package melange-jest

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Melange bindings for Jest


Dune Dependency






Melange bindings for Jest and jest-dom.

Based on @glennsl/bs-jest and bs-jest-dom.


Most of what's commonly used is very stable. But the more js-y parts should be considered experimental, such as mocking and some of the expects that don't transfer well, or just don't make sense for testing idiomatic Reason/OCaml code but could be useful for testing js interop.

  • Global: Fully implemented and tested, apart from require.*

  • Expect: Mostly implemented. Functionality that makes sense only for JS interop have been moved to ExpectJs. Some functionality does not make sense in a typed language, or is not possible to implement sensibly in ML.

  • Mock Functions: Experimental and unsafe implementation, very much in flux. The Jest bindings will most likely be relegated to the MockJs module as it's very quirky to use with native code. A separate native from-scratch implementation might suddenly appear as Mock.

  • The Jest Object: Fake timers are fully implemented and tested. Mock functionality has been moved to JestJs. It's mostly implemented, but experimental and largely untested.

  • Snapshotting: Expect functions exist and work, but there's currently no way to implement custom snapshot serializers.


open Jest;

describe("Expect", () => {
  open Expect;

  test("toBe", () =>
    expect(1 + 2) |> toBe(3))

describe("Expect.Operators", () => {
    open Expect;
    open! Expect.Operators;

    test("==", () =>
      expect(1 + 2) === 3)

See the jest tests or the jest-dom tests for more examples.


Install opam package manager.


opam install melange-jest

The bindings support the following versions of the jest npm package, which should be installed separately:

  "devDependencies": {
    "jest": "^26.5.2"

If you want to use jest-dom, you will need the following npm package:

  "devDependencies": {
    "@testing-library/jest-dom": "^5.10.0"


Add melange-jest.jest to the libraries field in your dune file:

; ...
  (libraries melange-jest.jest)
; ...

If you need jest-dom, add melange-jest.jest-dom to the libraries in your dune file:

; ...
  (libraries melange-jest.jest melange-jest.jest-dom)
; ...


Put tests in a __tests__ directory and use the suffix ** (Make sure to use valid module names. e.g. <name> is valid while <name> is not). When compiled they will be put in a __tests__ directory under lib, with a *test.js suffix, ready to be picked up when you run jest. If you're not already familiar with Jest, see the Jest documentation.

One very important difference from Jest is that assertions are not imperative. That is, expect(1 + 2) |> toBe(3), for example, will not "execute" the assertion then and there. It will instead return an assertion value which must be returned from the test function. Only after the test function has completed will the returned assertion be checked. Any other assertions will be ignored, but unless you explicitly ignore them, it will produce compiler warnings about unused values. This means there can be at most one assertion per test. But it also means there must be at least one assertion per test. You can't forget an assertion in a branch, and think the test passes when in fact it doesn't even test anything. It will also force you to write simple tests that are easy to understand and refactor, and will give you more information about what's wrong when something does go wrong.

At first sight this may still seem very limiting, and if you write very imperative code it really is, but I'd argue the real problem then is the imperative code. There are however some workarounds that can alleviate this:

  • Compare multiple values by wrapping them in a tuple: expect((this, that)) |> toBe((3, 4))

  • Use the testAll function to generate tests based on a list of data

  • Use describe and/or beforeAll to do setup for a group of tests. Code written in OCaml/Reason is immutable by default. Take advantage of it.

  • Write a helper function if you find yourself repeating code. That's what functions are for, after all. You can even write a helper function to generate tests.

  • If you're still struggling, make an issue on GitHub or bring it up in Discord. We'll either figure out a good way to do it with what we already have, or realize that something actually is missing and add it.


For the moment, please refer to Jest.mli.


1. __tests__ folder is being ignored by Dune

By default, Dune will ignore folders starting with underscore, so to add __tests__ as part of the sources, one has to indicate it explicitly.

In a dune file:

(dirs :standard __tests__)

2. Error Cannot use import statement outside a module

If you encounter the error SyntaxError: Cannot use import statement outside a module, it may be that you are trying to run Jest tests with es6 files generated by Melange.

As Melange allows to have generate both es6 and commonjs outputs in the same project, to solve this issue you can add a melange.emit stanza that only generates commonjs files, for testing purposes.

In a dune file:

  (target test)
  (module_systems commonjs)

Then, configure Jest rootDir to point to the output folder (see jest.config.js for an example).


git clone
cd melange-jest
make install

Then build and run tests with make test, run the tests in watch mode with make test-watch.



  • [BREAKING] Actually removed toThrowException, toThrowMessage and toThrowMessageRe as they relied on assumptions about BuckleScript internals that no longer hold.


  • Added Expect.toContainEqual

  • Updated to Jest 26.5.2

  • Upgraded bs-platform to 8.3.1


  • Added Expect.toMatchInlineSnapshot


  • Updated to Jest 25.1.0


  • Added Todo.test


  • Updated jest to 24.3.1

  • Fixed jest warnings not to return anything from describe callbacks by explicitly returning undefined (otherwise BuckleScript will return something else like (), which is represented as 0)

  • Fixed several newly uncovered uncurrying issues caused by surprise breaking changes in BuckleScript (Thanks again, Bob!)

  • Added Jest.advanceTimersByTime, which is basically just an alias of Jest.runTimersToTime


  • Added Expect.not__ for transitional compatibility with Reason syntax change of "unkeywording" not by mangling it into not_, and not_ into not__ and so on.


  • Made uncurrying explicit for afterAllPromise too.


  • Made uncurrying explicit to fix a breaking change in implicit uncurrying from bs-platform 4.0.7 (Thanks Bob!)


  • Removed some optimizations on skipped tests that Jest 23 suddenly started objecting to (#30)


  • Added MockJs.new0, new1 and new2

  • Added timeout argument to testAsync and testPromise functions

  • Added beforeEachAsync, beforeEachPromise, afterEachAsync and afterEachPromise

  • Added beforeAllAsync, beforeAllPromise, afterAllAsync and afterAllPromise


  • Moved repository from reasonml-community/bs-jest to glennsl/bs-jest

  • Renamed NPM package from bs-jest to @glennsl/bs-jest


  • Added toThrowException

  • Fixed an issue with custom Runner implementation shadowing the global test function from jest

  • Fixed a typo in the js boundary of not_ |> toBeLessThanEqual


  • Removed deprecations

  • Added testAll, Only.testAll, Skip.testAll that generates tests from a list of inputs

  • Fixed type signature of fail

  • Added expectFn


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