Alpha Version: You are viewing the ALPHA documentation. This is an experimental version and may contain breaking changes.
Skip to main content

Running tests

Scenarios are ReScript sources that compile to .res.mjs before Jest sees them, so a test run needs a build first. That is the one fact behind most confusing test failures: a stale build runs stale scenarios.

Set up a plugin package for tests​

Add Jest and the scenario DSL as dev dependencies:

pnpm add -D @reventlessdev/reventless-gwt jest

Then point Jest at the compiled output and give it the ESM flag:

{
  "scripts": {
    "build": "rescript build",
    "test": "NODE_OPTIONS='--experimental-vm-modules' npx jest",
    "dev": "npx jest --watchAll"
  },
  "jest": {
    "testMatch": [
      "<rootDir>/tests/**/*Test.res.mjs",
      "<rootDir>/tests/**/*_GWT.res.mjs"
    ],
    "moduleFileExtensions": ["js", "mjs"]
  }
}
The ESM flag is not optional

Compiled scenarios are ES modules, so Jest needs NODE_OPTIONS='--experimental-vm-modules' to load them at all. Without it every suite fails with SyntaxError: Cannot use import statement outside a module — which looks like a broken test suite rather than a missing flag. Always run tests through the test script that sets it, never a bare npx jest.

The loop​

pnpm run build     # compile .res → .res.mjs
pnpm test          # run every scenario in the package

For a tight loop, run the ReScript compiler in watch mode in one terminal and pnpm run dev (Jest --watchAll) in another: saving a .res file recompiles it and Jest re-runs the affected suites within seconds.

Where scenarios live​

Mirror src/ under tests/, one scenario file per spec:

src/Category/StateChangeSlice/AddCategory.res
tests/Category/StateChangeSlice/AddCategory_GWT.res

The _GWT suffix is what wires the file up: the file-level @@reventless.gwt annotation finds the matching spec from the filename and pulls in the Given/When/Then vocabulary, so the test body is only the scenarios themselves. See Writing scenarios for the DSL.

Running a subset​

Jest takes a path pattern, so any fragment of a filename works:

pnpm test AddCategory          # one spec's scenarios
pnpm test tests/Category       # one entity's folder

In a multi-package repository, run tests from the package directory rather than the root — the root run compiles and executes everything, which is the right thing before a commit and the wrong thing while iterating.

When a run looks wrong​

  • Every suite fails to load — the ESM flag is missing (see above).
  • A change you just made has no effect — the build did not run, or ran in a different package. Rebuild, then re-run.
  • A suite disappeared from the run — check the compiled .res.mjs exists; a compile error in one file leaves the previous output in place, so the suite silently runs an older version.

See also​