tokio_with_wasm – write async Rust once, run it natively AND in the browser (spawn_blocking included)
If you've ever tried to take an async Rust codebase to the web, you know the pain: tokio just doesn't work on wasm32-unknown-unknown. No threads, no timers, no runtime. You end up littering your code with #[cfg(target_arch = "wasm32")] and rewriting half your task logic around wasm-bindgen-futures.
We built tokio_with_wasm to fix this with one line:
use tokio_with_wasm::alias as tokio; // THIS LINE
use std::time::Duration;
let async_handle = tokio::task::spawn(async { fetch_data().await });
let blocking_handle = tokio::task::spawn_blocking(|| heavy_work());
tokio::time::sleep(Duration::from_secs(1)).await;
That's it. On native targets it resolves to real tokio. On the web, it swaps in a drop-in mimic built on JavaScript web APIs — same module names, same patterns, same code.
Highlights:
spawn_blocking()actually works in the browser. It runs your blocking/compute-heavy code in a pool of Web Workers, and the pool auto-scales with the number of parallel tasks. Real parallelism, in a browser tab, from unmodified tokio-style code.spawn**,**yield_now**,**sleep**,**interval**,**JoinSet— the familiar surface is there. Async tasks ride the JS event loop instead of a custom runtime, so nothing fights the browser.- We see this as a bridge, not a forever solution: once
wasm32-wasi+ wasi-threads mature, you shouldn't need it. Until then, this is how you ship.
It's MIT licensed, and there's a demo GIF in the README showing async + blocking tasks running side by side in a browser.
Repo: https://github.com/cunarist/tokio-with-wasm
Fair warning: spawn_blocking on the web needs nightly + atomics target features + COOP/COEP headers for SharedArrayBuffer, and you should avoid panics (no unwinding on wasm32-unknown-unknown). Small price for writing your async code exactly once.