as-test

as-test is a runtime-aware test runner for AssemblyScript.

You write ordinary specs, snapshots, mocks, and fuzzers once, then run them against the runtime you actually ship to — WASI, Node bindings, the browser, or a standalone runtime like wasmtime — through named modes. Tests are compiled to WebAssembly and executed there, so what you assert is what your users get.

It ships as a CLI (ast / as-test) and an npm dev dependency. There are no required peer dependencies; value formatting for assertions, snapshots, and log() is built in.

What you get

• Specs in assembly/__tests__/*.spec.ts with describe / test / it, lifecycle hooks, and focus/skip/todo.
• A full matcher set — deep equality, numeric comparisons, type and truthiness checks, string and collection matchers, snapshots, and toThrow.
• Snapshots stored under .as-test/ and updated from the CLI.
• Mocking — swap local functions (mockFn) or host imports (mockImport) at compile time.
• Fuzzing — property tests with deterministic, reproducible seeds in assembly/__fuzz__/*.fuzz.ts.
• Coverage — branch-aware instrumentation with project/dependency filtering.
• Runtimes & modes — one suite, many targets, selected with --mode.
• Reporters — a human-readable default and TAP, plus custom reporter modules.

Start here

1. Getting Started — install, scaffold, and run your first suite.
2. Writing Tests — suites, hooks, focus/skip, and logging.
3. Assertions — the matcher reference.
4. Add Snapshots, Mocking, or Fuzzing when you need them.
5. Runtimes & Modes — run the same specs across WASI, bindings, and the web.
6. CLI and Configuration — once the defaults stop being enough.

Project layout

A typical project looks like this:

assembly/
  __tests__/
    math.spec.ts
  __fuzz__/
    parser.fuzz.ts
.as-test/
  build/        compiled wasm artifacts
  runners/      generated runtime entry scripts
  logs/         captured log() output
  snapshots/    stored snapshots
  crashes/      fuzz crash artifacts
as-test.config.json

If you also need fast JSON for AssemblyScript data structures, see json-as.