3, callback: (err: any, result: T) => void) => void): (arg1: A1, arg2: A2, arg3: A3) => Promise<T>;
  <T, A1, A2, A3, A4>(func: (arg1: A1, arg2: A2, arg3: A3, arg4: A4, callback: (err: any, result: T) => void) => void): (arg1: A1, arg2: A2, arg3: A3, arg4: A4) => Promise<T>;
  <T, A1, A2, A3, A4, A5>(func: (arg1: A1, arg2: A2, arg3: A3, arg4: A4, arg5: A5, callback: (err: any, result: T) => void) => void): (arg1: A1, arg2: A2, arg3: A3, arg4: A4, arg5: A5) => Promise<T>;
}
/**
 * @example: const rReadFile = promisify<Buffer, string>(fs.readFile);
 * 
 */
export const promisify: Promisify = function(func: any) {
  return (...args: any[]) => {
    return new Promise((resolve, reject) => {
      func(...args, (err: any, response: any) => {
        if (err) {
          return reject(err);
        }
        resolve(response);
      });
    });
  };
};

readFile.ts

import * as fs from 'fs';
import promisify from './promisify';

const filePath = '/things/stuff';
const readFilePromised = promisify<Buffer, string>(fs.readFile);

(async function stuff () {
  let fileContents = await readFilePromised(filePath);
})();