Skip to content

How do I test the client?

Testing on the client is a little different than on the server.

This is because the code which is ultimately being executed in the browser is Javascript, translated from F# by Fable, and so it must be tested in a Javascript environment.

Furthermore, code that is shared between the Client and Server must be test in both a dotnet environment and a Javascript environment.

The SAFE template uses a library called Fable.Mocha which allows us to run the same tests in both environments. It mirrors the Expecto API and works in much the same way.

I'm using the standard template

If you are using the standard template then there is nothing more you need to do in order to start testing your Client.

You will find a folder in the solution named tests. Inside this there is a project, Client.Tests, which contains a single script demonstrating how to use Mocha to test the TODO sample.

Note the compiler directive here which makes sure that the Shared tests are only included when executing in a Javascript (Fable) context. They are covered by Expecto under dotnet as you can see in Server.Tests.fs.

1. Launch the test server

In order to run the tests, instead of starting your application using

dotnet fake build -t run
you should instead use
dotnet fake build -t runtests

2. View the results

Once the build is complete and the website is running, navigate to http://localhost:8081/ in a web browser. You should see a test results page that looks like this:

This command builds and runs the Server test project too. If you want to run the Client tests alone, you can simply launch the test server using npm run test:live, which executes a command stored in package.json.

I'm using the minimal template

If you are using the minimal template, you will need to first configure a test project as none are included.

1. Add a test project

In the src folder, create a create a .Net Standard library called Client.Tests.

cd src
dotnet new ClassLib -lang F# -o Client.Tests
cd ..
dotnet sln add src/Client.Tests

2. Reference the Client project

Reference the Client project from the Client.Tests project:

dotnet add Client.Tests reference Client

3. Add the Fable.Mocha package to Test project

Run the following command:

dotnet add Client.Tests package Fable.Mocha

4. Add a test

Delete the Library.fs file and replace it with a new file called Tests.fs to your test project. Add the following code to it:

module Tests

open Fable.Mocha
open Index

let client = testList "Client" [
    testCase "Hello received" <| fun _ ->
        let hello = "Hello Test!"
        let model, _ = init ()
        let model, _ = update (GotHello hello) model

        Expect.equal hello model.Hello "The hello should be set"

let all =
    testList "All"

let main _ = Mocha.runTests all

5. Add Test web page

Add a file called index.html to the root of the test project and add the following content to it:

<!DOCTYPE html>
        <title>SAFE Client Tests</title>

6. Add test webpack config

  • Add a new file root directory called webpack.tests.config.js.

  • Populate it with the contents of a Fable-compatible webpack config template such as this.

7. Update the test webpack config

  • Replace the CONFIG value in the webpack file you just created with the following:

    var CONFIG = {
        indexHtmlTemplate: './src/Client.Tests/index.html',
        fsharpEntry: './src/Client.Tests/Client.Tests.fsproj',
        outputDir: './src/Client.Tests',
        assetsDir: './src/Client.Tests',
        devServerPort: 8081,
        devServerProxy: undefined,
        babel: undefined

  • Remove all references and code from the webpack file that refers to MiniCssExtractPlugin or CONFIG.cssEntry.

8. Add launch command

Add the following entry to the scripts element in the Client project's package.json file:

"test:live": "webpack-dev-server --config webpack.tests.config.js"

9. Build the Tests

Save all changes and build the Client.Test project.

9. Launch the test website

Run the command

npm install
followed by
npm run test:live

Once the build is complete and the website is running, navigate to http://localhost:8081/ in a web browser. You should see a test results page that looks like this: