This is the unpublished documentation of wasm-bindgen, the published documentation is available on the main Rust and WebAssembly documentation site . Features documented here may not be available in released versions of wasm-bindgen.

Optimizing for Size with wasm-bindgen

The Rust and WebAssembly Working Group's Game of Life tutorial has an excellent section on shrinking wasm code size, but there's a few wasm-bindgen-specific items to mention as well!

First and foremost, wasm-bindgen is designed to be lightweight and a "pay only for what you use" mentality. If you suspect that wasm-bindgen is bloating your program that is a bug and we'd like to know about it! Please feel free to file an issue, even if it's a question!

What to profile

With wasm-bindgen there's a few different files to be measuring the size of. The first of which is the output of the compiler itself, typically at target/wasm32-unknown-unknown/release/foo.wasm. This file is not optimized for size and you should not measure it. The output of the compiler when linking with wasm-bindgen is by design larger than it needs to be, the wasm-bindgen CLI tool will automatically strip all unneeded functionality out of the binary.

This leaves us with two primary generated files to measure the size of:

  • Generated wasm - after running the wasm-bindgen CLI tool you'll get a file in --out-dir that looks like foo_bg.wasm. This file is the final fully-finished artifact from wasm-bindgen, and it reflects the size of the app you'll be publishing. All the optimizations mentioned in the code size tutorial will help reduce the size of this binary, so feel free to go crazy!

  • Generated JS - the other file after running wasm-bindgen is a foo.js file which is what's actually imported by other JS code. This file is already generated to be as small as possible (not including unneeded functionality). The JS, however, is not uglified or minified, but rather still human readable and debuggable. It's expected that you'll run an uglifier or bundler of the JS output to minimize it further in your application. If you spot a way we could reduce the output JS size further (or make it more amenable to bundler minification), please let us know!


As an example, the wasm-bindgen repository contains an example about generating small wasm binaries and shows off how to generate a small wasm file for adding two numbers.