var a={"balance":"1","busId":"1","shoId":"1"}, b={"userid":"1"};

var c = Object.assign({}, a);
var bKeys = Object.keys(b);
for (var i=0; i var bKey = bKeys[i];
c[bKey] = b[bKey];
}

JSON.stringify(c);

这个是符合你需要的。