feature: Add new function to single download audio

Implemented almost exact the same with the `batchDownload` function.

index.js

@@ -66,7 +66,7 @@ function getVideosInfo(...urls) {
 
         try {
             await Promise.all(urls.map(async (url) => {
-                ytdl.validateURL(url);               // Validate the URL
+                ytdl.validateURL(url);                     // Validate the URL
                 videosInfo.push(await ytdl.getInfo(url));  // Get the video info
             }));
         } catch (error) {
@@ -103,6 +103,85 @@ function argumentParser(args) {
 }
 
 
+function singleDownload(inputUrl) {
+    console.log(`[INFO] Input URL: ${inputUrl}`);
+
+    const illegalCharRegex = /[<>:"\/\\|?*\x00-\x1F]/g;
+
+    // Validate the given URL
+    ytdl.validateURL(normalizeYtMusicUrl(inputUrl));
+    const videosDataPromise = getVideosInfo(inputUrl)
+        .then((data) => {
+            const videoInfo = data[0];
+            return {
+                videoInfo,
+                format: ytdl.chooseFormat(videoInfo.formats, {
+                    quality: 140,
+                    filter: 'audioonly'
+                }),
+                title: videoInfo.videoDetails.title.replace(illegalCharRegex, '_'),
+                author: videoInfo.videoDetails.author.name,
+                videoUrl: videoInfo.videoDetails.video_url,
+                videoId: videoInfo.videoDetails.videoId,
+                viewers: videoInfo.videoDetails.viewCount
+            };
+        });
+
+    videosDataPromise.then((parsedData) => {
+        // Create output path and the write stream
+        const outFile = path.join('download', `${parsedData.title}.m4a`),
+              outFileBase = path.basename(outFile),
+              outStream = fs.createWriteStream(outFile);
+
+        const dlErrorLog = path.resolve(
+            __dirname, 'tmp', `dlError-${(new Date()).toISOString().split('.')[0]}.log`);
+
+        // Create the output directory asynchronously, if not exist
+        if (!fs.existsSync(path.dirname(outFile))) {
+            fs.mkdirSync(path.dirname(outFile), { recursive: true });
+        }
+
+        console.log(`[INFO] Downloading '${parsedData.title}'...`);
+        console.log('     ', {
+            author: parsedData.author,
+            viewers: parseInt(parsedData.viewers, 10).toLocaleString('en')
+        });
+        ytdl.downloadFromInfo(parsedData.videoInfo, { format: parsedData.format })
+            .on('progress', createDownloadProgress)
+            .on('end', () => {
+                console.log(`[DONE] Download finished: ${outFileBase}`);
+            })
+            .on('error', (err) => {
+                console.error(`[ERROR] Download error: ${outFileBase}`);
+                if (!fs.existsSync(path.dirname(dlErrorLog))) {
+                    fs.mkdirSync(path.dirname(dlErrorLog, { recursive: true }));
+                }
+                const dlErrorLogStream = fs.createWriteStream(
+                    dlErrorLog, { flags: 'a+', flush: true }
+                );
+
+                dlErrorLogStream.write(`[ERROR] ${err.message}${os.EOL}`);
+                dlErrorLogStream.write(`   Title: ${parsedData.title}${os.EOL}`);
+                dlErrorLogStream.write(`   Author: ${parsedData.author}${os.EOL}`);
+                dlErrorLogStream.write(`   URL: ${parsedData.videoUrl}${os.EOL}`);
+                console.error(err);
+            })
+            .pipe(outStream);
+
+        outStream
+            .on('finish', () => {
+                console.log(`[DONE] Written successfully.\n`);
+                convertToMp3(outFile);  // Convert to MP3
+            })
+            .on('error', (err) => {
+                console.error(`[ERROR] Unable to write to output file: ${outFile}\n`);
+                console.error(err);
+                process.exit(1);
+            });
+    })
+    .catch((err) => console.error(err));
+}
+
 function batchDownload(inputFile) {
     const urlsFile = path.resolve(inputFile);
     console.log(`[INFO] Input File: ${path.basename(urlsFile)}`);