Hey there,

This tutorial is for the productivity gurus who want their own functional apps to create content without worrying about learning complicated software or a paywall. Without further ado, here's how to build a Browser-Based Video Editor.

This guide teaches you how to build a complete browser-based video editor that supports multi-track timelines, text layers, shapes, images and full exports. It also includes an auto-caption feature powered by AssemblyAI. Everything runs in the browser, with only a lightweight FastAPI backend for uploading and transcription.

What You Are Building

This project gives users the ability to import videos, place them on a timeline, add overlay elements, preview in real time and export the final composition as an MP4 or WebM file. The editor supports text, shapes, images, basic transitions and AI captioning.

The system is modular so you can extend it into a full studio with more tracks, more effects and collaborative features later.

Exact Prompt to Use

Copy this into Emergent:

Build a browser-based video editor. Use React with Tailwind for the frontend. 
Implement a canvas preview that renders video frames, text, shapes and images using requestAnimationFrame. 
Create a multi-track timeline where users can drag and resize clips. 
Support text overlays with live editing, shapes, image layers and AI-generated captions. 
Import video files through the File API. 
Attach audio using the Web Audio API so audio is captured during export.

Use the Canvas API for all drawing. 
Use the MediaRecorder API to record the canvas and audio together for export. 
Include an inspector panel that shows properties for the selected element. 
Include toast notifications for errors or successful exports.

Structure the project in four phases: 
basic player, timeline, overlays and final export. 
Explain common problems and how to solve them such as frame sync, audio capture, canvas resolution and export quality

Core Features Overview

How the Editor Works

The video editor centers on a canvas loop. Each frame, the system draws the correct video timestamp, all text, shapes and image overlays in order. The timeline controls playback and clip timing. When users scrub, drag or resize, the preview updates immediately.

Auto captions are created by sending the audio or video file to the backend. The backend forwards it to AssemblyAI, receives a transcript with timestamps and returns clean caption data. The frontend then injects these captions into the timeline as text overlays.

Exporting runs the same canvas render loop, but sends the canvas stream to MediaRecorder along with an audio stream taken from the Web Audio API. When recording ends, the final video file is saved.

Step by Step Build Plan

1. Build a basic player that draws each video frame on a canvas.

2. Add a timeline with draggable clips and a movable playhead.

3. Add support for text, shapes and image layers.

4. Add auto captioning using AssemblyAI through a FastAPI endpoint.

5. Add export using MediaRecorder and Web Audio.

6. Polish with inspector panels, snapping, toasts and shortcuts.

The Value Add: Auto Captions Feature

AssemblyAI provides $50 in free credits for all new accounts. This is enough for roughly 200 hours of audio transcription, which is more than enough for development and testing.

Here is how a user gets the API key.

Step 1: Create an Account

1. Go to https://www.assemblyai.com

2. Click Sign Up

3. Choose email/password or sign in with Google

4. No payment method is required

Step 2: Activate the Free Credits

Immediately after signup, AssemblyAI automatically adds $50 free credits to your account.
There is nothing extra the user needs to do.

Step 3: Get the API Key

1. Once logged into the dashboard, click API Keys in the left menu

2. Copy your default API key

3. Add it to your environment variables.

Step 4: Add Key to FastAPI Backend

Step 5: You Are Ready

You can now transcribe audio or video files using their REST API or Python SDK at no cost until the free credits run out.

Common Issues and Fixes

1. Incorrect timing: Always sync the canvas frame to video.currentTime.

2. Blank exports: Confirm the export canvas uses full resolution.

3. Audio missing: Ensure audio connects to a MediaStreamDestination node.

4. Caption drift: Adjust caption start times after clip edits.

5. CORS issues: Update FastAPI CORS middleware.

6. Slow rendering: Cache measurements for text overlays.

Final Notes

This design keeps complexity low while providing professional results. The system can grow into a full editor with cuts, transitions, trimming, templates, filters and team collaboration. The auto-caption system turns your editor into a complete production tool out of the box.

Here's our version of this video editor for you to check out: