Targets & Runtimes

as-test compiles each spec to WebAssembly and runs it on a real runtime. The target decides how the wasm is built; the runtime decides what executes it. You pick the target in buildOptions.target and the runtime in runOptions.runtime.cmd.

Target	Built for	Default runner
wasi	WASI runtimes (Node's WASI, or standalone engines)	.as-test/runners/default.wasi.js
bindings	a JavaScript host (Node/Bun)	.as-test/runners/default.bindings.js
web	a browser	.as-test/runners/default.web.js

The runner scripts are generated by ast init and are thin: they call instantiate() from as-test/lib, which wires up host I/O and the WIPC channel the CLI uses to collect results.

WASI

The wasi target builds against @assemblyscript/wasi-shim and runs on any WASI preview1 runtime. The default runner uses Node's built-in WASI. The same artifact also runs on standalone engines — see Multiple Runtimes for wasmtime / wasmer / wazero modes.

{
  "buildOptions": { "target": "wasi" },
  "runOptions": { "runtime": { "cmd": "node ./.as-test/runners/default.wasi.js" } }
}

WASI is the default target and the most portable. Host I/O (including the result channel) goes through standard WASI file descriptors, so nothing extra needs to be injected at instantiation — which is why WASI can run on engines as-test doesn't control.

Bindings

The bindings target produces a JavaScript host (Node or Bun) plus an AssemblyScript bindings helper. This is the target to choose when your code calls @external host functions you implement in JS. Bindings come in two flavors, raw and esm, with different capabilities — see Bindings: raw & esm.

{
  "buildOptions": { "target": "bindings" },
  "runOptions": { "runtime": { "cmd": "node ./.as-test/runners/default.bindings.js" } }
}

Web

The web target runs specs in a real browser. The runner serves the wasm and helper, drives the browser, and streams results back. Choose the browser with runtime.browser (or --browser), and add --headless for CI.

{
  "buildOptions": { "target": "web" },
  "runOptions": {
    "runtime": {
      "cmd": "node ./.as-test/runners/default.web.js --headless",
      "browser": "chromium"
    }
  }
}

Supported browsers: chrome, chromium, firefox, webkit, or a path to an executable.

Choosing a target

• Default to wasi — most portable, runs anywhere, no host glue.
• Use bindings when specs depend on JS-provided @external imports, or when you ship to a JS host.
• Use web to validate browser-specific behavior.

You don't have to choose just one. Multiple Runtimes shows how to run the same specs across several targets in a single command.