JSFiddle - React, Tailwind, and code Playground

JavaScript

var ResultSet = {

  Result:[
    {
        "file_size":"722694",
        "desc":"description1"
    },

    {
        "file_size":"19754932",
        "desc":"description1"
    },

    {
        "file_size":"778174",
        "desc":"description2"
    },

    {
        "file_count":"3",
        "desc":"description1"
    },

    {
        "file_size":"466918",
        "desc":"description2"
    }
  ]

};

function findSum(description, array) {
    var i = 0;
    var sum = 0;

    for(i = 0; i < array.length; i++) {
        if(array[i]["desc"] == description && array[i].hasOwnProperty("file_size")) {
            sum += parseInt(array[i]["file_size"], 10);
        }
    }
    
    alert(sum);
}

findSum("description1", ResultSet.Result);