JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/lodash.js/2.1.0/lodash.min.js"></script>

JavaScript

var resp = {"findItemsAdvancedResponse":[
   {"ack":["Success"],
    "version":["1.13.0"],
    "timestamp":["2014-11-16T20:58:14.639Z"],
    "searchResult":[
     {"@count":"0"}],
    "paginationOutput":[
      {"pageNumber":["0"],
       "entriesPerPage":["100"],
       "totalPages":["0"],
       "totalEntries":["0"]}],
    "itemSearchURL":["http:\/\/www.ebay.co.uk\/sch\/i.html?&_nkw=mytest1"]
   }]
 };

var flatten = function( obj ) {
	var ret = {};

  if( String === obj.constructor || Number === obj.constructor ) {
    return obj;
  }

  for(var prop in obj) {
    if(!obj.hasOwnProperty(prop)) continue;
    
    if( String === obj[prop].constructor || Number === obj[prop].constructor ) {
    	ret[prop] = obj[prop];
      continue;
    }
    
    if( Object.prototype.toString.call( obj[prop] ) === '[object Array]' ) {
      if( obj[prop].length==0 )
        ret[prop] = null;
      if( obj[prop].length==1 && "0" in obj[prop] ) {
        ret[prop] = flatten(obj[prop][0]);
      } else {
        ret[prop] = flatten(obj[prop]);
      }
      continue; // skip below: all objects are Objects...!
    }
    
    if( Object === obj[prop].constructor ) {
      ret[prop] = flatten( obj[prop] );
      continue;
    }

  }
  return ret;
};

console.log( resp );
resp = flatten( resp );
console.log( resp );
console.log( resp.findItemsAdvancedResponse.ack );