Cheat Sheet

Quick reference for Rust + WebAssembly development

Project Setup

Cargo.toml

[lib]
crate-type = ["cdylib"]

[dependencies]
wasm-bindgen = "0.2"
web-sys = { version = "0.3", features = ["Window", "Document"] }
js-sys = "0.3"
wasm-bindgen-futures = "0.4"

Terminal

# Install
cargo install wasm-pack

# Build
wasm-pack build --target web

# Build for bundler (Webpack, Vite)
wasm-pack build --target bundler

#[wasm_bindgen] Attributes

Usage	Rust	JavaScript
Export function	`#[wasm_bindgen] pub fn foo()`	`foo()`
Struct as class	`#[wasm_bindgen] pub struct Foo`	`new Foo()`
Constructor	`#[wasm_bindgen(constructor)]`	`new Foo()`
Getter	`#[wasm_bindgen(getter)]`	`obj.name`
Setter	`#[wasm_bindgen(setter)]`	`obj.name = x`
Import JS function	`extern "C" { fn alert(s: &str); }`	`window.alert()`
JS namespace	`#[wasm_bindgen(js_namespace = console)]`	`console.log()`
Rename	`#[wasm_bindgen(js_name = "myFunc")]`	`myFunc()`
Skip export	`#[wasm_bindgen(skip)]`	Hidden

Type Mapping

Rust	JS	Cost
`i32, f64, bool`	`number, boolean`	Free
`i64, u64`	`BigInt`	Free
`&str`	`string` (JS→Rust)	Copy
`String`	`string` (Rust→JS)	Copy
`Vec<u8>, &[u8]`	`Uint8Array`	Copy
`Vec<f64>`	`Float64Array`	Copy
`JsValue`	`any`	Ref
`Option<T>`	`T \| undefined`	Varies
`Result<T, JsValue>`	`T` (throws on Err)	Varies
`#[wasm_bindgen] struct`	`class`	Pointer
`Closure<dyn FnMut()>`	`Function`	Alloc

Common Patterns

Export Function

#[wasm_bindgen]
pub fn add(a: i32, b: i32) -> i32 { a + b }

Call from JS

import init, { add } from './pkg/my_wasm.js';
await init();
add(2, 3); // → 5

Import console.log

#[wasm_bindgen]
extern "C" {
    #[wasm_bindgen(js_namespace = console)]
    fn log(s: &str);
}

Error Handling

#[wasm_bindgen]
pub fn run() -> Result<(), JsValue> {
    // ? converts Err to JS exception
    let el = document.create_element("div")?;
    Ok(())
}

Common web-sys Features

WindowDocumentElementHtmlElementHtmlCanvasElementCanvasRenderingContext2dconsoleNodeStorageMouseEventKeyboardEventRequestRequestInitResponseHeadersUrlHistoryLocation

Performance Tips

✓ Prefer primitives (i32, f64, bool) — zero cost
✓ Use flat arrays (Vec<f64>) for bulk data transfer
✓ Reuse buffers — don't allocate Vec per frame
✓ Structs pass as pointers — no data copy
✗ Avoid frequent string passing — each crossing copies data
✗ Don't pass structured data via JSON — use flat arrays