JSFiddle - React, Tailwind, and code Playground

JavaScript

var json = '[{"userId" : "123123", "password": "fafafa", "age": "21"}, {"userId" : "321321", "password" : "nana123", "age" : "34"}]';

var list = JSON.parse(json);
var output = {};

for(var i=0; i<list.length; i++)
{
    for(var key in list[i])
    {
        if(list[i].hasOwnProperty(key))
        {
            if(typeof output[key] == 'undefined')
            {
                output[key] = [];
            }
            output[key].push(list[i][key]);
        }
    }
}

document.write(JSON.stringify(output));