Writing WebAssembly with Rust

Rust is the most popular programming language for WebAssembly development. It offers excellent tooling, memory safety, and near-zero-cost abstractions.

Why Rust + WASM?

• Memory safety — No garbage collector, no undefined behavior
• Excellent performance — Comparable to C/C++
• First-class tooling — Top-notch wasm-pack and wasm-bindgen support
• Rich ecosystem — Large library ecosystem on crates.io

Project Setup

1. Initialize Project

cargo new --lib my-wasm-lib
cd my-wasm-lib

2. Configure Cargo.toml

[package]
name = "my-wasm-lib"
version = "0.1.0"
edition = "2021"

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

[dependencies]
wasm-bindgen = "0.2"

3. Install wasm-bindgen-cli

cargo install wasm-bindgen-cli

Basic Example

Rust Code

// src/lib.rs
use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub fn greet(name: &str) -> String {
    format!("Hello, {}!", name)
}

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

#[wasm_bindgen]
pub fn fibonacci(n: u32) -> u32 {
    match n {
        0 => 0,
        1 => 1,
        _ => fibonacci(n - 1) + fibonacci(n - 2),
    }
}

Build

wasm-pack build --target web

Generated files:

• pkg/my_wasm_lib_bg.wasm — Binary WASM module
• pkg/my_wasm_lib.js — JS bindings

JavaScript Usage

import init, { greet, add, fibonacci } from './pkg/my_wasm_lib.js';

await init();

console.log(greet('World'));     // "Hello, World!"
console.log(add(10, 20));      // 30
console.log(fibonacci(10));   // 55

Handling Complex Types

Passing Arrays

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub fn sum_array(arr: &[i32]) -> i32 {
    arr.iter().sum()
}

Returning Strings

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub fn reverse_string(s: &str) -> String {
    s.chars().rev().collect()
}

Using Structs

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub struct Counter {
    count: i32,
}

#[wasm_bindgen]
impl Counter {
    #[wasm_bindgen(constructor)]
    pub fn new() -> Self {
        Counter { count: 0 }
    }

    #[wasm_bindgen]
    pub fn increment(&mut self) -> i32 {
        self.count += 1;
        self.count
    }

    #[wasm_bindgen]
    pub fn decrement(&mut self) -> i32 {
        self.count -= 1;
        self.count
    }

    #[wasm_bindgen]
    pub fn get(&self) -> i32 {
        self.count
    }
}

impl Default for Counter {
    fn default() -> Self {
        Self::new()
    }
}

Performance Optimization

Optimization	Note
#[inline(always)]	Force inline small functions
release profile	Required for production
lto = true	Link-time optimization
opt-level = "s"	Optimize for size
opt-level = "z"	Smaller optimized size

Optimized Cargo.toml

[profile.release]
opt-level = 3
lto = true
codegen-units = 1
panic = "abort"
strip = true

What's Next

You've mastered Rust + WASM basics! Continue learning Core Concepts.

Resources

• Rust and WebAssembly Book
• wasm-bindgen Guide
• crates.io — Search for wasm-related packages