code: function promisifyAll(sourceObject, keys){
      // promisified output
      const promisifiedFunctions = {};
  
      // process each object key
      for (const name of keys){
          // function exist in source object ?
          if (name in sourceObject && typeof sourceObject[name] === 'function'){
              // create promisified named version
              promisifiedFunctions[name] = _promisify(sourceObject[name], name);
          }
      }
  
      return promisifiedFunctions;
  }
docs: apply promisify function to all elements of sourceObject included in key list