download.js

// ==UserScript==
// @name        Download songs
// @description Download songs from a specific website
// @namespace   Violentmonkey Scripts
// @match       REDACTED
// @version     0.1
// @author      Noah Betzen
// @grant       none
// ==/UserScript==

const STORAGE_KEY = `seen`

(function() {
    `use strict`;

    const seenString = localStorage.getItem(STORAGE_KEY);
    let seen = seenString ? JSON.parse(seenString) : {};
    const count = Object.keys(seen).length;

    if (count > 0) {
        console.log(`Seen URLs: ${count}`, seen);
    }

    const originalFetch = fetch;

    const queue = new DownloadQueue(originalFetch, 5);

    window.fetch = async function() {
        let response = await originalFetch.apply(this, arguments);

        // This is async but we don't await it on purpose so that the response returns quickly
        processNewData(response.clone(), seen, queue);

        return response;
    };
})();


async function processNewData(resp, seen, queue) {
    const responseData = await resp.json();

    responseData?.result?.servings?.forEach(async (serving) => {
        let originalDownloadUrl = serving.trackVariation?.tokenedUrl;
        if (originalDownloadUrl) {
            // Key url is stripped of query/hash params
            const url = new URL(originalDownloadUrl);
            const keyUrl = keyUrl.origin + keyUrl.pathname;

            if (!seen[keyUrl]) {
                console.log(`Unseen URL: ${keyUrl}`)
                const proxyDownloadUrl = `https://corsproxy.io/?${encodeURIComponent(originalDownloadUrl)}`;

                await queue.enqueue(originalDownloadUrl, proxyDownloadUrl, keyUrl);

                seen[keyUrl] = true;
                localStorage.setItem(STORAGE_KEY, JSON.stringify(seen));
            }
        }
    });
}

class DownloadQueue {
    constructor(originalFetch, concurrency) {
        this.queue = [];
        this.running = 0;
        this.originalFetch = originalFetch;
        this.concurrency = concurrency || 5;
    }

    // Add a file to the queue
    async enqueue(originalDownloadUrl, proxyDownloadUrl, keyUrl) {
        this.queue.push({
            originalDownloadUrl,
            proxyDownloadUrl,
            keyUrl
        });
        await this.run();
    }

    // Process the queue
    async run() {
        if (this.running >= this.concurrency || this.queue.length === 0) {
            return;
        }

        const {
            originalDownloadUrl,
            proxyDownloadUrl,
            keyUrl
        } = this.queue.shift();
        this.running++;

        try {
            await downloadFile(this.originalFetch, originalDownloadUrl, proxyDownloadUrl, keyUrl)
            this.running--;
            this.run();
        } catch (error) {
            console.error(`Error occurred in download queue`, {
                originalDownloadUrl,
                proxyDownloadUrl,
                keyUrl,
                error
            });
            this.running--;
            this.run();
        }
    }
}

async function downloadFile(originalFetch, originalDownloadUrl, proxyDownloadUrl, keyUrl) {
    const filename = new URL(keyUrl).pathname.split(`/`).pop();

    if (filename) {
        const response = await originalFetch(proxyDownloadUrl);

        if (!response.ok) {
            console.error(`Failed to fetch (response not ok) ${keyUrl}`, response);
            return;
        }

        const blob = await response.blob();

        // Create an object URL from the Blob
        const blobURL = URL.createObjectURL(blob);

        // Use an anchor element to trigger the download
        const a = document.createElement(`a`);
        a.style.display = `none`;
        document.body.appendChild(a);

        a.href = blobURL;
        a.download = filename;
        a.click();

        // Cleanup
        document.body.removeChild(a);
        URL.revokeObjectURL(blobURL);

        console.log(`Successfully downloaded file`, {
            filename,
            proxyDownloadUrl
        });
    } else {
        console.error(`Couldn not get filename for url ${keyUrl}`);
    }
}