WebAssembly Cheat Sheet

// lib.rsuse wasm_bindgen::prelude::*;#[wasm_bindgen]pub fn fibonacci(n: u32) -> u64 {    match n {        0 => 0,        1 => 1,        _ => {            let (mut a, mut b) = (0u64, 1u64);            for _ in 2..=n {                let c = a + b;                a = b;                b = c;            }            b        }    }}// Build with wasm-pack (produces .wasm + JS glue code)// $ wasm-pack build --target web

// Loading a hand-written/compiled .wasm module directlyconst { instance } = await WebAssembly.instantiateStreaming(  fetch('module.wasm'),  { env: { log: (val) => console.log('from wasm:', val) } } // imports);const result = instance.exports.add(2, 3); // call an exported function// Loading a wasm-bindgen (Rust) packageimport init, { fibonacci } from './pkg/my_module.js';await init(); // loads and instantiates the .wasm binaryconsole.log(fibonacci(20)); // 6765

const memory = new WebAssembly.Memory({ initial: 1, maximum: 10 }); // pages of 64KiBconst { instance } = await WebAssembly.instantiateStreaming(  fetch('module.wasm'),  { env: { memory } });// Grow memory on demand (returns previous size in pages, or -1 on failure)const prevPages = memory.grow(1);// Strings aren't a Wasm type -- you pass a (ptr, len) pair into linear memoryfunction writeString(str, ptr) {  const bytes = new TextEncoder().encode(str);  new Uint8Array(memory.buffer, ptr, bytes.length).set(bytes);  return bytes.length;}function readString(ptr, len) {  const view = new Uint8Array(memory.buffer, ptr, len);  return new TextDecoder('utf-8').decode(view);}// The module must export an allocator so JS knows where it's safe to writeconst ptr = instance.exports.alloc(64);const len = writeString('hola mundo', ptr);instance.exports.greet(ptr, len);instance.exports.dealloc(ptr, 64);

// A Table holds function references (Wasm has no raw function pointers into linear memory)const table = new WebAssembly.Table({ initial: 4, element: 'anyfunc' });const { instance } = await WebAssembly.instantiateStreaming(  fetch('dispatch.wasm'),  { env: { callback_table: table } });// Install a JS function at a table slot so Wasm can invoke it by indexfunction jsCallback(x) { return x * 2; }const wrapped = new WebAssembly.Function(  { parameters: ['i32'], results: ['i32'] },  jsCallback);table.set(0, wrapped);// Wasm side (WAT) resolves the call at runtime through the table:// (call_indirect (type $unary) (local.get $index))// Invoke a Wasm-side function stored in the table directly from JSconst fn = table.get(1);console.log(fn(21)); // whatever the Wasm function at slot 1 returns

// Memory must be `shared: true` and the page needs COOP/COEP headers// (Cross-Origin-Opener-Policy: same-origin, Cross-Origin-Embedder-Policy: require-corp)const memory = new WebAssembly.Memory({  initial: 1,  maximum: 10,  shared: true, // backed by SharedArrayBuffer instead of ArrayBuffer});// Main thread: spin up workers, each instantiating the SAME module + memoryconst worker = new Worker('worker.js');worker.postMessage({ memory }); // structured-clone shares the SAB, not a copy// Inside worker.js, after instantiating with the shared memory import:const i32 = new Int32Array(memory.buffer);Atomics.add(i32, 0, 1);           // atomic increment, race-free across threadsAtomics.store(i32, 1, 42);const result = Atomics.load(i32, 1);// Block a worker until another thread signals (never do this on the main thread)Atomics.wait(i32, 2, 0);   // sleep while i32[2] === 0Atomics.notify(i32, 2, 1); // wake one waiter

# Compile a Rust binary to the WASI target instead of the browser targetrustup target add wasm32-wasip1cargo build --release --target wasm32-wasip1# Run with Wasmtime, granting only the capabilities you explicitly pass inwasmtime run \  --dir=. \  --env API_KEY=secret \  target/wasm32-wasip1/release/my_tool.wasm -- --input data.csv# WASI is capability-based: the sandboxed module can ONLY touch the# directories/env vars explicitly preopened above -- no ambient filesystem access