JSFiddle - React, Tailwind, and code Playground

JavaScript

var some_array = [{
    ios: "110",
    rate_pl: "PlanC",
    ref_site: "N/A",
    reservation_id: "12709034"
},{
    ios: "121",
    rate_pl: "SomePlan",
    ref_site: "FB",
    reservation_id: "1273034"
},{
    ios: "141",
    rate_pl: "PlanC",
    ref_site: "Tweet",
    reservation_id: "143034"
}];

var result = some_array.reduce(function(prev, cur) {
    if (prev[cur.rate_pl] === undefined) {        
        prev[cur.rate_pl] = [];
        prev[cur.rate_pl].push({ ios : parseInt(cur.ios, 10) });
    } else {
        prev[cur.rate_pl][0].ios += parseInt(cur.ios, 10);
    }
    return prev;
}, []);

console.log(result);