movconverter

Browser-only MOV → MP4 conversion. No server, no upload — files never leave the user's machine.

Try it online → — the same converter as a free web tool, nothing to install.

• Zero config: detects the codecs inside the MOV and picks the fastest path automatically
• Lossless & instant for the most common case: iPhone/Mac footage (H.264/HEVC + AAC) is remuxed, not re-encoded — no wasm, constant memory, a 1GB file takes about as long as reading it from disk
• Camera footage (H.264 + PCM audio): video is copied verbatim, only the audio is re-encoded to AAC through the browser's native WebCodecs encoder
• Large-file safe: streaming design; memory usage does not grow with file size

Status: pre-release. The remux and audio-transcode paths are implemented; full transcoding (ProRes, MJPEG) is planned via a separate lazy-loaded wasm package.

Usage

import { convertMovToMp4 } from 'movconverter'

const { blob, method } = await convertMovToMp4(file, {
  onProgress: (p) => console.log(`${Math.round(p * 100)}%`),
})
// method is 'remux' | 'audio-transcode' | 'full-transcode' — show it off:
// 'remux' means the conversion was lossless and instant.

Preflight

Ask what would happen before converting (cheap — only the file's metadata is read):

import { canConvert } from 'movconverter'

const plan = await canConvert(file)
plan.method   // 'remux' | 'audio-transcode' | 'full-transcode'
plan.video    // { trackId, format: 'hvc1', action: 'copy' } ...
plan.dropped  // tracks MP4 cannot carry (timecode, ...), reported not errored

Off the main thread

import { convertInWorker } from 'movconverter'

const worker = new Worker(new URL('movconverter/worker', import.meta.url), { type: 'module' })
const result = await convertInWorker(worker, file, { onProgress })

Cancellation

const controller = new AbortController()
convertMovToMp4(file, { signal: controller.signal })
controller.abort()

How it works

MOV (QuickTime) is the direct ancestor of MP4 — both are ISO BMFF box structures. When the codecs inside are MP4-compatible, conversion is a container rewrite: build a new ftyp, rewrite moov (dropping what MP4 can't carry, re-tagging hev1 → hvc1, preserving rotation and color metadata), shift the chunk-offset tables, and reference the media bytes as lazy Blob slices. Nothing is decoded, nothing is copied through memory, and the output is faststart (moov first).

See the architecture document for the full design.

Browser support

• Remux path: any browser with Blob (everything current)
• Audio-transcode path: needs WebCodecs AudioEncoder with AAC (Chrome/Edge, Safari 16.4+; use isWebCodecsAacSupported() to check)

License

MIT