As an indie developer, one of my biggest everyday pet peeves is trying to share files between my iPhone and non-Apple devices. Apple’s proprietary formats—namely HEIC for photos and MOV for videos—are fantastic for storage efficiency on iOS, but they are a massive headache to open, edit, or upload anywhere else.
Normally, you'd head to an online conversion site, upload your private photos, wait for a server to process them, and download them. But as a developer, this workflow screamed three major red flags: user privacy risks, slow upload/download bottlenecks, and massive backend server costs for the creator.
I wanted to build a tool that solves this entirely within the browser. The result is iosconvert.com. Here is how I engineered a 100% client-side, zero-server converter using WebAssembly and modern browser APIs.
The Technical Challenge: Decoding HEIC in Javascript
Browsers cannot natively decode HEIC files. To render or convert them, we need a decoder. Building a decoder in raw JavaScript would be incredibly slow and inefficient.
To solve this, I compiled libheif (a widely used C++ library for HEIF files) into WebAssembly (WASM) using Emscripten. When a user drops a .heic file onto the browser:
- The file is read as an
ArrayBuffer. - The WASM module parses the buffer and extracts the raw RGB pixel data.
- The pixel data is written directly to an
OffscreenCanvas. - The canvas exports the image as a standard JPEG or PNG blob.
By leveraging Web Workers, this entire heavy-lifting process happens on a background thread, keeping the main UI thread completely responsive at 60 FPS.
Taming Video: MOV to MP4 Without the Overhead
Converting video is a different beast. Standard FFmpeg compiled to WASM (ffmpeg.wasm) is incredibly powerful, but it comes with a massive 30MB+ payload. Forcing a mobile user to download a 30MB framework just to convert a 5-second video is a terrible user experience.
Since most iPhone MOV files already use standard H.264 or HEVC codecs, we don't actually need to re-encode the entire video stream. Instead, we can perform container swapping (demuxing and remuxing). By parsing the MP4/MOV container structure in JavaScript, we can extract the raw video and audio tracks from the MOV container and pack them into a standard MP4 container in milliseconds—bypassing CPU-heavy transcoding entirely.
Scaling to Millions of Users for $0/Month
Because 100% of the computing power is offloaded to the user's browser, the server costs for the site are literally zero. The entire frontend is a static React application hosted on a global CDN.
More importantly, it’s a massive win for user privacy. Since no files are ever uploaded to a remote server, user data never leaves their local machine.
I'd love to get your feedback on the architecture! Try converting a few of your own Apple files at iosconvert.com and let me know how the client-side performance holds up on your machine.
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