Megamarket custom bonus activator

megamarket-bonus.user.js

// ==UserScript==
// @name         Megamarket custom bonus
// @namespace    http://tampermonkey.net/
// @version      0.2
// @description  Allows you to enter your own amount of bonuses for an order
// @author       yungd1plomat
// @grant none
// @match        https://megamarket.ru/multicart/checkout/*
// ==/UserScript==

var bonusAmount = null;
document.addEventListener("change", (e) => {
    if (e.target.value == 'true') {
        bonusAmount = prompt('Введите количество бонусов', 0);
        return;
    }
    bonusAmount = null;
});


const {
    fetch: originalFetch
} = window;

window.fetch = async (...args) => {
    let [resource, config] = args;

    if (resource.includes('checkoutService/checkout/calculate') && bonusAmount) {
        let body = config.body;
        let json = JSON.parse(body);
        if (json.discounts[0].amount) {
            json.discounts[0].amount = bonusAmount;
        }
        else {
            json.discounts[1].amount = bonusAmount;
        }
        config.body = JSON.stringify(json);
        console.log(`hooked calculate with ${bonusAmount} bonuses`);
    }

    const response = await originalFetch(resource, config);
    return response;
};