JSFiddle - React, Tailwind, and code Playground

JavaScript

var data = {
    "ResultSet": {
        "Result": [
            {
            "file_size": "722694",
            "cat_desc": "description1"},
        {
            "file_size": "19754932",
            "cat_desc": "description1"},
        {
            "file_size": "778174",
            "cat_desc": "description2"},
        {
            "file_count": "3",
            "cat_desc": "description1"},
        {
            "file_size": "466918",
            "cat_desc": "description2"}
        ]

    }
}

var result =  data.ResultSet.Result;
var stat = {};
for (var i = 0; i < result.length; i++) {    
    if (stat.hasOwnProperty(result[i].cat_desc)) {       
        if (result[i].hasOwnProperty('file_size')) {
        stat[result[i].cat_desc] += parseInt(result[i].file_size, 10);   
        }
    } else {
        stat[result[i].cat_desc] = parseInt(result[i].file_size, 10);
    }   
}


document.write('description 1 ' + stat['description1'] + '<br/>');
document.write('description 2 ' + stat['description2']);