function(target, source) {
    var skeys = _.keys(source);
    _.each(skeys, function(skey) {
      if (!target[skey]) {
        target[skey] = source[skey];
      }
    });
    return target;
  }
function(target, source) {
    var skeys = _.keys(source);
    _.each(skeys, function(skey) {
      if (!target[skey]) {
        target[skey] = source[skey];
      }
    });
    return target;
  }
function ( target , source ) { var skeys = _ . keys ( source ) ; _ . each ( skeys , function ( skey ) { if ( ! target [ skey ] ) { target [ skey ] = source [ skey ] ; } } ) ; return target ; }
merge in any properties to target that are excelusively only in source
merge in any properties to target that are excelusively only in source