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Elm's standard testing library is elm-test. You can install it by running the following command \ in your project. It will ask questions, just say yes to everything:

elm install elm-explorations/test

The next thing you need to do is set up a test framework. We have Lobo available internally. This is how you install it for your project:

npm install lobo --save

Now make sure you have a tests directory in the root of the project and that in your elm.json it is listed in source-directories.

Now you are ready to run the tests:

npx lobo --framework=elm-test

This will also ask a lot of questions, just say yes to everything (you only need to do this once). In the end it will show something like this:

==================================== Summary ===================================
Passed: 0
Failed: 0

runCount: 100
seed: 3473325025

This means we are ready to add tests. You can start by creating a tests/Tests.elm with the following content:

module Tests exposing (..)

import Expect
import Test exposing (Test, test)

testExpectTrue : Test
testExpectTrue =
test "Expect.true test" <|
\() ->
|> Expect.true "Expected true"

testExpectNotEqual : Test
testExpectNotEqual =
test "Expect Not Equal" <|
\() ->
Expect.notEqual "foo" "foobar"

Now you are ready to run the tests:

npx lobo --framework=elm-test

This should report 2 passing tests. Now you are ready to add real tests. For that follow the inestructions here: elm-explorations/test