function copyExistingProperties(src, dst) {
    Object.keys(dst).forEach(function(key) {
      if (dst.hasOwnProperty(key) && src.hasOwnProperty(key)) {
        dst[key] = src[key];
      }
    });
  }
Copies properties from source to dest only if a matching key is in dest

@param {Object.<string, ?>} src the source
@param {Object.<string, ?>} dst the dest