JSFiddle - React, Tailwind, and code Playground

JavaScript

var data1 = '{"foo": 123, "test":"foo"}';
var data2 = '{"bar": 456, "test":"bar"}';

var json1 = JSON.parse(data1);
var json2 = JSON.parse(data2);

var merged = {};
for(var i in json1) {
    if (json1.hasOwnProperty(i))
        merged[i] = json1[i];
}
for(var i in json2) {
    if (json2.hasOwnProperty(i))
        merged[i] = json2[i];
}

console.log(merged);