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There will be no more updates to Elmer. It was fun while it lasted, but I'm pretty sure Elmer won't work with Elm 0.19.1 or later versions, and I'm not planning to make any more updates.

Instead, you should use elm-spec.

Elm-spec is a test framework for Elm that lets you describe the behavior of Elm programs, much like Elmer did. Unlike Elmer, elm-spec doesn't use kernel/native code so it has been published to the Elm package repository and should be more resilient to future changes in Elm.


Elmer makes it easy to describe the behavior of Elm HTML applications. If you love TDD and you love Elm, then you'll probably appreciate Elmer.


Behavior-driven development is a great practice to follow when writing applications. If you describe the behavior of your app in code, it's easy to add new features or refactor the code with confidence: just run the tests to see if your app still has all the behavior you've described in those tests.

Elm is a really great language for writing web applications. However, practicing BDD in Elm can be difficult. The two main functions in the Elm architecture -- view and update -- return opaque types, which cannot be inspected for testing purposes. Even so, calling view or update directly requires knowledge of an application's implementation: the shape of its model, its messages, and so on. If writing tests requires knowledge of implementation details, you lose the biggest benefit of writing tests in the first place: the ability to change your code with confidence.

Elmer allows you to describe the behavior of your app without knowledge of implementation details. It simulates the Elm architecture, calling view and update as necessary throughout the course of your test. It lets you manage how commands and subscriptions are processed so you can describe the behavior of your app under whatever conditions you need. Elmer allows you to write tests first, which gives you the freedom and confidence to change your code later on.

Getting Started

Because Elmer uses some native Javascript code to accomplish its magic, you cannot install Elmer through the elm package repository. Instead, you can install Elmer with the elmer-test package on NPM. Follow these steps to TDD bliss ...


First, you'll need to install

  • Elm (0.19)
  • The latest version of the node test runner for the elm test package that works with Elm 0.19 (elm-test@elm0.19.0)
  • This package, which you'll install via npm

I recommend installing these dependencies locally in your project directory so you can track versions carefully. Here's the command to install all these at once:

$ npm install --save-dev elm elm-test@elm0.19.0 elmer-test

Now install the elm test library:

$ npx elm install elm-explorations/test

Update the elm.json file

In your elm.json file, you'll need to manually add elmer to the test-dependencies section like so:

"test-dependencies": {
  "direct": {
    "elm-explorations/test": "1.1.0",
    "elm-explorations/elmer": "6.0.0"
  "indirect": {}

The latest version of Elmer is 6.0.0. Make sure the version number of elmer matches the version number of the elmer-test NPM package.

Notice the indirect section under test-dependencies. Elmer itself has the following dependencies:

"dependencies": {
  "elm/browser": "1.0.0 <= v < 2.0.0",
  "elm/core": "1.0.0 <= v < 2.0.0",
  "elm/html": "1.0.0 <= v < 2.0.0",
  "elm/json": "1.0.0 <= v < 2.0.0",
  "elm/random": "1.0.0 <= v < 2.0.0",
  "elm/url": "1.0.0 <= v < 2.0.0",
  "elm-explorations/test": "1.0.0 <= v < 2.0.0"

If any of these dependencies are not already listed as direct or indirect dependencies of your app, you'll need to list these in the indirect section of your test-dependencies.

If you just try to run elm-test (see below) and you're missing any dependencies, the compiler will give you an error message. Take the missing dependencies it mentions and list them as indirect test dependencies.


Now that everything's in place, you're ready to write tests with Elmer. In order to run those tests, you'll need to set the ELM_HOME environment variable to the home directory under the elmer-test install. If you've installed elmer-test locally, the directory should look like this:

<Project Home>/node_modules/elmer-test/home

I recommend adding a test script to your package.json that sets the environment variable for you. The following will work on a Mac running bash:

"scripts": {
  "test": "ELM_HOME=$(pwd)/node_modules/elmer-test/home elm-test"

Note that ELM_HOME must be an absolute path (thus the $(pwd) in the test command).


The elm command searches for test dependencies any time you invoke it (so, even if you aren't running tests). This means that you will need to set the ELM_HOME environment variable as described above, any time you invoke the elm command. For example, to build your app, you'll need to do something like:

$ ELM_HOME=$(pwd)/node_modules/elmer-test/home elm make src/Main.elm



  • Removed Elmer.Http and the dependency on elm/http. Elmer.Http now lives in its own package so it can be updated independently.
  • Provided new APIs useful for creating extensions and custom matchers. See Elmer.Value, Elmer.Message, Elmer.Message.Failure,Elmer.Effects, and Elmer.Task


  • Support for calling a spy across multiple test states


  • Revised Elmer.Spy api to make it simpler and to allow the compiler to do type checking when injecting a spy or providing a fake implementation. This should provide better feedback when working with spies
  • See the section at the end of this document on migrating tests from 4.0.0 to 5.0.x


  • Updated Elmer to work with Elm 0.19
  • Revised api for targeting Html elements to allow the compiler to provide better feedback
  • See the section at the end of this document on migrating tests from 3.3.1 to 4.0.0


  • Last version for Elm 0.18


Read the latest documentation.

If you're interested in Elmer for Elm 0.18, you should read the documentation for Elmer 3.3.1, which you can find here.

Describing Behavior

While tests can be written with Elmer in a variety of ways, the goal is use Elmer to describe behavior rather than implementation details. What do I mean? Let's say that an application is a collection of behaviors. Each behavior has some pre-conditions -- these are characteristics of the world outside the application that must be true for the behavior to occur -- and some set of resulting post-conditions -- characteristics of the world outside the application that are a consequence of the behavior. To describe the behavior of an application, then, is to describe all the relationships that hold between relevant states of the world outside the application due to the use of that application.

Here's an example of a behavior from some game application that displays high scores:

  • Given that there is an HTTP web service that responds with a 200 status and a JSON document that lists the high scores in some format.
  • When the user starts the game application, then the high scores are displayed as list items in HTML.

This behavior links one state of the world -- where there is an HTTP web service that successfully returns a JSON document in some known format -- and another -- where some HTML document contains several <li> elements whose text shows the high scores from the web service.

To describe this behavior, we should not care how the application accomplishes the mapping between these two states. We only care about describing the two states. To make the test pass, we will need to provide some implementation, but the test gives us freedom to choose whatever implementation makes sense for us now. Most importantly, however, as we add new behaviors to our application, we will be able to refactor our code with confidence. No matter what implementation we end up with, we should still be able to run this test and ensure that the same mapping between pre-conditions and post-conditions still holds.

Like I've said, you can use Elmer to write tests in a variety of ways, but I encourage you to write tests that describe behavior so that you can refactor your code with confidence later on. This means your tests should know as little as they can about the implementation of your Elm application. Strive to write tests that do not know the shape of your model or the particular messages that flow through the update function. Don't unit test functions. Begin each test only with references to the functions that must exist -- view, update, init -- and use Elmer to describe the pre- and post-conditions associated with some behavior.

Create a TestState

To begin a test with Elmer, you need to generate a TestState value. There are a variety of ways to do this:

  • Use givenElement, givenApplication, givenDocument, or givenWorker from the Elmer.Program module to test particular kinds of programs. In these cases, you'll use Elmer.Program.init to provide an initial model and command.
  • Use Elmer.given to test an arbitrary model, view method, and update method.
  • Use Elmer.Command.given to test a command-generating function in isolation.

Working with HTML

Since Elm is primarily designed for writing HTML applications, much of the work that goes into describing the pre- and post-conditions that characterize some behavior will involve working with HTML elements.

Elmer allows you to simulate events on elements and examine the state of elements. In order to do either, you'll need to first target an element.

Targeting an Element

Use along with the functions from Elmer.Html.Selector to target an element. Here's a partial test that targets all the <li> elements that are children of an <ol> with a class scores in the current view:

allTests : Test
allTests =
  describe "My Fun Game"
  [ describe "High Score Screen"
    [ test "it shows the high scores" <|
      \() ->
        Elmer.Program.givenElement App.view App.update
          |> Elmer.Program.init (\_ -> App.init testFlags)
              << Elmer.Html.Selector.childrenOf 
                [ Elmer.Html.Selector.tag "ol"
                , Elmer.Html.Selector.class "score-list"
                [ Elmer.Html.Selector.tag "li" ]

See Elmer.Html.Selector for more examples of selectors. It's also possible to write your own.

Taking action on an element

Once you target an element, that element is the subject of subsequent actions, until you target another element. The following functions define actions on elements:

  • Click events: <testState>
  • Input events: Elmer.Html.Event.input <text> <testState>
  • Custom events: Elmer.Html.Event.trigger <eventName> <eventJson> <testState>
  • There are also events for mouse movements, and checking and selecting input elements. See the docs for more information.

Element Matchers

You can make expectations about targeted elements with the Elmer.Html.expect function.

First, specify whether you want to match against a single element (with element) or a list of elements (with elements). Then you provide the appropriate matchers for the element or the list. You can also expect that an element exists with the elementExists matcher.

See Elmer.Html.Matchers for a full list of matchers. Let's add to the example above to make an expectation about the elements we targeted. We'll use Elmer.expectAll to chain together several assertions. We'll expect that the list of <li> we've targeted has 2 elements, with the first containing text of "700 Points" and the second "900 Points".

allTests : Test
allTests =
  describe "My Fun Game"
  [ describe "High Score Screen"
    [ test "it shows the high scores" <|
      \() ->
        Elmer.Program.givenElement App.view App.update
          |> Elmer.Program.init (\_ -> App.init testFlags)
              << Elmer.Html.Selector.childrenOf 
                [ Elmer.Html.Selector.tag "ol"
                , Elmer.Html.Selector.class "score-list"
                [ Elmer.Html.Selector.tag "li" ]
          |> Elmer.Html.expect (Elmer.Html.Matchers.elements <|
              [ Elmer.hasLength 2
              , Elmer.atIndex 0 <| Elmer.Html.Matchers.hasText "700 Points"
              , Elmer.atIndex 1 <| Elmer.Html.Matchers.hasText "900 Points"


Commands describe actions to be performed by the Elm runtime; the result of a command depends on the state of the world outside the Elm application. Elmer simulates the Elm runtime in order to facilitate testing, but it is not intended to replicate the Elm runtime's ability to carry out commands. Instead, Elmer allows you to specify what effect should result from running a command. This is one important way that Elmer allows you to describe the conditions that characterize an application behavior.

Faking Effects

Suppose there is a function f : a -> b -> Cmd msg that takes two arguments and produces a command. In order to specify the effect of this command in our tests, we will override occurrences of f with a function we create in our tests. This function will generate a special command that specifies the intended effect, and Elmer will process the result as if the original command were actually performed.

For a more concrete example, check out this article, which discusses how to fake commands and subscriptions during a test.

Note that while Elmer is not capable of processing any commands, it does support the general operations on commands in the core Platform.Cmd module, namely, batch and map. So, you can use these functions as expected in your application and Elmer should do the right thing.

Elmer provides built-in support for navigation commands. If you want to work with Http during your tests, check out the Elmer.Http extension.


Elmer provides support for functions in the Browser.Navigation module that allow you to handle navigation for single-page web applications.

You'll need to begin your test with Elmer.Program.givenApplication since only Elm 'application' programs can handle navigation. Provide a reference to the messages that handle new url requests and url changes along with the view and update functions. Then provide Elmer.Spy.use with Elmer.Navigation.spy so that Elmer will be able to record and process location updates by overriding Browser.Navigation.pushUrl and Browser.Navigation.replaceUrl.

When you call Elmer.Program.init you'll need to use Elmer.Navigation.fakeKey to give your init function a Browser.Navigation.Key value. Here's an example of a test that expects the location to change when an element is clicked.

Elmer.Program.givenApplication App.OnUrlRequest App.OnUrlChange App.view App.update
  |> Elmer.Spy.use [ Elmer.Navigation.spy ]
  |> Elmer.Program.init (\_ -> App.init testFlags testUrl Elmer.Navigation.fakeKey)
  |> << by [ id "some-element" ]
  |> Elmer.Navigation.expectLocation ""

You can write an expectation about the current location with Elmer.Navigation.expectLocation.

See tests/src/Elmer/TestApps/NavigationTestApp.elm and tests/src/Elmer/NavigationTests.elm for examples.

Deferred Command Processing

It's often necessary to describe the behavior of an application while some command is running. For example, one might want to show a progress indicator while an HTTP request is in process. Elmer provides general support for deferred commands. Use Elmer.Command.defer to create a command that will not be processed until Elmer.resolveDeferred is called. Note that all currently deferred commands will be resolved when this function is called.

Testing Commands in Isolation

You might want to test a command independently of any module that might use it. In that case, use Elmer.Command.given and provide it with a function that generates the command you want to test. This will initiate a TestState that simply records any messages that result when the given command is processed. You can use the Elmer.Command.expectMessages function to make any expectations about the messages received. For example, here's a test that expects a certain message when a certain command is processed:

Elmer.Command.given (\_ -> MyModule.myCommand MyTagger withSomeArgument)
  |> Elmer.Command.expectMessages (\messages ->
    Expect.equal [ MyTagger "Fun Result" ]

You can use Elmer.Command.given with spies as it makes sense. So, you might write a test that exercises a module with some function that needs to be stubbed (like a port command):

Elmer.Command.given (\_ -> MyModule.sendRequest MyTagger someArgument)
  |> Elmer.Spy.use [ someSpy ]
  |> Elmer.Command.expectMessages (\messages ->
    Expect.equal [ MyTagger "Fun Result" ]


Using subscriptions, your application can register to be notified when certain effects occur. To describe the behavior of an application that has subscriptions, you'll need to do these things:

  1. Override the function that generates the subscription using Elmer.Spy.create along with Elmer.Spy.andCallFake and replace it with a fake subscription using Elmer.Subscription.fake
  2. Register the subscriptions using Elmer.Subscription.with
  3. Simulate the effect you've subscribed to receive with Elmer.Subscription.send

Here's an example test:

timeSubscriptionTest : Test
timeSubscriptionTest =
  describe "when a time effect is received"
  [ test "it prints the number of seconds" <|
    \() ->
        timeSpy =
          Elmer.Spy.observe (\_ -> Time.every)
            |> Elmer.Spy.andCallFake (\_ tagger ->
              Elmer.Subscription.fake "timeEffect" tagger
        Elmer.given App.defaultModel App.view App.update
          |> Elmer.Spy.use [ timeSpy ]
          |> Elmer.Subscription.with (\() -> App.subscriptions)
          |> Elmer.Subscription.send "timeEffect" (Time.millisToPosix 3000)
          |> << by [ id "num-seconds" ]
          |> Elmer.Html.expect (
              Elmer.Html.Matchers.element <|
                Elmer.Html.Matchers.hasText "3 seconds"

For a more complete example, check out this article.


You can manage ports during your test in just the same way you would manage any command or subscription.

Suppose you have a port that sends data to Javascript:

port module MyModule exposing (..)

port sendData : String -> Cmd msg

You can create a spy for this function just like you would for any command-generating function:

Elmer.Spy.observe (\_ -> MyModule.sendData)
  |> Elmer.Spy.andCallFake (\_ -> Cmd.none)

Note that you will need to provide a fake implementation of this method since otherwise Elmer will not know how to handle the generated command.

A port that receives data from Javascript works just the same as any subscription.

port receiveData : (String -> msg) -> Sub msg

type Msg = ReceivedData String

subscriptions : Module -> Sub Msg
subscriptions model =
  receiveData ReceivedData

We can create a spy for this subscription-generating function and provide a fake subscription that will allow us to send data tagged with the appropriate message during our test.

  spy =
    Elmer.Spy.observe (\_ -> MyModule.receiveData)
      |> Elmer.Spy.andCallFake (\tagger ->
           Elmer.Subscription.fake "fake-receive" tagger
  Elmer.given MyModule.defaultModel MyModule.view MyModule.update
    |> Elmer.Spy.use [ spy ]
    |> Elmer.Subscription.with (\_ -> MyModule.subscriptions)
    |> Elmer.Subscription.send "fake-receive" "some fake data"
    |> ...

Testing Tasks

Elm uses tasks to describe asynchronous operations at a high-level. You can use Elmer to describe the behavior of applications that use the Task API. To do so:

  1. Stub any task-generating functions to return a task created with Task.succeed or and the value you want as necessary for the behavior you want to describe.

  2. That's it.

Elmer does not know how to run any tasks other than Task.succeed and However, Elmer does know how to properly apply all the functions from the Task API. In this way, Elmer allows you to describe the behavior that results from operations with tasks without actually running those tasks during your test.

Here's an example. Suppose when a button is clicked, your app creates a task that gets the current time, formats it (using some function called formatTime : Time -> String), and tags the resulting string with TagFormattedTime like so:
  |> formatTime
  |> Task.perform TagFormattedTime

You can test this behavior by replacing with a Task.succeed that resolves to the time you want.

  timeSpy =
    Task.succeed (Time.millisToPosix 1515281017615)
      |> Spy.replaceValue (\_ ->
    |> Elmer.Spy.use [ timeSpy ]
    |> << by [ id "get-current-time" ]
    |> << by [ id "current-time" ]
    |> Elmer.Html.expect (
      element <| hasText "1/6/2018 23:23:37"

See the Elmer.Task module for more tasks that are useful when writing tests or building extensions to Elmer.

Spies and Fakes

Elmer generalizes the pattern for managing the effects of Subs and Cmds, allowing you to spy on any function you like. NOTE You should use Elmer spies sparingly and with care. Each spy that you add to your test couples that test to implementation details.

Suppose you need to write a test that expects a certain function to be called, but you don't need to describe the resulting behavior. You can spy on a function with Elmer.Spy.observe and make expectations about it with Elmer.Spy.expect.

For example, suppose you want to ensure that a component is calling a specific function in another module for parsing some string. You have tests for the parsing function itself; you just need to know that your component is using it.

parseTest : Test
parseTest =
  describe "when the string is submitted"
  [ test "it passes it to the parsing module" <|
    \() ->
        spy = 
          Elmer.Spy.observe (\_ -> MyParserModule.parse)
            |> Elmer.Spy.andCallThrough
        Elmer.given App.defaultModel App.view App.update
          |> Elmer.Spy.use [ spy ]
          |> << by [ tag "input", attribute ("type", "text") ]
          |> Elmer.Html.Event.input "A string to be parsed"
          |> Elmer.Spy.expect (\_ -> MyParserModule.parse) (
            wasCalled 1

Elmer also allows you to provide a fake implementation for any function. Suppose that you want to stub the result of the parsing function:

parseTest : Test
parseTest =
  describe "when the string is submitted"
  [ test "it displays the parsed result" <|
    \() ->
        spy =
          Elmer.Spy.observe (\_ -> MyParserModule.parse)
            |> Elmer.Spy.andCallFake (\_ ->
              "Some Parsed String"
        Elmer.given App.defaultModel App.view App.update
          |> Elmer.Spy.use [ spy ]
          |> << by [ tag "input", attribute ("type", "text") ]
          |> Elmer.Html.Event.input "A string to be parsed"
          |> << by [ id "parsing-result" ]
          |> Elmer.Html.expect (Elmer.Html.element <|
            Elmer.Html.Matchers.hasText "Some Parsed String

For any spy, you can make an expectation about how many times it was called like so:

Elmer.Spy.expect (\_ -> MyModule.someFunction) (wasCalled 3)

You can also expect that the spy was called with some list of arguments at least once:

Elmer.Spy.expect (\_ -> MyModule.someFunction) (
    [ Elmer.Spy.Matchers.stringArg "someString"
    , Elmer.Spy.Matchers.anyArg
    , Elmer.Spy.Matcher.intArg 23

See Elmer.Spy.Matchers for a full list of argument matchers.

Elmer.Spy.observe is good for spying on named functions in your production code. Sometimes, though, it would be nice to provide the code you are testing with a 'fake' function for testing purposes only. Suppose that you are testing a module that takes a function as an argument, and you want to expect that the function is called with a certain argument. You can create a 'fake' function in your test module, observe it with a spy, and provide it to the code you're testing using Elmer.Spy.inject. For example:

someFake tagger data =
  Command.fake <| tagger data

myTest =
    spy =
      Spy.observe (\_ -> someFake)
        |> Spy.andCallThrough
    Elmer.given testModel MyModule.view (MyModule.updateUsing <| Spy.inject (\_ -> someFake))
      |> Elmer.Spy.use [ spy ]
      |> << by [ tag "input" ]
      |> Elmer.Html.Event.input "some text"
      |> << by [ tag "button" ]
      |> Elmer.Spy.expect (\_ -> someFake) (
          [ Elmer.Spy.Matchers.anyArg
          , Elmer.Spy.Matchers.stringArg "some text"

Use Elmer.Spy.inject to provide the function you want to observe so that Elmer has time to install the associated spy during the test.

Finally, you can use Spy.replaceValue to replace the value returned by a no-argument function (such as during a test. You can't make expectations about spies created in this way; Spy.replaceValue is just a convenient way to inject fake values during a test.


It's easy to build extensions on top of Elmer to provide custom matchers or extra functions that help describe the behavior of an Elm application. In particular, Elmer.Value, Elmer.Message, and Elmer.Effects provide functions that are useful when writing custom matchers or extension modules.

For a good example of an extension, see the Elmer.Http package, which adds support for describing the behavior of apps that use HTTP.

Upgrading from Elmer 5.x

The Elmer.Http api has been removed and moved to its own package.

Elmer.Command.resolveDeferred has moved to Elmer.resolveDeferred to accomodate the fact that Tasks can also be deferred.

Upgrading from Elmer 4.x

If you've written tests with Elmer 4.x, the Elmer.Spy api has changed:

  • Elmer.Spy.create is now Elmer.Spy.observe and no longer needs a string identifier.

  • When using Elmer.Spy.observe you must call either Elmer.Spy.andCallThrough or Elmer.Spy.andCallFake so that Elmer knows what to do when the observed function is called.

  • Elmer.Spy.expect now uses a reference to the observed function (rather than a string) to identify the function you'd like to assert about.

  • Elmer.Spy.createWith has been removed. Create a 'fake' function in your test module and observe it with Elmer.Spy.observe instead.

  • Elmer.Spy.callable has been removed. Instead, use Elmer.Spy.inject to provide a 'fake' function to the code under test.

Upgrading from Elmer 3.x

If you've written tests with Elmer 3.x and plan to upgrade them to Elmer 4.0.0, here are some things you'll need to consider:

  • Elmer.Platform.Command has changed to Elmer.Command.

  • Elmer.Platform.Subscription has changed to Elmer.Subscription.

  • Elmer.Headless.givenCommand has been replaced with Elmer.Command.given.

  • Elmer.Headless.expectMessages has been replaced with Elmer.Command.expectMessages.

  • Elmer.Headless.given has been replaced with Elmer.Program.givenWorker.

  • Elmer.Http.expectThat has been replaced with Elmer.Http.expect.

  • Elmer.Http.expect has been replaced with Elmer.Http.expectRequest.

  • The <&&> operator has been replaced with Elmer.expectAll.

  • Elmer.Html.Matchers.hasProperty has been removed. Use Elmer.Html.Matchers.hasAttribute instead.

  • now has a new api. Combine functions in the Elmer.Html.Selector module to build a an element selector.


To run the tests:

$ cd tests
$ ./


Describe the behavior of Elm HTML applications





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