Testing
WebAssembly gives you near-native performance in the browser and on the server, but testing Wasm modules has historically been awkward. AssemblyScript — a TypeScript-like language that compiles to Wasm — changes that by bringing a familiar type system and a purpose-built test runner called as-pect. This post covers
Wasmtime
Wasmtime is a fast, standards-compliant WebAssembly runtime from the Bytecode Alliance. While wasm-bindgen connects Rust Wasm to browsers, Wasmtime targets server-side use cases: plugin systems, sandboxed computation, edge functions, and language-agnostic module hosting. WASI (WebAssembly System Interface) gives Wasm modules controlled access to OS resources — files,
wasm-bindgen
wasm-bindgen is the bridge between Rust WebAssembly and JavaScript. It generates the glue code that lets JS call Rust functions, pass complex types across the boundary, and use JavaScript APIs from Rust. That bridge is a rich source of bugs: type mismatches, ownership errors at the boundary, and behavior
WebAssembly
WebAssembly is no longer a browser novelty. Wasm modules power plugin systems, edge compute functions, serverless runtimes, and embedded logic in languages from Rust to C++ to Go. And like any production code, Wasm modules need tests — but testing Wasm is different from testing native code. The core challenge: Wasm
WebAssembly
WebAssembly is fast — often within 10–20% of native performance — but "fast" is relative. Whether your Wasm module is fast enough depends on the baseline, the workload, and the runtime. Benchmarking Wasm requires measuring both the Wasm execution and comparing it against native, and tracking performance over time
Rust
Wasmtime is a fast, standards-compliant WebAssembly runtime from the Bytecode Alliance. Its Rust crate lets you load, instantiate, and call WASM modules from a Rust host — making it ideal for host-side unit tests that verify WASM module behavior, test WASI system calls in a controlled sandbox, and validate
WebAssembly
WebAssembly performance is close to native — but "close" needs to be measured. Whether you're evaluating the overhead of crossing the WASM/JS boundary, comparing execution speed between runtimes, or profiling where your WASM module spends its time, you need specific tools: Criterion for Rust-level benchmarks,
Rust
wasm-pack is the standard toolchain for building and testing Rust code compiled to WebAssembly. Its wasm-pack test command runs your Rust tests inside a real browser (headless Chrome, Firefox, or Safari) or under Node.js, giving you confidence that your WASM module behaves correctly in the environments where
Go
Go has had WebAssembly support since Go 1.11 via GOARCH=wasm GOOS=js, and since Go 1.21 via the WASI target GOOS=wasip1. Testing Go WASM code requires a different test runner than the standard go test command — either the wasm_exec.js Node.js harness for browser-
WebAssembly
The WebAssembly Component Model is a standard for composing WASM modules with typed interfaces defined in WIT (WebAssembly Interface Types). It replaces ad-hoc ABI conventions with a formal type system — records, variants, options, results — and generates host and guest bindings automatically via wit-bindgen. Testing components means verifying these
WebAssembly
WebAssembly modules in browsers are instantiated via JavaScript and expose their functionality through JS exports. End-to-end and integration tests for WASM web apps use the same tools as regular web tests — Playwright for browser automation and Jest for unit-level JS/WASM integration — but with a few WASM-