Dfyjkt

var savedData = 
 "a": 
 "x": 
 "foo": 1,
 "foofoo": 11
 ,
 "y": 
 "bar": 2,
 "barbar": 22
 
 ,
 "b": 
 //...
 ,
 "c": 
 //...
 
;


var newData = 
 "a": 
 "x": 
 "foo": 7 //<== new value to be changed;
 //<== notice there were "foofoo" key here, we need to keep this key and does NOT remove it;
 ,
 "y": 
 "bar": 8 //<== new value to be changed;
 //<== notice there were "barbar" key here, we need to keep this key and does NOT remove it;
 ,
 "z": //<== this is a brand new child object to be added to the savedData;
 "baz": 9
 
 
;


updateSavedData(newData);


function updateSavedData(newData) 

 if (savedData) 
 Object.keys(savedData).forEach(function(savedKeyLevel1) 
 Object.keys(newData).forEach(function(newKeyLevel1) 

 if (savedKeyLevel1 === newKeyLevel1) 
 console.log('the key [' + savedKeyLevel1 + '] exist among the saved and the new data!');
 if (jQuery.type(savedData[savedKeyLevel1]) === "object") 
 console.log('the key [' + savedKeyLevel1 + '] is an object!');
 //start looping again, but this time in a deeper level of this child object...
 Object.keys(savedData[savedKeyLevel1]).forEach(function(savedKeyLevel2) 
 Object.keys(newData[newKeyLevel1]).forEach(function(newKeyLevel2) 
 if (savedKeyLevel2 === newKeyLevel2) 
 console.log('the key [' + savedKeyLevel1 + ' -> ' + savedKeyLevel2 + '] exist among the saved and the new data!');
 //...
 //<== my question is about what to do here?
 //if I don't know how much deeper the savedData is.
 else 
 //this is a brand new child object, add it to the tree;
 savedData[savedKeyLevel1][newKeyLevel2] = newData[newKeyLevel1][newKeyLevel2];
 
 );
 );

 
 else 
 //this is a brand new child object, add it to the tree;
 savedData[newKeyLevel1] = newData[newKeyLevel1];
 
 );
 );

 else 
 savedData = newData;
 

 console.log('The savedData after update is:n', JSON.stringify(savedData));

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

var savedData = "a": "x": "foo": 1,"foofoo": 11,"y": "bar": 2,"barbar": 22,"b": ,"c": ;
var newData = "a": "x": "foo": 7,"y": "bar": 8 ,"z": "baz": 9;
 
function merge(data, newdata)
 for (let key in newdata)
 if (typeof data[key] == 'object') merge(data[key], newdata[key])
 else data[key] = newdata[key]
 

merge(savedData, newData)
console.log(savedData)