JSFiddle - React, Tailwind, and code Playground

HTML

<pre id="pre"></pre>

JavaScript

function parse(object) {
    var result = {};
    var temp   = {};
    var keys;

    for (var property in object) {
        keys = property.split('_');
        temp = result;

        for (var i = 0; i < keys.length - 1; i++) {
            if (typeof temp[keys[i]] == 'undefined') {
                temp[keys[i]] = {};
            }
            temp = temp[keys[i]];
        };

        temp[keys[i]] = object[property];
    }

    return result;

}

var json = '{"generics_utenteID":"1","generics_elencoID":"1","mainbar_ade_stepID_0":"1","mainbar_ade_stepID_1":"3","mainbar_ade_stepTitle_0":"blablabla", "mainbar_ade_stepTitle_1" : "quiquoqua"}';
var obj  = JSON.parse(json);

document.getElementById('pre').innerHTML = JSON.stringify(parse(obj), null, 4);