0.19.1-revision12 • Public • Published

node-test-runner Version

Runs elm-explorations/test suites in Node.js.

When people say “elm-test” they usually refer to either:

  • This CLI tool for running tests.
  • elm-explorations/test – an Elm package for defining tests that this CLI tool can run.


Not all versions of elm-explorations/test and this CLI tool work together!

elm-explorations/test elm-test CLI
>= 2.0.0 >= 0.19.1-revision10
<= 1.2.2 <= 0.19.1-revision9

Unfortunate behavior of 0.19.1-revision9 and older

  • elm-test init always installs the latest elm-explorations/test. This means that if you run elm-test init on version 0.19.1-revision9 or older, you will get elm-explorations/test 2.0.0 or later, which don’t work 100 % together (see the next point).
  • elm-test 0.19.1-revision9 or older do not validate that elm-explorations/test in your elm.json has a compatible version. If you upgrade to elm-explorations/test 2.0.0 or later but forget to upgrade the elm-test CLI, most things will still work, but test distribution diagrams (new in elm-explorations/test 2.0.0) won’t show up. So if you use Test.fuzzWith and wonder why distribution diagrams never show up – check your elm-test CLI version!
  • There exists an elm-test CLI version called just "0.19.1". It should have been called "0.19.1-revision1", but unfortunately isn’t. Don’t make the mistake thinking it’s the latest version! You always want "0.19.1-revisionX".


npm install --save-dev elm-test

Quick start

Install elm-explorations/test and create tests/Example.elm:

npx elm-test init

Run tests in the tests/ folder:

npx elm-test

Run tests in one particular file:

npx elm-test tests/Example.elm

Run tests in files matching a glob:

npx elm-test "src/**/*Tests.elm"

Note: The double quotes are important! Without quotes, your shell might expand the globs for you. With quotes, elm-test expands the globs. This way the watcher can pick up new tests matching the globs, and it will work cross-platform.

Run in watch mode:

npx elm-test --watch

Where to put tests

Locating files containing tests

There are 3 places you could put your tests:

  1. In the tests/ folder.

    This is the default and requires no extra setup.

  2. In any source directory ("source-directories" in elm.json for applications, src/ for packages) as separate files.

    A convention is to put test files next to the file it tests with a Tests suffix. For example, you could have src/LoginForm.elm and src/LoginFormTests.elm.

    This requires telling elm-test which folders/files to run. Examples:

    npx elm-test "src/**/*Tests.elm"
    npx elm-test test/frontend/elm

    You might also need to configure your editor to understand that the "test-dependencies" in your elm.json are available in these files.

  3. In already existing source files.

    This allows testing internal functions without exposing them. (Be aware that testing implementation details can sometimes be counter-productive.)

    This requires moving everything in "test-dependencies" in your elm.json into regular "dependencies", so your project still compiles. This also helps your editor. Note that this approach isn’t suitable for packages, since you don’t want your package to unnecessarily depend on elm-explorations/test.

You can mix all three variants if you want:

npx elm-test tests "src/**/*Tests.elm" app

In this example, "src" and "app" need to be in "source-directories" in elm.json.

Locating tests within files

For elm-test to find tests in your files you need to:

  1. Create top-level values of the type Test. You can name the values anything – the only thing that matters is that their type is Test.
  2. Expose them.


module LoginForm exposing (alreadyLoggedInTests, tests)

import Test exposing (Test)

tests : Test
tests =
    -- ...

alreadyLoggedInTests : Test
alreadyLoggedInTests =
    -- ...

Some prefer to expose a single Test value and group everything using describe. Some prefer to expose several Test values.

Also check out the elm-explorations/test quick-start guide!

Command Line Arguments

These are the most common commands and flags. Run elm-test --help for an exhaustive list.

Note: Throughout this section, the npx prefix is omitted for brevity.


Like elm install, except elm-test will install to "test-dependencies" in your elm.json instead of to "dependencies".

elm-test install elm/regex


Runs elm-test install elm-explorations/test and then creates a tests/Example.elm example test to get you started.

elm-test init requires an elm.json file up the directory tree, so you will need to run elm init first if you don’t already have one.

After initializing elm-test in your project, try out the example by running elm-test with no arguments.

elm init
elm-test init


Start the runner in watch mode. Your tests will automatically rerun whenever your project changes.

elm-test --watch


Run with a specific fuzzer seed, rather than a randomly generated seed. This allows reproducing a failing fuzz-test. The command needed to reproduce (including the --seed flag) is printed after each test run. Copy, paste and run it!

elm-test --seed 336948560956134


Define how many times each fuzz-test should run. Defaults to 100.

elm-test --fuzz 500


Specify which format to use for reporting test results. Valid options are:

  • console (default): pretty, human readable formatted output.
  • json: newline-delimited json with an object for each event.
  • junit: junit-compatible xml.
elm-test --report json


Disable colored console output.

Colors are also disabled when you pipe the output of elm-test to another program. You can use --color to force the colors back.

Alternatively, you can set the environment variable FORCE_COLOR to 0 to disable colors, or to any other value to force them.

See chalk.supportsColor for more information.


If elm is not in your $PATH when elm-test runs, or the Elm executable is called something other than elm, you can use this flag to point to your installation.

elm-test --compiler /path/to/elm

To run a tool installed locally using npm you can use npx:

npx elm-test

npx adds the local node_modules/.bin/ folder to $PATH when it executes the command passed to it. This means that if you have installed elm locally, elm-test will automatically find that local installation.

As mentioned in Installation we recommend installing elm-test locally in every project. This ensures all contributors and CI use the same version, to avoid nasty “works on my computer” issues.

Travis CI

If you want to run your tests on Travis CI, here's a good starter .travis.yml:

language: elm
  - 0.19.1

Here is an example travis.yml configuration file for running tests in CI.



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