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word number: 15011

Time: 2026-04-13 07:48:43 +0800

User: You said

Moving on to next project, I want to build a video cropper in just html css js. How would you recommend me to build?

Gemini: Building a client-side video cropper is a fantastic project that bridges the gap between simple UI work and complex media processing.

Since you want to stick to HTML, CSS, and JS, the architecture effectively splits into two distinct problems:

The UI (The “Ghost” Crop): A visual overlay that lets users select where to crop.

The Engine (The “Real” Crop): The actual processing tool that slices the video file, for which FFmpeg.wasm is the industry standard.

Here is the recommended blueprint for building this.

  1. The Architecture

Engine: FFmpeg.wasm. This is a port of the famous FFmpeg tool to WebAssembly. It runs directly in the browser, allowing you to edit videos without a backend server. +1

UI: Cropper.js (via CDN). While you can write your own resize/drag logic, Cropper.js is battle-tested, handles aspect ratios, and touch support perfectly. We will “trick” it into working with video.

  1. The Core Challenge: Coordinates

The hardest part of this project isn’t the cropping itself; it’s the math. The video displayed on your screen (e.g., 500px wide) is rarely the size of the actual video file (e.g., 1920px wide 4k video).

You must calculate a Scale Factor: Scale= Displayed Video Width Original Video Width ​

When the user draws a crop box, you multiply their coordinates by this scale factor before sending them to FFmpeg.

  1. Step-by-Step Implementation Guide

Here is a complete, copy-pasteable structure to get you started.

Step A: The Setup (HTML)

You need a container to stack the video and the crop overlay perfectly.

HTML <!DOCTYPE html>

JS Video Cropper
Download Cropped Video

Step B: The Logic (app.js) JavaScript const { createFFmpeg, fetchFile } = FFmpeg; const ffmpeg = createFFmpeg({ log: true });

const video = document.getElementById(‘video-element’); const upload = document.getElementById(‘upload’); const cropBtn = document.getElementById(‘crop-btn’); const message = document.getElementById(‘message’);

let cropper;

// 1. Initialize FFmpeg const loadFFmpeg = async () => { message.innerText = ‘Loading FFmpeg…’; await ffmpeg.load(); message.innerText = ‘FFmpeg Ready’; }; loadFFmpeg();

// 2. Handle Video Upload upload.addEventListener(‘change’, (e) => { const file = e.target.files[0]; const url = URL.createObjectURL(file); video.src = url;

// Initialize Cropper.js once video metadata is loaded
video.onloadedmetadata = () => {
    if (cropper) cropper.destroy();
    
    // We initialize cropper on the VIDEO element directly
    cropper = new Cropper(video, {
        viewMode: 1,
        movable: false, // Don't move the video, move the crop box
        zoomable: false,
        rotatable: false,
        scalable: false,
        crop(event) {
            // This updates strictly on UI interaction
        },
    });
}; });

// 3. The Crop Logic cropBtn.addEventListener(‘click’, async () => { if (!ffmpeg.isLoaded()) { alert(“FFmpeg is still loading…”); return; }

message.innerText = 'Processing... This may take time.';

// A. Get Crop Data from UI
const cropData = cropper.getData(); // Returns {x, y, width, height}

// B. Calculate "Real" Coordinates
// Note: Cropper.js usually handles the scaling for you if initialized on the element
// But if you see offsets, use: 
// const scaleX = video.videoWidth / video.offsetWidth;
// const realX = cropData.x * scaleX;

// For this example, we trust cropper.getData() (which usually returns natural dimensions)
const w = Math.round(cropData.width);
const h = Math.round(cropData.height);
const x = Math.round(cropData.x);
const y = Math.round(cropData.y);

// C. Write File to Memory
const file = upload.files[0];
const fileName = 'input.mp4';
ffmpeg.FS('writeFile', fileName, await fetchFile(file));

// D. Run FFmpeg Command
// -vf "crop=w:h:x:y" is the magic filter
await ffmpeg.run('-i', fileName, '-vf', `crop=${w}:${h}:${x}:${y}`, '-c:a', 'copy', 'output.mp4');

// E. Read Result & Download
const data = ffmpeg.FS('readFile', 'output.mp4');
const videoBlob = new Blob([data.buffer], { type: 'video/mp4' });
const videoUrl = URL.createObjectURL(videoBlob);

const outputLink = document.getElementById('output-link');
outputLink.href = videoUrl;
outputLink.style.display = 'block';
message.innerText = 'Done!'; });
  1. Critical: The “Server” Header Issue

This is the #1 blocker for beginners with FFmpeg.wasm. WebAssembly video processing requires a feature called SharedArrayBuffer. Browsers disable this by default for security unless your server sends two specific headers:

Cross-Origin-Opener-Policy: same-origin

Cross-Origin-Embedder-Policy: require-corp

How to fix this for local development:

You cannot just open index.html file directly in the browser. You must use a local server (like Live Server in VS Code).

You need to use a “drop-in” script like coi-serviceworker (linked in the HTML above). You must download the coi-serviceworker.js file from this GitHub repo and place it next to your index.html. It magically adds the headers for you in the browser.

  1. Performance Tips

File Size: Client-side processing is heavy. Processing a 500MB video might crash the browser tab. Limit inputs to small clips (<50MB) for your first version.

Trim First: If you add a “Trim” feature (cut start/end time) before cropping, the file becomes smaller and cropping is faster.

UI Feedback: Show a spinner. FFmpeg processing freezes the UI unless run in a worker, so visual feedback is vital.

Next Step

Would you like me to explain how to add a “Trim” (start/end time) slider to this, so users can shorten the video before processing to make the crop faster?

User: You said

I asked the wrong question but this is great to put it for later. I am actually asking about just trimming. How would you recommend me to build?

Gemini: No problem! That previous guide is great for spatial editing (cropping dimensions), but you are asking for temporal editing (trimming time).

This is actually much easier and significantly faster to build. Because you are just cutting time, you often don’t need to re-encode the video (re-draw every pixel); you just copy the existing data stream.

Here is how to build a Client-Side Video Trimmer using HTML, CSS, JS, and FFmpeg.wasm.

  1. The Strategy: “Stream Copy”

The secret weapon for this build is the FFmpeg flag -c copy.

Normal Render: Decodes video -> Edits -> Encodes video. (Slow, heavy CPU usage).

Stream Copy: Locates the start/end bytes in the file -> Copies them to a new file. (Instant, near-zero CPU usage).

  1. The Architecture

Engine: FFmpeg.wasm (Still the standard).

UI Component: noUiSlider.

Why? HTML5 only has one handle. A trimmer needs two handles (Start and End). Writing a dual-handle slider from scratch is painful; this tiny library handles the touch/drag physics perfectly.

  1. Step-by-Step Implementation

You will need the coi-serviceworker.js file mentioned in the previous answer (to enable SharedArrayBuffer), but the code is much simpler.

Step A: HTML Structure

We need a video player and a slider bar underneath it.

HTML <!DOCTYPE html>

JS Video Trimmer

Start: 0s | End: 0s
Download

Step B: The Logic (app.js)

The trick here is linking the slider events to the video.currentTime so the user sees exactly what frame they are cutting.

JavaScript const { createFFmpeg, fetchFile } = FFmpeg; const ffmpeg = createFFmpeg({ log: true });

const video = document.getElementById(‘video-element’); const upload = document.getElementById(‘upload’); const slider = document.getElementById(‘slider-container’); const trimBtn = document.getElementById(‘trim-btn’); const message = document.getElementById(‘message’); const lblStart = document.getElementById(‘lbl-start’); const lblEnd = document.getElementById(‘lbl-end’);

let fileData = null; // Store the file for FFmpeg

// 1. Initialize FFmpeg const init = async () => { message.innerText = ‘Loading Engine…’; await ffmpeg.load(); message.innerText = ‘’; trimBtn.disabled = false; }; init();

// 2. Initialize Slider (Hidden initially) noUiSlider.create(slider, { start: [0, 100], connect: true, range: { ‘min’: 0, ‘max’: 100 }, step: 0.1 // Allow 0.1s precision }); slider.setAttribute(‘disabled’, true);

// 3. Handle Upload upload.addEventListener(‘change’, (e) => { const file = e.target.files[0]; fileData = file; video.src = URL.createObjectURL(file); });

// 4. Once Video Metadata Loads (We know duration) video.addEventListener(‘loadedmetadata’, () => { const duration = video.duration;

// Update Slider Range to match Video Duration
slider.noUiSlider.updateOptions({
    range: { 'min': 0, 'max': duration },
    start: [0, duration] // Start with full selection
});
slider.removeAttribute('disabled');
updateLabels([0, duration]); });

// 5. Sync Slider to Video (The “Preview” Logic) slider.noUiSlider.on(‘update’, (values, handle) => { // Update text labels updateLabels(values); });

slider.noUiSlider.on(‘slide’, (values, handle) => { // When user drags a handle, seek video to that time // handle 0 = start handle, handle 1 = end handle video.currentTime = values[handle]; video.pause(); // Pause so they can see the specific frame });

function updateLabels(values) { lblStart.innerText = parseFloat(values[0]).toFixed(1) + ‘s’; lblEnd.innerText = parseFloat(values[1]).toFixed(1) + ‘s’; }

// 6. The Trimming Logic (FFmpeg) trimBtn.addEventListener(‘click’, async () => { if(!fileData) return;

message.innerText = 'Trimming...';
const values = slider.noUiSlider.get();
const startTime = values[0]; // String, e.g., "10.50"
const endTime = values[1]; 
const duration = endTime - startTime;

// A. Write file to memory
ffmpeg.FS('writeFile', 'input.mp4', await fetchFile(fileData));

// B. Run Command
// -ss: Seek to start
// -t: Duration to keep (End - Start)
// -c copy: STREAM COPY (Fast! No re-encoding)
// -avoid_negative_ts 1: Helps with sync issues
await ffmpeg.run(
    '-ss', startTime.toString(), 
    '-i', 'input.mp4', 
    '-t', duration.toString(), 
    '-c', 'copy', 
    'output.mp4'
);

// C. Get Result
const data = ffmpeg.FS('readFile', 'output.mp4');
const url = URL.createObjectURL(new Blob([data.buffer], { type: 'video/mp4' }));

const dl = document.getElementById('download-link');
dl.href = url;
dl.style.display = 'inline-block';
message.innerText = 'Done!'; });
  1. Technical Nuance: The “Keyframe” Problem

If you use -c copy (Stream Copy), FFmpeg can only cut at “Keyframes” (Intra-frames). These are complete images stored in the video stream.

The Good: It is instant.

The Bad: It is not “frame perfect.” If you ask to cut at 10.5s, but the nearest keyframe is at 8s, the video might start at 8s, or start with a bit of black screen until the 10.5s mark.

How to fix it? If you need frame-perfect cuts (exactly where the user pointed), you must remove -c copy from the command.

Change: [‘-ss’, …, ‘-c’, ‘copy’, ‘output.mp4’]

To: [‘-ss’, …, ‘output.mp4’]

Trade-off: This forces FFmpeg to re-encode the video. It will take 10x-20x longer to process.

Next Step

For a project like this, the UI is usually the tricky part. Would you like me to show you how to add a “Play Selection” button? (A button that plays only the part of the video inside the slider handles, so the user can verify their cut before downloading?)

User: You said

I am using live server in vscode windows and below is what in the console:

’’’

createFFmpeg.js:43 [info] use ffmpeg.wasm v0.11.6

createFFmpeg.js:43 [info] load ffmpeg-core

createFFmpeg.js:43 [info] loading ffmpeg-core

1baa0d2c-5342-4d35-8b29-5245b06214e1:22 Uncaught (in promise) ReferenceError: SharedArrayBuffer is not defined

    at 1baa0d2c-5342-4d35-8b29-5245b06214e1:22:175

    at createFFmpeg.js:117:20

    at u (runtime.js:63:40)

    at Generator._invoke (runtime.js:293:22)

    at Generator.next (runtime.js:118:21)

    at o (ffmpeg.min.js:1:6011)

    at c (ffmpeg.min.js:1:6214)

(anonymous) @ 1baa0d2c-5342-4d35-8b29-5245b06214e1:22

(anonymous) @ createFFmpeg.js:117

u @ runtime.js:63

(anonymous) @ runtime.js:293

(anonymous) @ runtime.js:118

o @ ffmpeg.min.js:1

c @ ffmpeg.min.js:1

Promise.then

o @ ffmpeg.min.js:1

c @ ffmpeg.min.js:1

(anonymous) @ ffmpeg.min.js:1

(anonymous) @ ffmpeg.min.js:1

(anonymous) @ createFFmpeg.js:103

init @ app.js:17

(anonymous) @ app.js:21

’’’

Gemini: This is the “Boss Fight” of WebAssembly development. Don’t worry, you aren’t doing anything wrong in your code logic.

The error SharedArrayBuffer is not defined means your browser has disabled the feature for security reasons.

Browsers (Chrome, Edge, Firefox) lock this feature by default to prevent security vulnerabilities (like Spectre). To unlock it, your server must send two specific “Cross-Origin” headers. The default VS Code Live Server does not send these headers.

Here are the two ways to fix this.

Fix 1: The “Drop-in” Fix (Easiest)

You likely added the

Download the file: Go to this GitHub repository: coi-serviceworker.js raw file. (Right-click the page and “Save As…”, or copy all the text and paste it into a new file).

Save it correctly: Save this file as coi-serviceworker.js inside the same folder as your index.html.

Verify your HTML: Make sure this script is at the very top of your <body> or <head>, before your app.js or FFmpeg scripts run.

HTML

Hard Reload: Go to your page, clear cache, and reload (Ctrl + Shift + R). The page might reload itself automatically once; this is normal (it’s the service worker activating the headers).

Fix 2: The “Node Server” Fix (More Robust)

If Fix 1 feels “hacky” or isn’t working, the professional way is to run a tiny custom server instead of the VS Code extension.

Create a file named server.js in your project folder.

Paste this code (requires Node.js installed):

JavaScript const http = require(‘http’); const fs = require(‘fs’); const path = require(‘path’);

const PORT = 3000;

const mimeTypes = { ‘.html’: ‘text/html’, ‘.js’: ‘text/javascript’, ‘.css’: ‘text/css’, ‘.json’: ‘application/json’, ‘.png’: ‘image/png’, ‘.jpg’: ‘image/jpg’, ‘.gif’: ‘image/gif’, ‘.svg’: ‘image/svg+xml’, ‘.wav’: ‘audio/wav’, ‘.mp4’: ‘video/mp4’, ‘.woff’: ‘application/font-woff’, ‘.ttf’: ‘application/font-ttf’, ‘.eot’: ‘application/vnd.ms-fontobject’, ‘.otf’: ‘application/font-otf’, ‘.wasm’: ‘application/wasm’ };

http.createServer(function (request, response) { console.log(‘request ‘, request.url);

let filePath = '.' + request.url;
if (filePath == './') {
    filePath = './index.html';
}

const extname = String(path.extname(filePath)).toLowerCase();
const contentType = mimeTypes[extname] || 'application/octet-stream';

fs.readFile(filePath, function(error, content) {
    if (error) {
        if(error.code == 'ENOENT') {
            response.writeHead(404, { 'Content-Type': 'text/html' });
            response.end("404 Not Found", 'utf-8');
        }
        else {
            response.writeHead(500);
            response.end('Sorry, check with the site admin for error: '+error.code+' ..\n');
        }
    }
    else {
        // HERE IS THE MAGIC PART
        response.writeHead(200, { 
            'Content-Type': contentType,
            'Cross-Origin-Opener-Policy': 'same-origin',
            'Cross-Origin-Embedder-Policy': 'require-corp'
        });
        response.end(content, 'utf-8');
    }
});

}).listen(PORT);

console.log(Server running at http://127.0.0.1:${PORT}/);

Open your VS Code terminal and run: node server.js

Open your browser to http://localhost:3000. The error will be gone.

Summary

The error is not in your JavaScript logic. It is strictly a server configuration issue.

Try Fix 1 first. Ensure the coi-serviceworker.js file physically exists in your folder.

Use Fix 2 if you want a reliable dev environment without relying on a service worker hack.

User: You said

Thanks it works with fix 1. I would still like to report the console:

(index):40  GET http://localhost:5500/coi-serviceworker.js net::ERR_ABORTED 404 (Not Found)

(index):1 Refused to execute script from ‘http://localhost:5500/coi-serviceworker.js’ because its MIME type (‘text/html’) is not executable, and strict MIME type checking is enabled.

createFFmpeg.js:43 [info] use ffmpeg.wasm v0.11.6

createFFmpeg.js:43 [info] load ffmpeg-core

createFFmpeg.js:43 [info] loading ffmpeg-core

createFFmpeg.js:43 [info] ffmpeg-core loaded

createFFmpeg.js:43 [info] run FS.writeFile input.mp4 <583593963 bytes binary file>

createFFmpeg.js:43 [info] run ffmpeg command: -ss 1024.10 -i input.mp4 -t 417.72 -c copy output.mp4

createFFmpeg.js:43 [fferr] ffmpeg version 9e96b1c Copyright (c) 2000-2020 the FFmpeg developers

createFFmpeg.js:43 [fferr]   built with emcc (Emscripten gcc/clang-like replacement) 2.0.8 (d059fd603d0b45b584f634dc2365bc9e9a6ec1dd)

createFFmpeg.js:43 [fferr]   configuration: –target-os=none –arch=x86_32 –enable-cross-compile –disable-x86asm –disable-inline-asm –disable-stripping –disable-programs –disable-doc –disable-debug –disable-runtime-cpudetect –disable-autodetect –extra-cflags=’-O3 –closure 1 -I/src/build/include -s USE_PTHREADS=1’ –extra-cxxflags=’-O3 –closure 1 -I/src/build/include -s USE_PTHREADS=1’ –extra-ldflags=’-O3 –closure 1 -I/src/build/include -s USE_PTHREADS=1 -L/src/build/lib’ –pkg-config-flags=–static –nm=llvm-nm –ar=emar –ranlib=emranlib –cc=emcc –cxx=em++ –objcc=emcc –dep-cc=emcc –enable-gpl –enable-nonfree –enable-zlib –enable-libx264 –enable-libx265 –enable-libvpx –enable-libwavpack –enable-libmp3lame –enable-libfdk-aac –enable-libtheora –enable-libvorbis –enable-libfreetype –enable-libopus –enable-libwebp –enable-libass –enable-libfribidi

createFFmpeg.js:43 [fferr]   libavutil      56. 51.100 / 56. 51.100

createFFmpeg.js:43 [fferr]   libavcodec     58. 91.100 / 58. 91.100

createFFmpeg.js:43 [fferr]   libavformat    58. 45.100 / 58. 45.100

createFFmpeg.js:43 [fferr]   libavdevice    58. 10.100 / 58. 10.100

createFFmpeg.js:43 [fferr]   libavfilter     7. 85.100 /  7. 85.100

createFFmpeg.js:43 [fferr]   libswscale      5.  7.100 /  5.  7.100

createFFmpeg.js:43 [fferr]   libswresample   3.  7.100 /  3.  7.100

createFFmpeg.js:43 [fferr]   libpostproc    55.  7.100 / 55.  7.100

createFFmpeg.js:43 [fferr] Input #0, mov,mp4,m4a,3gp,3g2,mj2, from ‘input.mp4’:

createFFmpeg.js:43 [fferr]   Metadata:

createFFmpeg.js:43 [fferr]     major_brand     : mp42

createFFmpeg.js:43 [fferr]     minor_version   : 0

createFFmpeg.js:43 [fferr]     compatible_brands: isommp42

createFFmpeg.js:43 [fferr]     creation_time   : 2015-11-29T05:18:44.000000Z

createFFmpeg.js:43 [fferr]   Duration: 00:24:01.82, start: 0.000000, bitrate: 3238 kb/s

createFFmpeg.js:43 [fferr]     Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 3043 kb/s, 23.98 fps, 23.98 tbr, 24k tbn, 47.95 tbc (default)

createFFmpeg.js:43 [fferr]     Metadata:

createFFmpeg.js:43 [fferr]       handler_name    : VideoHandler

createFFmpeg.js:43 [fferr]     Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 191 kb/s (default)

createFFmpeg.js:43 [fferr]     Metadata:

createFFmpeg.js:43 [fferr]       creation_time   : 2015-11-29T05:19:08.000000Z

createFFmpeg.js:43 [fferr]       handler_name    : IsoMedia File Produced by Google, 5-11-2011

createFFmpeg.js:43 [fferr] Output #0, mp4, to ‘output.mp4’:

createFFmpeg.js:43 [fferr]   Metadata:

createFFmpeg.js:43 [fferr]     major_brand     : mp42

createFFmpeg.js:43 [fferr]     minor_version   : 0

createFFmpeg.js:43 [fferr]     compatible_brands: isommp42

createFFmpeg.js:43 [fferr]     encoder         : Lavf58.45.100

createFFmpeg.js:43 [fferr]     Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 3043 kb/s, 23.98 fps, 23.98 tbr, 24k tbn, 24k tbc (default)

createFFmpeg.js:43 [fferr]     Metadata:

createFFmpeg.js:43 [fferr]       handler_name    : VideoHandler

createFFmpeg.js:43 [fferr]     Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 191 kb/s (default)

createFFmpeg.js:43 [fferr]     Metadata:

createFFmpeg.js:43 [fferr]       creation_time   : 2015-11-29T05:19:08.000000Z

createFFmpeg.js:43 [fferr]       handler_name    : IsoMedia File Produced by Google, 5-11-2011

createFFmpeg.js:43 [fferr] Stream mapping:

createFFmpeg.js:43 [fferr]   Stream #0:0 -> #0:0 (copy)

createFFmpeg.js:43 [fferr]   Stream #0:1 -> #0:1 (copy)

createFFmpeg.js:43 [fferr] frame= 2112 fps=0.0 q=-1.0 size=   34816kB time=00:01:27.60 bitrate=3255.8kbits/s speed= 173x    

createFFmpeg.js:43 [fferr] frame= 5539 fps=5413 q=-1.0 size=   88832kB time=00:03:50.75 bitrate=3153.6kbits/s speed= 226x    

createFFmpeg.js:43 [fferr] frame= 8275 fps=5432 q=-1.0 size=  139520kB time=00:05:44.78 bitrate=3314.9kbits/s speed= 226x    

createFFmpeg.js:43 [fferr] frame=10030 fps=5727 q=-1.0 Lsize=  158647kB time=00:06:57.69 bitrate=3111.4kbits/s speed= 239x    

createFFmpeg.js:43 [fferr] video:148604kB audio:9805kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.150325%

createFFmpeg.js:43 [ffout] FFMPEG_END

createFFmpeg.js:43 [info] run FS.readFile output.mp4

Gemini: This is a success!

Despite the red error at the top, your FFmpeg logs confirm that the video trimmer is working, and it is working incredibly fast.

Here is the breakdown of what your logs are telling you:

  1. The “Red” Error (The Warning)

GET http://localhost:5500/coi-serviceworker.js net::ERR_ABORTED 404 (Not Found)

This means the browser cannot find the coi-serviceworker.js file.

Why is it working then? Chrome sometimes relaxes security rules for localhost (your local computer), allowing SharedArrayBuffer to work even without the headers.

The Fix: Make sure coi-serviceworker.js is in the exact same folder as your index.html. Currently, the browser thinks it’s missing. You should fix this before you try to put this website online, or it will break.

  1. The “Green” Success (The Logic)

Your FFmpeg logs are beautiful. Look at this line:

speed= 239x

This confirms that the Stream Copy strategy (-c copy) is active.

Normal processing: Usually runs at 1x or 0.5x speed (slower than real-time).

Your processing: Ran at 239x speed. You processed a 7-minute chunk (time=00:06:57) in just a couple of seconds.

Why was it so fast?

You successfully bypassed the “Re-encoding” process. Instead of redrawing every pixel, you just told the computer to copy the data packets from A to B.

Summary of your Logs Log Segment Meaning Verdict ERR_ABORTED 404 The security script file is missing from your folder. Fix this (Check file location) -c copy output.mp4 You are using “Stream Copy” mode. Perfect speed= 239x The engine is flying. Extremely efficient. Perfect FFMPEG_END The process finished without crashing. Success Export to Sheets Next Step: User Experience

Since your engine is working perfectly, you might notice that when you trim the video, the browser freezes for a second or two while it works.

Would you like to add a Progress Bar? FFmpeg.wasm has a built-in logger that reports progress (like time=00:03:50). We can use that to show a percentage bar (e.g., “Processing: 45%”) so the user doesn’t think the app crashed.

User: You said

That’s done, back to the content, sometimes sliding the bar is hard to be precise. Can we add to be also to edit the timer?

Gemini: Yes, absolutely. Sliders are great for “feeling” the edit, but terrible for precision (e.g., trying to land exactly on 10.50s).

Adding editable input boxes creates a Two-Way Binding:

Slider moves -> Updates the numbers.

Typing numbers -> Jumps the slider (and the video frame) to that exact spot.

Here is how to upgrade your UI and Code.

  1. Update the HTML

Replace your old time-display div with this new structure using fields.

HTML

  1. Update the Javascript (app.js)

We need to connect these inputs to the slider. The logic must handle “collisions” (so the start time cannot be set higher than the end time).

Add/Replace this logic in your app.js:

JavaScript // 1. Select the new inputs const valStart = document.getElementById(‘val-start’); const valEnd = document.getElementById(‘val-end’);

// … [Existing init code] …

// 2. UPDATED: Sync Slider -> Inputs (Add this inside your existing slider setup) slider.noUiSlider.on(‘update’, (values, handle) => { const value = parseFloat(values[handle]).toFixed(2);

if (handle === 0) {
    valStart.value = value;
} else {
    valEnd.value = value;
} });

// 3. NEW: Sync Inputs -> Slider // We listen for ‘change’ (when user hits enter or clicks away) const inputs = [valStart, valEnd];

inputs.forEach((input, handle) => { input.addEventListener(‘change’, function () { // A. Get the current slider values let currentValues = slider.noUiSlider.get(); let newVal = parseFloat(this.value);

    // B. Basic Validation
    // If changing Start (handle 0), it can't be >= End
    if (handle === 0 && newVal >= parseFloat(currentValues[1])) {
        newVal = parseFloat(currentValues[1]) - 0.1; // Push it back slightly
    }
    // If changing End (handle 1), it can't be <= Start
    if (handle === 1 && newVal <= parseFloat(currentValues[0])) {
        newVal = parseFloat(currentValues[0]) + 0.1;
    }

    // C. Update the Slider (This automatically updates the video preview too!)
    // 'handle' is 0 for start, 1 for end.
    // We pass 'null' for the handle we aren't changing.
    if (handle === 0) {
        slider.noUiSlider.set([newVal, null]);
    } else {
        slider.noUiSlider.set([null, newVal]);
    }
    
    // D. Force Video Preview Update immediately
    video.currentTime = newVal;
}); });

How it works now

Precision: You can now type 12.45 and hit Enter. The slider handles will jump exactly to that position.

Safety: If the video ends at 20s and the user tries to type 25s as the start time, noUiSlider (which knows the max range) will automatically clamp it to the max duration, preventing errors.

Next Step: Visual Polish?

Right now, you are building a very functional “Dev” tool. To make it feel like a real product (like Instagram or TikTok’s trimmer), we could display thumbnails underneath the slider track.

Is that something you want to explore, or would you prefer to focus on adding the “Crop” functionality back in so you have both Trimming + Cropping?

User: You said

write me the full app.js

Gemini: Here is the consolidated app.js. This script combines the FFmpeg engine, the dual-handle slider logic, the two-way numerical input binding, and the video preview sync.

Make sure you have coi-serviceworker.js in your folder and your HTML elements match the IDs used here.

JavaScript /**

const { createFFmpeg, fetchFile } = FFmpeg; const ffmpeg = createFFmpeg({ log: true });

// UI Element Selectors const video = document.getElementById(‘video-element’); const upload = document.getElementById(‘upload’); const slider = document.getElementById(‘slider-container’); const trimBtn = document.getElementById(‘trim-btn’); const message = document.getElementById(‘message’); const valStart = document.getElementById(‘val-start’); const valEnd = document.getElementById(‘val-end’); const downloadLink = document.getElementById(‘download-link’);

let fileData = null;

/**

/**

/**

/**

/**

// Seek video preview when dragging slider slider.noUiSlider.on(‘slide’, (values, handle) => { video.currentTime = parseFloat(values[handle]); video.pause(); });

/**

/**

Pro Tip for Windows Users

If you find that the Stream Copy (-c copy) makes the start of your trimmed video slightly “frozen” for a second, it’s because it’s waiting for the first keyframe. To fix this, you can change the order of the command to:

JavaScript // Re-encoding version (Slower, but frame-accurate) await ffmpeg.run(‘-i’, inputName, ‘-ss’, start, ‘-to’, end, outputName);

Next Step

Your trimmer is now fully functional with precision controls! Would you like to add a “Status Progress” listener so the user can see exactly how many frames have been processed in real-time?

User: You said

Is there something wrong?

index.html:

’’’

<!DOCTYPE html>

            JS Video Trimmer            

       
       
       
                               
       
                               
   
   

               

’’’

app.js:

’’’

/**

 * VIDEO TRIMMER ENGINE (HTML5 + FFmpeg.wasm)

 */

const { createFFmpeg, fetchFile } = FFmpeg;

const ffmpeg = createFFmpeg({ log: true });

// UI Element Selectors

const video = document.getElementById(‘video-element’);

const upload = document.getElementById(‘upload’);

const slider = document.getElementById(‘slider-container’);

const trimBtn = document.getElementById(‘trim-btn’);

const message = document.getElementById(‘message’);

const valStart = document.getElementById(‘val-start’);

const valEnd = document.getElementById(‘val-end’);

const downloadLink = document.getElementById(‘download-link’);

let fileData = null;

/**

 * 1. INITIALIZE FFMPEG

 */

const initFFmpeg = async () => {

    try {

        message.innerText = ‘Loading FFmpeg Engine…’;

        await ffmpeg.load();

        message.innerText = ‘Engine Ready.’;

        trimBtn.disabled = false;

    } catch (err) {

        message.innerText = ‘Error loading FFmpeg. Check console.’;

        console.error(err);

    }

};

initFFmpeg();

/**

 * 2. INITIALIZE SLIDER (noUiSlider)

 */

noUiSlider.create(slider, {

    start: [0, 10], // Initial dummy values

    connect: true,

    range: { ‘min’: 0, ‘max’: 10 },

    step: 0.01,

    tooltips: false

});

slider.setAttribute(‘disabled’, true); // Disable until video loads

/**

 * 3. FILE UPLOAD HANDLER

 */

upload.addEventListener(‘change’, (e) => {

    const file = e.target.files[0];

    if (!file) return;

    fileData = file;

    const url = URL.createObjectURL(file);

    video.src = url;

    downloadLink.style.display = ‘none’;

    message.innerText = ‘Video loaded. Set your trim range.’;

});

/**

 * 4. VIDEO METADATA HANDLER

 * Triggered when video duration is known

 */

video.addEventListener(‘loadedmetadata’, () => {

    const duration = video.duration;

    // Update slider to match video length

    slider.noUiSlider.updateOptions({

        range: { ‘min’: 0, ‘max’: duration },

        start: [0, duration]

    });

    slider.removeAttribute(‘disabled’);

   

    valStart.value = 0;

    valEnd.value = duration.toFixed(2);

});

/**

 * 5. SLIDER -> UI BINDING

 * When slider moves, update input boxes and video frame

 */

slider.noUiSlider.on(‘update’, (values, handle) => {

    const value = parseFloat(values[handle]);

   

    if (handle === 0) {

        valStart.value = value.toFixed(2);

    } else {

        valEnd.value = value.toFixed(2);

    }

});

// Seek video preview when dragging slider

slider.noUiSlider.on(‘slide’, (values, handle) => {

    video.currentTime = parseFloat(values[handle]);

    video.pause();

});

/**

 * 6. INPUT BOX -> SLIDER BINDING (Precision Editing)

 * When user types a number, update the slider and video frame

 */

[valStart, valEnd].forEach((input, handle) => {

    input.addEventListener(‘change’, function() {

        let newVal = parseFloat(this.value);

        let currentValues = slider.noUiSlider.get().map(Number);

        const duration = video.duration;

        // Validation: Clamp to video duration

        if (newVal < 0) newVal = 0;

        if (newVal > duration) newVal = duration;

        // Validation: Prevent handles from crossing

        if (handle === 0 && newVal >= currentValues[1]) {

            newVal = currentValues[1] - 0.1;

        } else if (handle === 1 && newVal <= currentValues[0]) {

            newVal = currentValues[0] + 0.1;

        }

        this.value = newVal.toFixed(2);

        // Update Slider handles

        let setArray = [null, null];

        setArray[handle] = newVal;

        slider.noUiSlider.set(setArray);

        // Update Video Preview

        video.currentTime = newVal;

    });

});

/**

 * 7. THE TRIM ACTION (FFmpeg Execution)

 */

trimBtn.addEventListener(‘click’, async () => {

    if (!fileData   !ffmpeg.isLoaded()) return;

    trimBtn.disabled = true;

    message.innerText = ‘Processing… Please wait.’;

    const start = valStart.value;

    const end = valEnd.value;

    const duration = (parseFloat(end) - parseFloat(start)).toFixed(2);

    // Write file to FFmpeg’s Virtual File System

    const inputName = ‘input.mp4’;

    const outputName = ‘output.mp4’;

    ffmpeg.FS(‘writeFile’, inputName, await fetchFile(fileData));

    /**

     * COMMAND EXPLAINED:

     * -ss : Start time (placed before -i for faster seeking)

     * -i  : Input file

     * -t  : Duration to cut

     * -c copy : Stream Copy (Fastest, no re-encoding)

     */

    await ffmpeg.run(

        ‘-ss’, start,

        ‘-i’, inputName,

        ‘-t’, duration,

        ‘-c’, ‘copy’,

        outputName

    );

    // Read resulting file

    const data = ffmpeg.FS(‘readFile’, outputName);

   

    // Create download URL

    const blob = new Blob([data.buffer], { type: ‘video/mp4’ });

    const url = URL.createObjectURL(blob);

   

    downloadLink.href = url;

    downloadLink.style.display = ‘inline-block’;

    message.innerText = ‘Trim complete!’;

    trimBtn.disabled = false;

   

    // Cleanup virtual FS to save memory

    ffmpeg.FS(‘unlink’, inputName);

    ffmpeg.FS(‘unlink’, outputName);

});

’’’

Gemini: You’re very close! The logic in your app.js is solid, but you have a few missing pieces in your HTML that are causing the JavaScript to crash.

Specifically, your script is looking for a Trim Button and a Download Link that don’t exist in your HTML file yet. Because those variables come back as null, the script stops running as soon as it tries to disable the button.

The Fix

You need to add the button and the download link to your index.html. Here is the corrected body section:

HTML



What was happening?

In your app.js, the very first thing initFFmpeg() does is: trimBtn.disabled = false;

Because the button wasn’t in the HTML, trimBtn was null. Trying to set a property on null throws a TypeError, which kills the rest of your script, meaning the slider and upload functionality never even get a chance to initialize.

A Quick Note on the Service Worker

You switched to the CDN version of the service worker:

This is a clever way to do it! Just remember that if you see the SharedArrayBuffer error again, it’s usually because the service worker hasn’t “registered” yet. Simply refresh the page twice and it should clear up.

Next Step

Now that the UI is linked, would you like me to show you how to add a Loading Spinner or a Progress Percentage? Since FFmpeg can take a few seconds on larger files, it’s helpful to show the user that the app is actually working!

User: You said

I tried to put it on github page but failed to load, can you check?

’’’

layout: post

title: Video Trimmer in HTML CSS JS

description: A video trimmer using only html css js through ffmpeg wasm.

featuredimage: /assets/images/icon/ftcut.png

category: tools

tags: [tools, video, trimmer, html css, js, json]

canonicalurl: https://0fajarpurnama0.github.io/tools/2022/12/15/video-trimmer



   
                   
       
                   

Reference

Source Code

<!DOCTYPE html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <meta name="viewport" content="width=device-width, initial-scale=1.0">

    <title>JS Video Trimmer</title>

    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/noUiSlider/15.7.1/nouislider.min.css">

    <style>

        body { font-family: sans-serif; padding: 20px; max-width: 800px; margin: 0 auto; }

        video { width: 100%; margin-bottom: 20px; border-radius: 8px; background: #000; }

        /* The Slider Container */

        #slider-container { margin: 0 10px 30px 10px; }

        /* Visual Styles */

        .controls { display: flex; gap: 10px; align-items: center; }

        .time-display { font-family: monospace; font-weight: bold; }

        #download-link { display: none; color: white; background: #007bff; padding: 10px 20px; text-decoration: none; border-radius: 4px;}

        button { padding: 10px 20px; cursor: pointer; }

        /* Add this simple CSS to make them look nice */

        .controls-wrapper {

            display: flex;

            gap: 20px;

            margin-bottom: 20px;

            align-items: center;

            justify-content: center;

        }

        .input-group {

            display: flex;

            flex-direction: column;

        }

        .input-group input {

            width: 80px;

            padding: 5px;

            font-size: 16px;

            font-family: monospace;

        }

    </style>

</head>

<body>

    <input type="file" id="upload" accept="video/*"><br><br>

    <video id="video-element" controls playsinline></video>

    <div id="slider-container"></div>

    <div class="controls-wrapper">

        <div class="input-group">

            <label>Start (sec)</label>

            <input type="number" id="val-start" step="0.01" min="0">

        </div>

        <button id="trim-btn" disabled>Trim Video</button>

        <div class="input-group">

            <label>End (sec)</label>

            <input type="number" id="val-end" step="0.01" min="0">

        </div>

    </div>

    <a id="download-link" download="trimmed.mp4" style="display:none; margin-top: 10px; text-align: center;">Download Trimmed Video</a>

    <p id="message"></p>

    <script src="https://cdnjs.cloudflare.com/ajax/libs/noUiSlider/15.7.1/nouislider.min.js"></script>

    <script src="https://unpkg.com/coi-serviceworker@0.1.7/coi-serviceworker.min.js"></script>

    <script src="https://unpkg.com/@ffmpeg/ffmpeg@0.11.6/dist/ffmpeg.min.js"></script>

    <script src="app.js"></script>




<script>

/**

 * VIDEO TRIMMER ENGINE (HTML5 + FFmpeg.wasm)

 */




const { createFFmpeg, fetchFile } = FFmpeg;

const ffmpeg = createFFmpeg({ log: true });




// UI Element Selectors

const video = document.getElementById('video-element');

const upload = document.getElementById('upload');

const slider = document.getElementById('slider-container');

const trimBtn = document.getElementById('trim-btn');

const message = document.getElementById('message');

const valStart = document.getElementById('val-start');

const valEnd = document.getElementById('val-end');

const downloadLink = document.getElementById('download-link');




let fileData = null;




/**

 * 1. INITIALIZE FFMPEG

 */

const initFFmpeg = async () => {

    try {

        message.innerText = 'Loading FFmpeg Engine...';

        await ffmpeg.load();

        message.innerText = 'Engine Ready.';

        trimBtn.disabled = false;

    } catch (err) {

        message.innerText = 'Error loading FFmpeg. Check console.';

        console.error(err);

    }

};

initFFmpeg();




/**

 * 2. INITIALIZE SLIDER (noUiSlider)

 */

noUiSlider.create(slider, {

    start: [0, 10], // Initial dummy values

    connect: true,

    range: { 'min': 0, 'max': 10 },

    step: 0.01,

    tooltips: false

});

slider.setAttribute('disabled', true); // Disable until video loads




/**

 * 3. FILE UPLOAD HANDLER

 */

upload.addEventListener('change', (e) => {

    const file = e.target.files[0];

    if (!file) return;




    fileData = file;

    const url = URL.createObjectURL(file);

    video.src = url;

    downloadLink.style.display = 'none';

    message.innerText = 'Video loaded. Set your trim range.';

});




/**

 * 4. VIDEO METADATA HANDLER

 * Triggered when video duration is known

 */

video.addEventListener('loadedmetadata', () => {

    const duration = video.duration;




    // Update slider to match video length

    slider.noUiSlider.updateOptions({

        range: { 'min': 0, 'max': duration },

        start: [0, duration]

    });

    slider.removeAttribute('disabled');

   

    valStart.value = 0;

    valEnd.value = duration.toFixed(2);

});




/**

 * 5. SLIDER -> UI BINDING

 * When slider moves, update input boxes and video frame

 */

slider.noUiSlider.on('update', (values, handle) => {

    const value = parseFloat(values[handle]);

   

    if (handle === 0) {

        valStart.value = value.toFixed(2);

    } else {

        valEnd.value = value.toFixed(2);

    }

});




// Seek video preview when dragging slider

slider.noUiSlider.on('slide', (values, handle) => {

    video.currentTime = parseFloat(values[handle]);

    video.pause();

});




/**

 * 6. INPUT BOX -> SLIDER BINDING (Precision Editing)

 * When user types a number, update the slider and video frame

 */

[valStart, valEnd].forEach((input, handle) => {

    input.addEventListener('change', function() {

        let newVal = parseFloat(this.value);

        let currentValues = slider.noUiSlider.get().map(Number);

        const duration = video.duration;




        // Validation: Clamp to video duration

        if (newVal < 0) newVal = 0;

        if (newVal > duration) newVal = duration;




        // Validation: Prevent handles from crossing

        if (handle === 0 && newVal >= currentValues[1]) {

            newVal = currentValues[1] - 0.1;

        } else if (handle === 1 && newVal <= currentValues[0]) {

            newVal = currentValues[0] + 0.1;

        }




        this.value = newVal.toFixed(2);




        // Update Slider handles

        let setArray = [null, null];

        setArray[handle] = newVal;

        slider.noUiSlider.set(setArray);




        // Update Video Preview

        video.currentTime = newVal;

    });

});




/**

 * 7. THE TRIM ACTION (FFmpeg Execution)

 */

trimBtn.addEventListener('click', async () => {

    if (!fileData || !ffmpeg.isLoaded()) return;




    trimBtn.disabled = true;

    message.innerText = 'Processing... Please wait.';




    const start = valStart.value;

    const end = valEnd.value;

    const duration = (parseFloat(end) - parseFloat(start)).toFixed(2);




    // Write file to FFmpeg's Virtual File System

    const inputName = 'input.mp4';

    const outputName = 'output.mp4';

    ffmpeg.FS('writeFile', inputName, await fetchFile(fileData));




    /**

     * COMMAND EXPLAINED:

     * -ss : Start time (placed before -i for faster seeking)

     * -i  : Input file

     * -t  : Duration to cut

     * -c copy : Stream Copy (Fastest, no re-encoding)

     */

    await ffmpeg.run(

        '-ss', start,

        '-i', inputName,

        '-t', duration,

        '-c', 'copy',

        outputName

    );




    // Read resulting file

    const data = ffmpeg.FS('readFile', outputName);

   

    // Create download URL

    const blob = new Blob([data.buffer], { type: 'video/mp4' });

    const url = URL.createObjectURL(blob);

   

    downloadLink.href = url;

    downloadLink.style.display = 'inline-block';

    message.innerText = 'Trim complete!';

    trimBtn.disabled = false;

   

    // Cleanup virtual FS to save memory

    ffmpeg.FS('unlink', inputName);

    ffmpeg.FS('unlink', outputName);

});

</script>

</body>

</html>

’’’

Gemini: The issue is almost certainly due to where you placed the coi-serviceworker.js file.

The Problem: Service Worker Scope

You placed the file in /assets/js/3rdparty/coi-serviceworker-0.1.7.js. Your page is located at /tools/2022/12/15/video-trimmer.

Rule of the Web: A Service Worker can only control pages that are inside the same folder or deeper.

A worker at /assets/js/… cannot control a page at /tools/….

Therefore, the worker never activates, the security headers are never added, and SharedArrayBuffer remains broken.

The Fix

To fix this on GitHub Pages, you must move the service worker file to the Root of your repository.

Step 1: Move the File

Move coi-serviceworker-0.1.7.js (or rename it to just coi-serviceworker.js for simplicity) to the very top level of your GitHub repo, so it is accessible at: https://0fajarpurnama0.github.io/coi-serviceworker.js

Step 2: Update Your HTML

Change the script reference in your Jekyll post to point to the root:

HTML

Why this works

When the file is at the root (/), it has “Scope” over the entire domain (/). It can now intercept the request for your /tools/… page and inject the required Cross-Origin-Embedder-Policy headers.

One more Jekyll Tip

Since you are using Jekyll, sometimes the liquid template engine tries to parse JavaScript syntax (like ). If you see weird rendering errors, wrap your JavaScript block in raw tags:

HTML

// Your FFmpeg and Slider code here…

</script>

User: You said

The engine loads now but when uploading video it does load to the player:

’’’

layout: post

title: Video Trimmer in HTML CSS JS

description: A video trimmer using only html css js through ffmpeg wasm.

featuredimage: /assets/images/icon/ftcut.png

category: tools

tags: [tools, video, trimmer, html css, js, json]

canonicalurl: https://0fajarpurnama0.github.io/tools/2022/12/15/video-trimmer



   
                   
       
                   

Reference

Source Code

<!DOCTYPE html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <meta name="viewport" content="width=device-width, initial-scale=1.0">

    <title>JS Video Trimmer</title>

    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/noUiSlider/15.7.1/nouislider.min.css">

    <style>

        body { font-family: sans-serif; padding: 20px; max-width: 800px; margin: 0 auto; }

        video { width: 100%; margin-bottom: 20px; border-radius: 8px; background: #000; }

        /* The Slider Container */

        #slider-container { margin: 0 10px 30px 10px; }

        /* Visual Styles */

        .controls { display: flex; gap: 10px; align-items: center; }

        .time-display { font-family: monospace; font-weight: bold; }

        #download-link { display: none; color: white; background: #007bff; padding: 10px 20px; text-decoration: none; border-radius: 4px;}

        button { padding: 10px 20px; cursor: pointer; }

        /* Add this simple CSS to make them look nice */

        .controls-wrapper {

            display: flex;

            gap: 20px;

            margin-bottom: 20px;

            align-items: center;

            justify-content: center;

        }

        .input-group {

            display: flex;

            flex-direction: column;

        }

        .input-group input {

            width: 80px;

            padding: 5px;

            font-size: 16px;

            font-family: monospace;

        }

    </style>

</head>

<body>

    <input type="file" id="upload" accept="video/*"><br><br>

    <video id="video-element" controls playsinline></video>

    <div id="slider-container"></div>

    <div class="controls-wrapper">

        <div class="input-group">

            <label>Start (sec)</label>

            <input type="number" id="val-start" step="0.01" min="0">

        </div>

        <button id="trim-btn" disabled>Trim Video</button>

        <div class="input-group">

            <label>End (sec)</label>

            <input type="number" id="val-end" step="0.01" min="0">

        </div>

    </div>

    <a id="download-link" download="trimmed.mp4" style="display:none; margin-top: 10px; text-align: center;">Download Trimmed Video</a>

    <p id="message"></p>

    <script src="https://cdnjs.cloudflare.com/ajax/libs/noUiSlider/15.7.1/nouislider.min.js"></script>

    <script src="https://unpkg.com/coi-serviceworker@0.1.7/coi-serviceworker.min.js"></script>

    <script src="https://unpkg.com/@ffmpeg/ffmpeg@0.11.6/dist/ffmpeg.min.js"></script>

    <script src="app.js"></script>




<script>

/**

 * VIDEO TRIMMER ENGINE (HTML5 + FFmpeg.wasm)

 */




const { createFFmpeg, fetchFile } = FFmpeg;

const ffmpeg = createFFmpeg({ log: true });




// UI Element Selectors

const video = document.getElementById('video-element');

const upload = document.getElementById('upload');

const slider = document.getElementById('slider-container');

const trimBtn = document.getElementById('trim-btn');

const message = document.getElementById('message');

const valStart = document.getElementById('val-start');

const valEnd = document.getElementById('val-end');

const downloadLink = document.getElementById('download-link');




let fileData = null;




/**

 * 1. INITIALIZE FFMPEG

 */

const initFFmpeg = async () => {

    try {

        message.innerText = 'Loading FFmpeg Engine...';

        await ffmpeg.load();

        message.innerText = 'Engine Ready.';

        trimBtn.disabled = false;

    } catch (err) {

        message.innerText = 'Error loading FFmpeg. Check console.';

        console.error(err);

    }

};

initFFmpeg();




/**

 * 2. INITIALIZE SLIDER (noUiSlider)

 */

noUiSlider.create(slider, {

    start: [0, 10], // Initial dummy values

    connect: true,

    range: { 'min': 0, 'max': 10 },

    step: 0.01,

    tooltips: false

});

slider.setAttribute('disabled', true); // Disable until video loads




/**

 * 3. FILE UPLOAD HANDLER

 */

upload.addEventListener('change', (e) => {

    const file = e.target.files[0];

    if (!file) return;




    fileData = file;

    const url = URL.createObjectURL(file);

    video.src = url;

    downloadLink.style.display = 'none';

    message.innerText = 'Video loaded. Set your trim range.';

});




/**

 * 4. VIDEO METADATA HANDLER

 * Triggered when video duration is known

 */

video.addEventListener('loadedmetadata', () => {

    const duration = video.duration;




    // Update slider to match video length

    slider.noUiSlider.updateOptions({

        range: { 'min': 0, 'max': duration },

        start: [0, duration]

    });

    slider.removeAttribute('disabled');

   

    valStart.value = 0;

    valEnd.value = duration.toFixed(2);

});




/**

 * 5. SLIDER -> UI BINDING

 * When slider moves, update input boxes and video frame

 */

slider.noUiSlider.on('update', (values, handle) => {

    const value = parseFloat(values[handle]);

   

    if (handle === 0) {

        valStart.value = value.toFixed(2);

    } else {

        valEnd.value = value.toFixed(2);

    }

});




// Seek video preview when dragging slider

slider.noUiSlider.on('slide', (values, handle) => {

    video.currentTime = parseFloat(values[handle]);

    video.pause();

});




/**

 * 6. INPUT BOX -> SLIDER BINDING (Precision Editing)

 * When user types a number, update the slider and video frame

 */

[valStart, valEnd].forEach((input, handle) => {

    input.addEventListener('change', function() {

        let newVal = parseFloat(this.value);

        let currentValues = slider.noUiSlider.get().map(Number);

        const duration = video.duration;




        // Validation: Clamp to video duration

        if (newVal < 0) newVal = 0;

        if (newVal > duration) newVal = duration;




        // Validation: Prevent handles from crossing

        if (handle === 0 && newVal >= currentValues[1]) {

            newVal = currentValues[1] - 0.1;

        } else if (handle === 1 && newVal <= currentValues[0]) {

            newVal = currentValues[0] + 0.1;

        }




        this.value = newVal.toFixed(2);




        // Update Slider handles

        let setArray = [null, null];

        setArray[handle] = newVal;

        slider.noUiSlider.set(setArray);




        // Update Video Preview

        video.currentTime = newVal;

    });

});




/**

 * 7. THE TRIM ACTION (FFmpeg Execution)

 */

trimBtn.addEventListener('click', async () => {

    if (!fileData || !ffmpeg.isLoaded()) return;




    trimBtn.disabled = true;

    message.innerText = 'Processing... Please wait.';




    const start = valStart.value;

    const end = valEnd.value;

    const duration = (parseFloat(end) - parseFloat(start)).toFixed(2);




    // Write file to FFmpeg's Virtual File System

    const inputName = 'input.mp4';

    const outputName = 'output.mp4';

    ffmpeg.FS('writeFile', inputName, await fetchFile(fileData));




    /**

     * COMMAND EXPLAINED:

     * -ss : Start time (placed before -i for faster seeking)

     * -i  : Input file

     * -t  : Duration to cut

     * -c copy : Stream Copy (Fastest, no re-encoding)

     */

    await ffmpeg.run(

        '-ss', start,

        '-i', inputName,

        '-t', duration,

        '-c', 'copy',

        outputName

    );




    // Read resulting file

    const data = ffmpeg.FS('readFile', outputName);

   

    // Create download URL

    const blob = new Blob([data.buffer], { type: 'video/mp4' });

    const url = URL.createObjectURL(blob);

   

    downloadLink.href = url;

    downloadLink.style.display = 'inline-block';

    message.innerText = 'Trim complete!';

    trimBtn.disabled = false;

   

    // Cleanup virtual FS to save memory

    ffmpeg.FS('unlink', inputName);

    ffmpeg.FS('unlink', outputName);

});

</script>

</body>

</html>

’’’

Gemini: This is a Syntax Error in your HTML that is breaking the dependency chain.

The reason the video player isn’t loading is likely not related to FFmpeg or the Service Worker anymore. It is because your Javascript crashes before it can ever “listen” for your file upload.

The Error

Look closely at line 54 of your code:

HTML

What happens:

The browser tries to load nouislider. It fails (404 Not Found) because of the typo js;.

Your app.js runs.

It reaches line 106: noUiSlider.create(slider, {….

CRASH: Uncaught ReferenceError: noUiSlider is not defined.

Result: The script stops immediately. The code that comes after that (specifically upload.addEventListener) is never executed. Your upload button is dead.

The Fix

Remove the semicolon ; from the path:

HTML

A Safety Check for “Strict Mode”

Since you are now using the “Service Worker” (Strict Security Mode), there is a chance the browser might block the video from playing even after you fix the typo above.

If the video loads but shows a black screen or refuses to play, add crossorigin to your video tag:

HTML

This tells the browser: “This video blob is safe to play in strict mode.”

User: You said

Thanks, it works well now. Next is the opposite. What about video concat or combine?

Gemini: Building a video joiner (concatenator) is the perfect logical next step.

Just like with trimming, there are two ways to do this in FFmpeg: the Fast Way (Stream Copy) and the Slow Way (Re-encoding).

Since you are building a web tool, we want the Fast Way (Stream Copy) whenever possible. This method is called the “Demuxer Concatenation.” It stitches videos together without re-compressing them, so it’s instant.

The Challenge: The “List File”

FFmpeg cannot just take file1.mp4 and file2.mp4 as arguments for joining. It requires a special text file that lists the videos.

You must virtually create a file named list.txt inside the browser’s memory.

The content must look like this:

Plaintext file ‘input0.mp4’ file ‘input1.mp4’ file ‘input2.mp4’

Step-by-Step Implementation

Here is how to build it. I will keep the UI simple: a multi-file uploader and a “Merge” button.

  1. The HTML Structure

We need an input that accepts multiple files (multiple attribute).

HTML <!DOCTYPE html>

JS Video Merger

Video Merger (Concat)

No files selected.

  1. The Logic (app.js)

The logic here loops through the uploaded files, renames them sequentially (input0.mp4, input1.mp4…), writes them to memory, creates the list.txt, and runs the command.

JavaScript const { createFFmpeg, fetchFile } = FFmpeg; const ffmpeg = createFFmpeg({ log: true });

const upload = document.getElementById(‘upload’); const fileListDisplay = document.getElementById(‘file-list’); const mergeBtn = document.getElementById(‘merge-btn’); const message = document.getElementById(‘message’); const video = document.getElementById(‘video-element’);

let selectedFiles = [];

// 1. Initialize FFmpeg const init = async () => { message.innerText = ‘Loading Engine…’; await ffmpeg.load(); message.innerText = ‘Ready.’; }; init();

// 2. Handle File Selection upload.addEventListener(‘change’, (e) => { selectedFiles = Array.from(e.target.files);

if (selectedFiles.length < 2) {
    fileListDisplay.innerHTML = '<span style="color:red">Please select at least 2 videos.</span>';
    mergeBtn.disabled = true;
    return;
}

// Display list names
fileListDisplay.innerHTML = selectedFiles
    .map((f, i) => `<div class="file-item">${i + 1}. ${f.name}</div>`)
    .join('');
    
mergeBtn.disabled = false;
message.innerText = `${selectedFiles.length} videos ready to merge.`; });

// 3. The Merge Logic mergeBtn.addEventListener(‘click’, async () => { if (!ffmpeg.isLoaded()) return;

mergeBtn.disabled = true;
message.innerText = 'Merging... (This might take a moment)';

// A. Write all files to memory & Build list.txt content
let listContent = '';

for (let i = 0; i < selectedFiles.length; i++) {
    const fname = `input${i}.mp4`;
    // Write the video file to memory
    ffmpeg.FS('writeFile', fname, await fetchFile(selectedFiles[i]));
    // Add entry to list.txt string
    listContent += `file '${fname}'\n`;
}

// B. Write the list.txt file to memory
ffmpeg.FS('writeFile', 'list.txt', listContent);

// C. Run FFmpeg Concatenation Command
// -f concat : Use the concat format
// -safe 0   : Allow reading absolute paths (standard requirement for concat)
// -i list.txt : Input the list file
// -c copy   : Stream Copy (Instant join, no re-encoding)
await ffmpeg.run('-f', 'concat', '-safe', '0', '-i', 'list.txt', '-c', 'copy', 'output.mp4');

// D. Read Result & Play
const data = ffmpeg.FS('readFile', 'output.mp4');
const url = URL.createObjectURL(new Blob([data.buffer], { type: 'video/mp4' }));

video.src = url;
video.style.display = 'block';
message.innerText = 'Merge Complete!';
mergeBtn.disabled = false;

// Cleanup (Important for memory management!)
for (let i = 0; i < selectedFiles.length; i++) {
    ffmpeg.FS('unlink', `input${i}.mp4`);
}
ffmpeg.FS('unlink', 'list.txt');
ffmpeg.FS('unlink', 'output.mp4'); });

Important Constraints (The “Gotcha”)

The -c copy method is fast, but it is strict.

Resolution: All videos must have the exact same resolution (e.g., all 1080p).

Codec: All videos must use the same codec (e.g., all H.264).

Frame Rate: All videos must have the same frame rate.

If a user tries to merge a 720p vertical video with a 1080p horizontal video, the output will look glitchy or fail.

How to handle mixed formats? If you want to support mixing different videos, you must remove -c copy.

Command: await ffmpeg.run(‘-f’, ‘concat’, ‘-safe’, ‘0’, ‘-i’, ‘list.txt’, ‘output.mp4’); (Note: removed -c copy)

Result: FFmpeg will automatically re-encode everything to match the first video.

Trade-off: It will be much slower (rendering time) instead of instant.

Next Step

For a robust tool, you usually want to give the user the ability to reorder the list (drag and drop order) before merging. Would you like to see how to make that file list sortable?

User: You said

Yes please, here are the current:

index.html:


<!DOCTYPE html>

<html lang="en">

<head>

<meta charset="UTF-8">

<meta name="viewport" content="width=device-width, initial-scale=1.0">

<title>JS Video Merger</title>

<link rel="stylesheet" href="styles.css">

</head>

<body>




<h2>Video Merger (Concat)</h2>

<input type="file" id="upload" accept="video/*" multiple>

<div class="file-list" id="file-list">

No files selected.

</div>




<button id="merge-btn" disabled>Merge Videos</button>

<p id="message"></p>




<video id="video-element" controls playsinline></video>




<script src="../coi-serviceworker.js"></script>

<script src="../lib/ffmpeg-0.11.6.min.js"></script>




<script src="script.js"></script>

</body>

</html>

styles.css:


body { font-family: sans-serif; padding: 20px; max-width: 800px; margin: 0 auto; }

.file-list { margin: 20px 0; border: 1px solid #ddd; padding: 10px; min-height: 50px; }

.file-item { padding: 5px; border-bottom: 1px solid #eee; }

button { padding: 10px 20px; cursor: pointer; margin-right: 10px;}

#video-element { width: 100%; margin-top: 20px; background: #000; display: none; }

scripts.js:


const { createFFmpeg, fetchFile } = FFmpeg;

const ffmpeg = createFFmpeg({ log: true });




const upload = document.getElementById('upload');

const fileListDisplay = document.getElementById('file-list');

const mergeBtn = document.getElementById('merge-btn');

const message = document.getElementById('message');

const video = document.getElementById('video-element');




let selectedFiles = [];




// 1. Initialize FFmpeg

const init = async () => {

message.innerText = 'Loading Engine...';

await ffmpeg.load();

message.innerText = 'Ready.';

};

init();




// 2. Handle File Selection

upload.addEventListener('change', (e) => {

selectedFiles = Array.from(e.target.files);

if (selectedFiles.length < 2) {

fileListDisplay.innerHTML = '<span style="color:red">Please select at least 2 videos.</span>';

mergeBtn.disabled = true;

return;

}




// Display list names

fileListDisplay.innerHTML = selectedFiles

.map((f, i) => `<div class="file-item">${i + 1}. ${f.name}</div>`)

.join('');

mergeBtn.disabled = false;

message.innerText = `${selectedFiles.length} videos ready to merge.`;

});




// 3. The Merge Logic

mergeBtn.addEventListener('click', async () => {

if (!ffmpeg.isLoaded()) return;




mergeBtn.disabled = true;

message.innerText = 'Merging... (This might take a moment)';




// A. Write all files to memory & Build list.txt content

let listContent = '';

for (let i = 0; i < selectedFiles.length; i++) {

const fname = `input${i}.mp4`;

// Write the video file to memory

ffmpeg.FS('writeFile', fname, await fetchFile(selectedFiles[i]));

// Add entry to list.txt string

listContent += `file '${fname}'\n`;

}




// B. Write the list.txt file to memory

ffmpeg.FS('writeFile', 'list.txt', listContent);




// C. Run FFmpeg Concatenation Command

// -f concat : Use the concat format

// -safe 0 : Allow reading absolute paths (standard requirement for concat)

// -i list.txt : Input the list file

// -c copy : Stream Copy (Instant join, no re-encoding)

await ffmpeg.run('-f', 'concat', '-safe', '0', '-i', 'list.txt', '-c', 'copy', 'output.mp4');




// D. Read Result & Play

const data = ffmpeg.FS('readFile', 'output.mp4');

const url = URL.createObjectURL(new Blob([data.buffer], { type: 'video/mp4' }));

video.src = url;

video.style.display = 'block';

message.innerText = 'Merge Complete!';

mergeBtn.disabled = false;




// Cleanup (Important for memory management!)

for (let i = 0; i < selectedFiles.length; i++) {

ffmpeg.FS('unlink', `input${i}.mp4`);

}

ffmpeg.FS('unlink', 'list.txt');

ffmpeg.FS('unlink', 'output.mp4');

});

Gemini: Adding drag-and-drop sorting is a fantastic upgrade. It changes the app from a basic script into a tool that actually feels good to use.

Since you are using modern web standards, we don’t even need any external libraries for this. We can use the native HTML5 Drag and Drop API.

The logic works like this:

Turn each file item into a draggable=”true” element.

When the user drops an item onto another item, we update the selectedFiles array by moving the file to its new index.

We re-render the list visually. Because the selectedFiles array is physically reordered, your FFmpeg merge logic doesn’t need to change at all!

Here are the updates for your CSS and JS files. Your index.html stays exactly the same.

  1. Update styles.css

We need to add a few styles to make the items look “grabbable” and provide visual feedback when an item is being dragged.

CSS body { font-family: sans-serif; padding: 20px; max-width: 800px; margin: 0 auto; } .file-list { margin: 20px 0; border: 1px solid #ddd; padding: 10px; min-height: 50px; background: #fafafa; border-radius: 4px; }

/* Updated File Item Styles for Drag & Drop */ .file-item { padding: 10px; border: 1px solid #ccc; margin-bottom: 5px; background: #fff; cursor: grab; display: flex; align-items: center; border-radius: 4px; transition: transform 0.1s ease; } .file-item:active { cursor: grabbing; }

/* The little hamburger icon to indicate dragging */ .drag-handle { margin-right: 15px; color: #888; font-weight: bold; cursor: grab; }

/* Style for the item currently being dragged */ .file-item.dragging { opacity: 0.4; border: 1px dashed #007bff; background: #e9f5ff; }

button { padding: 10px 20px; cursor: pointer; margin-right: 10px; } #video-element { width: 100%; margin-top: 20px; background: #000; display: none; }

  1. Update script.js

Replace your entire script.js with this. I’ve added a renderFileList() function that handles all the drag-and-drop state, array sorting, and re-rendering automatically.

JavaScript const { createFFmpeg, fetchFile } = FFmpeg; const ffmpeg = createFFmpeg({ log: true });

const upload = document.getElementById(‘upload’); const fileListDisplay = document.getElementById(‘file-list’); const mergeBtn = document.getElementById(‘merge-btn’); const message = document.getElementById(‘message’); const video = document.getElementById(‘video-element’);

let selectedFiles = []; let draggedIndex = null; // Keeps track of what we are currently dragging

// 1. Initialize FFmpeg const init = async () => { message.innerText = ‘Loading Engine…’; await ffmpeg.load(); message.innerText = ‘Ready.’; }; init();

// 2. NEW: Function to render the draggable list const renderFileList = () => { fileListDisplay.innerHTML = ‘’; // Clear current list

selectedFiles.forEach((file, index) => {
    // Create the element
    const div = document.createElement('div');
    div.className = 'file-item';
    div.draggable = true;
    div.innerHTML = `<span class="drag-handle">☰</span> ${index + 1}. ${file.name}`;

    // -- DRAG EVENTS --
    
    // When drag starts, remember this item's index and add visual class
    div.addEventListener('dragstart', (e) => {
        draggedIndex = index;
        setTimeout(() => div.classList.add('dragging'), 0); // Timeout allows the drag ghost image to render normally first
    });

    // When drag ends, remove visual class
    div.addEventListener('dragend', () => {
        div.classList.remove('dragging');
    });

    // Allow items to be dropped on top of this element
    div.addEventListener('dragover', (e) => {
        e.preventDefault(); 
    });

    // When an item is dropped here, swap it in the array and re-render
    div.addEventListener('drop', (e) => {
        e.preventDefault();
        const targetIndex = index;
        
        if (draggedIndex === targetIndex) return; // Didn't move

        // Extract the dragged file from the array
        const draggedFile = selectedFiles[draggedIndex];
        selectedFiles.splice(draggedIndex, 1); 
        // Insert it at the new dropped position
        selectedFiles.splice(targetIndex, 0, draggedFile);

        // Re-render the UI with the newly sorted array
        renderFileList();
    });

    fileListDisplay.appendChild(div);
}); };

// 3. Handle File Selection upload.addEventListener(‘change’, (e) => { // Add newly selected files to our master array const newFiles = Array.from(e.target.files); selectedFiles = selectedFiles.concat(newFiles);

if (selectedFiles.length < 2) {
    fileListDisplay.innerHTML = '<span style="color:red">Please select at least 2 videos.</span>';
    mergeBtn.disabled = true;
    return;
}

renderFileList(); // Draw the sortable list
    
mergeBtn.disabled = false;
message.innerText = `${selectedFiles.length} videos ready to merge. Drag to reorder.`; });

// 4. The Merge Logic (Stays exactly the same!) mergeBtn.addEventListener(‘click’, async () => { if (!ffmpeg.isLoaded()) return;

mergeBtn.disabled = true;
message.innerText = 'Merging... (This might take a moment)';

let listContent = '';

// selectedFiles is now guaranteed to be in the order the user dragged them!
for (let i = 0; i < selectedFiles.length; i++) {
    const fname = `input${i}.mp4`;
    ffmpeg.FS('writeFile', fname, await fetchFile(selectedFiles[i]));
    listContent += `file '${fname}'\n`;
}

ffmpeg.FS('writeFile', 'list.txt', listContent);

// Run Concat
await ffmpeg.run('-f', 'concat', '-safe', '0', '-i', 'list.txt', '-c', 'copy', 'output.mp4');

// Read and Output
const data = ffmpeg.FS('readFile', 'output.mp4');
const url = URL.createObjectURL(new Blob([data.buffer], { type: 'video/mp4' }));

video.src = url;
video.style.display = 'block';
message.innerText = 'Merge Complete!';
mergeBtn.disabled = false;

// Cleanup Memory
for (let i = 0; i < selectedFiles.length; i++) {
    ffmpeg.FS('unlink', `input${i}.mp4`);
}
ffmpeg.FS('unlink', 'list.txt');
ffmpeg.FS('unlink', 'output.mp4'); });

Try it out!

When you load the page now, upload a few videos. You’ll see a small “☰” icon next to each file. Click and hold anywhere on the file row to drag it up or down. The list will automatically snap into place, and when you click “Merge,” it will respect that exact order.

Next Step

Right now, the result just plays in the browser. Would you like to add a Download Output button so the user can actually save their combined video to their computer?

User: You said

Yes please, here are the current:

index.html:


<!DOCTYPE html>

<html lang="en">

<head>

<meta charset="UTF-8">

<meta name="viewport" content="width=device-width, initial-scale=1.0">

<title>JS Video Merger</title>

<link rel="stylesheet" href="styles.css">

</head>

<body>




<h2>Video Merger (Concat)</h2>

<input type="file" id="upload" accept="video/*" multiple>

<div class="file-list" id="file-list">

No files selected.

</div>




<button id="merge-btn" disabled>Merge Videos</button>

<p id="message"></p>




<video id="video-element" controls playsinline></video>




<script src="../coi-serviceworker.js"></script>

<script src="../lib/ffmpeg-0.11.6.min.js"></script>




<script src="scripts.js"></script>

</body>

</html>

styles.css:


body { font-family: sans-serif; padding: 20px; max-width: 800px; margin: 0 auto; }

.file-list { margin: 20px 0; border: 1px solid #ddd; padding: 10px; min-height: 50px; background: #fafafa; border-radius: 4px; }




/* Updated File Item Styles for Drag & Drop */

.file-item {

padding: 10px;

border: 1px solid #ccc;

margin-bottom: 5px;

background: #fff;

cursor: grab;

display: flex;

align-items: center;

border-radius: 4px;

transition: transform 0.1s ease;

}

.file-item:active { cursor: grabbing; }




/* The little hamburger icon to indicate dragging */

.drag-handle { margin-right: 15px; color: #888; font-weight: bold; cursor: grab; }




/* Style for the item currently being dragged */

.file-item.dragging { opacity: 0.4; border: 1px dashed #007bff; background: #e9f5ff; }




button { padding: 10px 20px; cursor: pointer; margin-right: 10px; }

#video-element { width: 100%; margin-top: 20px; background: #000; display: none; }

scripts.js:


const { createFFmpeg, fetchFile } = FFmpeg;

const ffmpeg = createFFmpeg({ log: true });




const upload = document.getElementById('upload');

const fileListDisplay = document.getElementById('file-list');

const mergeBtn = document.getElementById('merge-btn');

const message = document.getElementById('message');

const video = document.getElementById('video-element');




let selectedFiles = [];

let draggedIndex = null; // Keeps track of what we are currently dragging




// 1. Initialize FFmpeg

const init = async () => {

message.innerText = 'Loading Engine...';

await ffmpeg.load();

message.innerText = 'Ready.';

};

init();




// 2. NEW: Function to render the draggable list

const renderFileList = () => {

fileListDisplay.innerHTML = ''; // Clear current list

selectedFiles.forEach((file, index) => {

// Create the element

const div = document.createElement('div');

div.className = 'file-item';

div.draggable = true;

div.innerHTML = `<span class="drag-handle">☰</span> ${index + 1}. ${file.name}`;




// -- DRAG EVENTS --

// When drag starts, remember this item's index and add visual class

div.addEventListener('dragstart', (e) => {

draggedIndex = index;

setTimeout(() => div.classList.add('dragging'), 0); // Timeout allows the drag ghost image to render normally first

});




// When drag ends, remove visual class

div.addEventListener('dragend', () => {

div.classList.remove('dragging');

});




// Allow items to be dropped on top of this element

div.addEventListener('dragover', (e) => {

e.preventDefault();

});




// When an item is dropped here, swap it in the array and re-render

div.addEventListener('drop', (e) => {

e.preventDefault();

const targetIndex = index;

if (draggedIndex === targetIndex) return; // Didn't move




// Extract the dragged file from the array

const draggedFile = selectedFiles[draggedIndex];

selectedFiles.splice(draggedIndex, 1);

// Insert it at the new dropped position

selectedFiles.splice(targetIndex, 0, draggedFile);




// Re-render the UI with the newly sorted array

renderFileList();

});




fileListDisplay.appendChild(div);

});

};




// 3. Handle File Selection

upload.addEventListener('change', (e) => {

// Add newly selected files to our master array

const newFiles = Array.from(e.target.files);

selectedFiles = selectedFiles.concat(newFiles);

if (selectedFiles.length < 2) {

fileListDisplay.innerHTML = '<span style="color:red">Please select at least 2 videos.</span>';

mergeBtn.disabled = true;

return;

}




renderFileList(); // Draw the sortable list

mergeBtn.disabled = false;

message.innerText = `${selectedFiles.length} videos ready to merge. Drag to reorder.`;

});




// 4. The Merge Logic (Stays exactly the same!)

mergeBtn.addEventListener('click', async () => {

if (!ffmpeg.isLoaded()) return;




mergeBtn.disabled = true;

message.innerText = 'Merging... (This might take a moment)';




let listContent = '';

// selectedFiles is now guaranteed to be in the order the user dragged them!

for (let i = 0; i < selectedFiles.length; i++) {

const fname = `input${i}.mp4`;

ffmpeg.FS('writeFile', fname, await fetchFile(selectedFiles[i]));

listContent += `file '${fname}'\n`;

}




ffmpeg.FS('writeFile', 'list.txt', listContent);




// Run Concat

await ffmpeg.run('-f', 'concat', '-safe', '0', '-i', 'list.txt', '-c', 'copy', 'output.mp4');




// Read and Output

const data = ffmpeg.FS('readFile', 'output.mp4');

const url = URL.createObjectURL(new Blob([data.buffer], { type: 'video/mp4' }));

video.src = url;

video.style.display = 'block';

message.innerText = 'Merge Complete!';

mergeBtn.disabled = false;




// Cleanup Memory

for (let i = 0; i < selectedFiles.length; i++) {

ffmpeg.FS('unlink', `input${i}.mp4`);

}

ffmpeg.FS('unlink', 'list.txt');

ffmpeg.FS('unlink', 'output.mp4');

});

Gemini: Adding a download button is straightforward since you already have the Blob and the URL.createObjectURL() generated for the video player. We just need to attach that exact same URL to an anchor () tag with a download attribute.

Here are the specific updates to make to your three files.

  1. Update index.html

Add the download link right below your message paragraph. I’ve given it an id and set it to hide by default.

HTML

  1. Update styles.css

Let’s style that tag so it looks like a nice, clickable button, distinct from the Merge button. Add this to the bottom of your CSS:

CSS /* Style for the download link to make it look like a button / #download-btn { display: inline-block; padding: 10px 20px; background-color: #28a745; / Green color to indicate success */ color: white; text-decoration: none; border-radius: 4px; font-family: sans-serif; font-size: 14px; margin-bottom: 20px; cursor: pointer; } #download-btn:hover { background-color: #218838; }

  1. Update scripts.js

You need to grab the new button, reset its visibility when new files are uploaded, and finally attach the video data to it when the merge finishes.

Here is your updated JavaScript. Look for the // NEW: comments to see exactly what changed.

JavaScript const { createFFmpeg, fetchFile } = FFmpeg; const ffmpeg = createFFmpeg({ log: true });

const upload = document.getElementById(‘upload’); const fileListDisplay = document.getElementById(‘file-list’); const mergeBtn = document.getElementById(‘merge-btn’); const message = document.getElementById(‘message’); const video = document.getElementById(‘video-element’); // NEW: Grab the download button const downloadBtn = document.getElementById(‘download-btn’);

let selectedFiles = []; let draggedIndex = null;

// 1. Initialize FFmpeg const init = async () => { message.innerText = ‘Loading Engine…’; await ffmpeg.load(); message.innerText = ‘Ready.’; }; init();

// 2. Function to render the draggable list const renderFileList = () => { fileListDisplay.innerHTML = ‘’;

selectedFiles.forEach((file, index) => {
    const div = document.createElement('div');
    div.className = 'file-item';
    div.draggable = true;
    div.innerHTML = `<span class="drag-handle">☰</span> ${index + 1}. ${file.name}`;

    div.addEventListener('dragstart', (e) => {
        draggedIndex = index;
        setTimeout(() => div.classList.add('dragging'), 0); 
    });

    div.addEventListener('dragend', () => {
        div.classList.remove('dragging');
    });

    div.addEventListener('dragover', (e) => {
        e.preventDefault(); 
    });

    div.addEventListener('drop', (e) => {
        e.preventDefault();
        const targetIndex = index;
        
        if (draggedIndex === targetIndex) return; 

        const draggedFile = selectedFiles[draggedIndex];
        selectedFiles.splice(draggedIndex, 1); 
        selectedFiles.splice(targetIndex, 0, draggedFile);

        renderFileList();
    });

    fileListDisplay.appendChild(div);
}); };

// 3. Handle File Selection upload.addEventListener(‘change’, (e) => { const newFiles = Array.from(e.target.files); selectedFiles = selectedFiles.concat(newFiles);

// NEW: Hide the download button and video if user adds more files
downloadBtn.style.display = 'none';
video.style.display = 'none';

if (selectedFiles.length < 2) {
    fileListDisplay.innerHTML = '<span style="color:red">Please select at least 2 videos.</span>';
    mergeBtn.disabled = true;
    return;
}

renderFileList(); 
    
mergeBtn.disabled = false;
message.innerText = `${selectedFiles.length} videos ready to merge. Drag to reorder.`; });

// 4. The Merge Logic mergeBtn.addEventListener(‘click’, async () => { if (!ffmpeg.isLoaded()) return;

mergeBtn.disabled = true;
message.innerText = 'Merging... (This might take a moment)';
downloadBtn.style.display = 'none'; // Hide button during new merge

let listContent = '';

for (let i = 0; i < selectedFiles.length; i++) {
    const fname = `input${i}.mp4`;
    ffmpeg.FS('writeFile', fname, await fetchFile(selectedFiles[i]));
    listContent += `file '${fname}'\n`;
}

ffmpeg.FS('writeFile', 'list.txt', listContent);

// Run Concat
await ffmpeg.run('-f', 'concat', '-safe', '0', '-i', 'list.txt', '-c', 'copy', 'output.mp4');

// Read and Output
const data = ffmpeg.FS('readFile', 'output.mp4');
const url = URL.createObjectURL(new Blob([data.buffer], { type: 'video/mp4' }));

video.src = url;
video.style.display = 'block';

// NEW: Setup and show the download button
downloadBtn.href = url;
downloadBtn.download = 'merged_video.mp4';
downloadBtn.style.display = 'inline-block';

message.innerText = 'Merge Complete!';
mergeBtn.disabled = false;

// Cleanup Memory
for (let i = 0; i < selectedFiles.length; i++) {
    ffmpeg.FS('unlink', `input${i}.mp4`);
}
ffmpeg.FS('unlink', 'list.txt');
ffmpeg.FS('unlink', 'output.mp4'); });

Would you like me to show you how to add a small “X” or “Remove” button next to each file in the list so users can delete a file if they selected the wrong one?