extendRecursive.js

function extendRecursive() {
  const dst = {};
  let src;
  const args = [].splice.call(arguments, 0);
  const toString = ({}).toString;

  while (args.length > 0) {
    src = args.splice(0, 1)[0];
    if (toString.call(src) === '[object Object]') {
      Object.keys(src).forEach(p => {
        if (toString.call(src[p]) === '[object Object]') {
          dst[p] = extendRecursive(dst[p] || {}, src[p]);
        } else {
          dst[p] = src[p];
        }
      });
    }
  }
  return dst;
}