duration?: number;
      };
    };
  };
}];

}; }

export class ManifestResolver { async resolveManifestUrl(postUrl: string): Promise<string> { const jsonUrl = postUrl.replace(//$/, '') + '.json'; const response = await fetch(jsonUrl, { headers: { 'Accept': 'application/json' } });

if (!response.ok) throw new Error(`Manifest fetch failed: ${response.status}`);

const payload: RedditMediaPayload = await response.json();
const media = payload.data.children[0]?.data?.secure_media?.reddit_video;

if (!media?.dash_url) {
  throw new Error('No DASH manifest found in payload');
}

return media.dash_url;

} }

### 2. CORS-Bypass Streaming Proxy
Browsers block cross-origin fetches to CDN domains like `v.redd.it`. A Node.js proxy must intercept segment requests, inject trusted headers, strip restrictive CDN responses, and stream data back to the client without buffering the entire file.

```typescript
import { createServer, IncomingMessage, ServerResponse } from 'http';
import { request as httpsRequest } from 'https';

const PROXY_PORT = 3001;
const CDN_BASE = 'https://v.redd.it';

createServer((req: IncomingMessage, res: ServerResponse) => {
  const targetPath = req.url?.replace('/proxy/', '');
  if (!targetPath) {
    res.writeHead(400);
    return res.end('Missing target path');
  }

  const proxyReq = httpsRequest(
    `${CDN_BASE}${targetPath}`,
    {
      method: 'GET',
      headers: {
        'User-Agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36',
        'Referer': 'https://www.reddit.com/',
        'Accept': '*/*'
      }
    },
    (proxyRes) => {
      res.writeHead(proxyRes.statusCode || 200, {
        'Content-Type': proxyRes.headers['content-type'] || 'application/octet-stream',
        'Access-Control-Allow-Origin': '*',
        'Access-Control-Allow-Methods': 'GET, OPTIONS',
        'Access-Control-Allow-Headers': 'Content-Type'
      });
      
      // Stream directly to avoid memory accumulation
      proxyRes.pipe(res);
    }
  );

  proxyReq.on('error', (err) => {
    console.error('Proxy request failed:', err.message);
    res.writeHead(502);
    res.end('Bad Gateway');
  });

  proxyReq.end();
}).listen(PROXY_PORT, () => {
  console.log(`Streaming proxy active on port ${PROXY_PORT}`);
});

3. Parallel Segment Retrieval

DASH manifests reference dozens to hundreds of segment URLs. Fetching them sequentially creates a bottleneck. A concurrency-limited async pool ensures maximum throughput without overwhelming the network stack or triggering CDN rate limits.

export class ConcurrentSegmentFetcher {
  private concurrency: number;

  constructor(concurrencyLimit: number = 8) {
    this.concurrency = concurrencyLimit;
  }

  async fetchAll(segmentUrls: string[]): Promise<ArrayBuffer[]> {
    const results: ArrayBuffer[] = new Array(segmentUrls.length);
    let currentIndex = 0;

    const worker = async () => {
      while (currentIndex < segmentUrls.length) {
        const index = currentIndex++;
        const url = segmentUrls[index];
        try {
          const response = await fetch(`/proxy/${url.split('/').pop()}`);
          results[index] = await response.arrayBuffer();
        } catch (err) {
          console.warn(`Segment ${index} failed, retrying...`);
          currentIndex--; // Push back for retry
          await new Promise(r => setTimeout(r, 500));
        }
      }
    };

    await Promise.all(Array.from({ length: this.concurrency }, () => worker()));
    return results;
  }
}

4. Client-Side Transmuxing with FFmpeg.wasm

WebAssembly enables FFmpeg to run natively in the browser. The critical optimization is using -c copy to remux streams without decoding. This preserves quality and executes in milliseconds.

import { createFFmpeg, fetchFile } from '@ffmpeg/ffmpeg';

export class BrowserTransmuxer {
  private ffmpeg: ReturnType<typeof createFFmpeg>;

  constructor() {
    this.ffmpeg = createFFmpeg({ log: false, mainName: 'main' });
  }

  async initialize(): Promise<void> {
    if (!this.ffmpeg.isLoaded()) {
      await this.ffmpeg.load();
    }
  }

  async transmux(videoBuffer: ArrayBuffer, audioBuffer: ArrayBuffer): Promise<Uint8Array> {
    await this.initialize();
    
    const { fetchFile } = await import('@ffmpeg/ffmpeg');
    this.ffmpeg.FS('writeFile', 'input_video.mp4', fetchFile(videoBuffer));
    this.ffmpeg.FS('writeFile', 'input_audio.mp4', fetchFile(audioBuffer));

    await this.ffmpeg.run(
      '-i', 'input_video.mp4',
      '-i', 'input_audio.mp4',
      '-c', 'copy',
      '-movflags', '+faststart',
      'output.mp4'
    );

    const data = this.ffmpeg.FS('readFile', 'output.mp4');
    
    // Cleanup virtual filesystem to prevent memory leaks
    this.ffmpeg.FS('unlink', 'input_video.mp4');
    this.ffmpeg.FS('unlink', 'input_audio.mp4');
    this.ffmpeg.FS('unlink', 'output.mp4');

    return data;
  }
}

Architecture Rationale:

• Proxy over Direct Fetch: Browsers enforce CORS. A streaming proxy handles header emulation and CORS injection while piping data directly to the client, avoiding server-side memory accumulation.
• Concurrency Pool: Network latency dominates segment downloads. Limiting concurrency to 6–10 threads balances throughput with CDN rate-limit thresholds.
• WASM Transmuxing: Server-side FFmpeg requires temporary storage, increases egress costs, and introduces privacy risks. Client-side -c copy remuxing is instantaneous, privacy-preserving, and quality-identical to the source.

Pitfall Guide

1. Ignoring Split-Stream Architecture

Explanation: Fetching only the video track manifest results in a playable but silent file. Many developers assume the primary stream contains both tracks. Fix: Always parse the DASH manifest to identify separate Representation IDs for video and audio content types. Fetch and process both tracks independently before remuxing.

2. Sequential Segment Downloads

Explanation: DASH manifests can contain 200+ segments. Sequential await fetch() calls create a linear bottleneck, causing timeouts and poor UX. Fix: Implement a concurrency-controlled async pool. Limit parallel requests to 6–10 to avoid triggering CDN throttling while maximizing bandwidth utilization.

3. Re-encoding Instead of Transmuxing

Explanation: Running FFmpeg without -c copy forces a full decode/encode cycle. This degrades quality, spikes CPU usage, and increases processing time by 10–50x. Fix: Always use -c copy for container conversion. Only re-encode when format conversion (e.g., WebM to MP4) or resolution scaling is explicitly required.

4. Proxy Memory Leaks

Explanation: Buffering entire video files in the proxy before sending them to the client causes Node.js heap exhaustion, especially for long-form content. Fix: Use ReadableStream piping (proxyRes.pipe(res)). Never accumulate chunks in memory. Set appropriate highWaterMark values if backpressure becomes an issue.

5. WASM Virtual Filesystem Accumulation

Explanation: FFmpeg.wasm stores files in a browser-side virtual filesystem. Failing to clean up after transmuxing causes memory leaks that crash the tab on subsequent runs. Fix: Explicitly call FS('unlink', filename) for all input and output files after reading the result. Consider resetting the FS instance if processing multiple videos in a single session.

6. CORS Header Omission

Explanation: The proxy returns data but omits Access-Control-Allow-Origin. The browser blocks the response, causing silent fetch failures. Fix: Always inject Access-Control-Allow-Origin: * (or specific origins) and handle OPTIONS preflight requests. Strip any X-Frame-Options or Strict-Transport-Security headers that interfere with client-side consumption.

7. Audio/Video Timestamp Drift

Explanation: If audio and video segments are fetched out of order or mismatched by index, the remuxed file exhibits sync drift or playback stuttering. Fix: Maintain strict index alignment between audio and video segment arrays. Validate segment durations against the manifest's SegmentTemplate before remuxing.

Production Bundle

Action Checklist

• Validate manifest structure: Ensure the JSON endpoint returns a valid DASH URL before proceeding.
• Configure proxy headers: Emulate a trusted browser User-Agent and Referer to bypass CDN 403 blocks.
• Implement concurrency limits: Set parallel fetch count to 6–10 based on target CDN rate limits.
• Initialize WASM early: Load FFmpeg.wasm during app idle time to avoid UI blocking during transmux.
• Use -c copy exclusively: Verify FFmpeg command flags to prevent accidental re-encoding.
• Clean up WASM FS: Unlink all virtual files post-transmux to prevent memory leaks.
• Handle segment failures: Implement retry logic with exponential backoff for dropped network requests.
• Stream proxy responses: Pipe CDN responses directly to client without server-side buffering.

Decision Matrix

Scenario	Recommended Approach	Why	Cost Impact
High-traffic public tool	Client-Side WASM + Streaming Proxy	Zero server storage, scales with users, privacy-compliant	Near-zero infrastructure cost
Enterprise archiving (compliance)	Server-Side Transcoding with Ephemeral Storage	Centralized logging, audit trails, controlled environment	High egress + storage costs
Low-end mobile devices	Server-Side Transcoding	WASM initialization and memory overhead may crash low-RAM browsers	Moderate compute cost
Real-time preview generation	Client-Side WASM (thumbnail extraction)	Instant feedback, no network roundtrip for processing	Minimal bandwidth usage

Configuration Template

// proxy-server.ts
import { createServer } from 'http';
import { request as httpsRequest } from 'https';

const PROXY_PORT = process.env.PROXY_PORT || 3001;
const CDN_ORIGIN = 'https://v.redd.it';

createServer((req, res) => {
  if (req.method === 'OPTIONS') {
    res.writeHead(204, {
      'Access-Control-Allow-Origin': '*',
      'Access-Control-Allow-Methods': 'GET, OPTIONS',
      'Access-Control-Allow-Headers': 'Content-Type'
    });
    return res.end();
  }

  const target = req.url?.replace('/stream/', '');
  if (!target) return res.writeHead(400).end('Invalid path');

  const proxyReq = httpsRequest(`${CDN_ORIGIN}${target}`, {
    method: 'GET',
    headers: {
      'User-Agent': 'Mozilla/5.0 (compatible; MediaProxy/1.0)',
      'Referer': 'https://www.reddit.com/',
      'Accept': '*/*'
    }
  }, (proxyRes) => {
    res.writeHead(proxyRes.statusCode || 200, {
      'Content-Type': proxyRes.headers['content-type'] || 'application/octet-stream',
      'Access-Control-Allow-Origin': '*',
      'Cache-Control': 'no-cache'
    });
    proxyRes.pipe(res);
  });

  proxyReq.on('error', () => res.writeHead(502).end('Proxy Error'));
  proxyReq.end();
}).listen(PROXY_PORT);

// transmux-service.ts
import { createFFmpeg, fetchFile } from '@ffmpeg/ffmpeg';

export const createTransmuxer = () => {
  const ffmpeg = createFFmpeg({ log: false, mainName: 'main' });
  
  return {
    async init() {
      if (!ffmpeg.isLoaded()) await ffmpeg.load();
    },
    async merge(videoBuf: ArrayBuffer, audioBuf: ArrayBuffer): Promise<Uint8Array> {
      await this.init();
      ffmpeg.FS('writeFile', 'vid.mp4', fetchFile(videoBuf));
      ffmpeg.FS('writeFile', 'aud.mp4', fetchFile(audioBuf));
      
      await ffmpeg.run('-i', 'vid.mp4', '-i', 'aud.mp4', '-c', 'copy', '-movflags', '+faststart', 'out.mp4');
      
      const result = ffmpeg.FS('readFile', 'out.mp4');
      ['vid.mp4', 'aud.mp4', 'out.mp4'].forEach(f => ffmpeg.FS('unlink', f));
      return result;
    }
  };
};

Quick Start Guide

1. Deploy the streaming proxy: Run the Node.js proxy script on a lightweight container or serverless function. Ensure it forwards requests to the target CDN while injecting CORS and browser-emulation headers.
2. Install FFmpeg.wasm: Add @ffmpeg/ffmpeg and @ffmpeg/util to your frontend project. Load the WASM core and shared library during application initialization.
3. Fetch and parse the manifest: Query the platform's JSON endpoint, extract the DASH URL, and parse the XML manifest to isolate audio and video segment paths.
4. Download segments concurrently: Use a concurrency pool to fetch audio and video segments through the proxy. Maintain strict index alignment between the two arrays.
5. Transmux and deliver: Pass the segment buffers to FFmpeg.wasm with -c copy. Read the resulting Uint8Array, trigger a browser download, and clean up the virtual filesystem.