JSFiddle - React, Tailwind, and code Playground

JavaScript

document.write(add("RUST-PROOFING (9),SPOILER (1),ALARM (2),SIDE-SKIRT (1)"));

function add(str) {
    var retVal = 0;

    var strNumber = "";

    for (var i = 0; i < str.length; i++) {
        if (isNumber(str[i]) && str[i-1]=="(" && str[i+1]==")") {
            strNumber += str[i];
        } else if (strNumber != "") {
            retVal += Number(strNumber);
            strNumber = "";
        }
    }
    return retVal;
}

function isNumber(n) {
    if (n == "") return false;
    return !isNaN(n);
}