Test paths

@xstate/test generates test paths that it walks through to execute your tests. Knowing how these paths are generated will make your tests more predictable.

Coverage

The following example models a checkbox:

ts
import { createTestMachine } from "@xstate/test";
const machine = createTestMachine({
  initial: "notChecked",
  states: {
    notChecked: {
      on: {
        CLICK: "checked",
      },
    },
    checked: {
      on: {
        CLICK: "notChecked",
      },
    },
  },
});

ts
import { createTestMachine } from "@xstate/test";
const machine = createTestMachine({
  initial: "notChecked",
  states: {
    notChecked: {
      on: {
        CLICK: "checked",
      },
    },
    checked: {
      on: {
        CLICK: "notChecked",
      },
    },
  },
});

You could take a few different approaches to ensure everything in this machine works:

State coverage

The first approach is to ensure full coverage of states, where you would want to test:

When checked is reached, the checkbox is displaying a checkbox.
When notChecked is reached, the checkbox is NOT displaying a checkbox.

Event coverage

State coverage is a good start, but we also want to ensure all the events are working. To do this, we can add a new test:

Ensure that CLICK changes the checkbox.

Putting these tests together, you’d end up with a single test path:

Assert we’re in the notChecked state.
Run the CLICK event.
Assert we’re in the checked state.

The test path above feels complete, but it’s not quite there. We now know that clicking the checkbox can change it from notChecked to checked. But we don’t know that the same will happen when we go the other way! That means our full test should be:

Assert we’re in the notChecked state.
Run the CLICK event.
Assert we’re in the checked state.
Run the CLICK event.
Assert we’re in the notChecked state.

In @xstate/test, we achieve the test path above by checking all transitions are covered, which means you get full coverage out of the box.

Multiple paths

Test setup can be expensive, whether you’re loading up a browser or just setting up a database. @xstate/test will speed up your tests by attempting to walk through your test model in as few paths as possible.

The following example models a login form:

ts
import { createTestMachine } from "@xstate/test";
const loginMachine = createTestMachine({
  initial: "showingLoginForm",
  states: {
    showingLoginForm: {
      on: {
        SUBMIT_VALID_FORM: 'loggedIn',
        SUBMIT_INVALID_FORM: 'passwordInvalid',
      }
    },
    loggedIn: {}
    passwordInvalid: {},
  },
});

ts
import { createTestMachine } from "@xstate/test";
const loginMachine = createTestMachine({
  initial: "showingLoginForm",
  states: {
    showingLoginForm: {
      on: {
        SUBMIT_VALID_FORM: 'loggedIn',
        SUBMIT_INVALID_FORM: 'passwordInvalid',
      }
    },
    loggedIn: {}
    passwordInvalid: {},
  },
});

This example would generate two test paths:

txt
showingLoginForm -> SUBMIT_VALID_FORM -> loggedIn
showingLoginForm -> SUBMIT_INVALID_FORM -> passwordInvalid

txt
showingLoginForm -> SUBMIT_VALID_FORM -> loggedIn
showingLoginForm -> SUBMIT_INVALID_FORM -> passwordInvalid

Two test paths are generated because the test model can’t transition away from the loggedIn state or the passwordInvalid state.

Condensing to a single path

If we were to model the machine slightly differently, the test model would generate a single path:

ts
import { createTestMachine } from "@xstate/test";
const loginMachine = createTestMachine({
  initial: "showingLoginForm",
  states: {
    showingLoginForm: {
      on: {
        SUBMIT_VALID_FORM: 'loggedIn',
        SUBMIT_INVALID_FORM: 'passwordInvalid',
      }
    },
    loggedIn: {
      on: {
        LOG_OUT: 'showingLoginForm'
      }
    }
    passwordInvalid: {},
  },
});

ts
import { createTestMachine } from "@xstate/test";
const loginMachine = createTestMachine({
  initial: "showingLoginForm",
  states: {
    showingLoginForm: {
      on: {
        SUBMIT_VALID_FORM: 'loggedIn',
        SUBMIT_INVALID_FORM: 'passwordInvalid',
      }
    },
    loggedIn: {
      on: {
        LOG_OUT: 'showingLoginForm'
      }
    }
    passwordInvalid: {},
  },
});

In the example above, we’ve added a LOG_OUT transition to loggedIn, which means the test model can navigate away from the loggedIn state. Now, the test model will run a single path:

txt
showingLoginForm
  -> SUBMIT_VALID_FORM -> loggedIn
  -> LOG_OUT -> showingLoginForm
  -> SUBMIT_INVALID_FORM -> passwordInvalid

txt
showingLoginForm
  -> SUBMIT_VALID_FORM -> loggedIn
  -> LOG_OUT -> showingLoginForm
  -> SUBMIT_INVALID_FORM -> passwordInvalid

The test above requires less setup while also testing more behavior.

Note: we don’t necessarily recommend running fewer test paths, but understanding this behavior is useful when using @xstate/test.