Find undefine keys in plain object

keysUndefined

/**
* @param {object} obj plain object with { key: value }
* @param {array} keysToCheck List of keys to check for undefined values e.g. ['a','b','c']
* @example
*
*  const obj = { a: undefined, b: 1, c: 2, d: undefined }
*
*  console.log(keysUndefined(obj))
*  returns ['a','d' ]
**/

function keysUndefined( obj ) {
   return Object.keys(obj).filter(x => obj[x] === undefined)
}