Maybe I am not clear on what exactly you want to do but it sounds to me as if you want have a JSON and want to merge it with another JSON, creating just a JSON file.
let Json1 = {'Superman': 'Favorite' };
let Json2 = {'Supergirl': 'Greatest'};
let Json3 = {'IronFist': 'Top 10' };
You now want to add Supergirl (the new element) to Superman (the old element) I assume. Take a look here @ merge-json a simple package which does its job well. You would code as follows:
use strict;
var mergeJSON = require("merge-json");
let Json1 = {'Superman': 'Favorite' };
let Json2 = {'Supergirl': 'Greatest'};
let Json3 = {'IronFist': 'Top 10' };
let Json6 = mergeJSON(Json1,Json2);
Json6=mergeJSON(Json6,Json3);
You would end up with as follows:
Json6 = {'Superman': 'Favorite', 'Supergirl': 'Greatest', 'IronFist': 'Top 10'}
This is how I make use of combining JSON information or text information into a JSON file. You can get much more sophisticated with the module mentioned above. (Just do not confuse merge-json with json-merge and other modules.)
If this is not what you are looking for my apologies, then I did not understand the question correctly.