/*
    Recursively merge properties of two objects 
    ONLY for properties that exist in obj1
*/
var obj1 = { x:'',z:'' };
function merge(obj1,obj2) {
    for( var p in obj2 )
        if( obj1.hasOwnProperty(p) )
            obj1[p] = typeof obj2[p] === 'object' ? merge(obj1[p],obj2[p]) : obj2[p];
    return obj1;
}
merge(obj1,obj2 );
console.dir( obj1 ); // { x:1,b:2 } }