# Tests

Unit tests for Google Tag Manager custom templates help you validate the functionality of your templates. You can create a set of tests for each template
that can be run without needing to deploy your tag, which allows you to
continuously test your template's behavior during development. Each test can
provide sample input values, mock function calls, and assert code behavior.

## Limitations

- Unit tests do not check validation rules but you can manually check validation using the **Run Code** button.
- Permission checks do not happen on mocked APIs in unit tests.

This guide will walk you through how to write unit tests for your custom template. This example creates a variable template that takes an input string and returns
the uppercase version of that string.

1. Create a new variable template. Click **Templates** in the left navigation and click **New** under the **Variable Templates** section.

2. Click **Fields**.

3. Click **Add Field** and select **Text input** . Name the field `text1` and set the display name to *"Text 1"*.

4. In the **Code** tab, replace the default code with this sandboxed JavaScript:

       let input = data.text1;
       return input.toUpperCase();

5. Click **Tests** to open the testing tab.

6. Click **Add Test** and change the test's name from *"Untitled test 1"* to *"Handles strings"*.

7. Click on the expand icon () to reveal the test's sandboxed JavaScript editor. Replace the code with this sandboxed JavaScript:

       // Call runCode to run the template's code with a lowercase string
       let variableResult = runCode({text1: 'this is a test'});
       // Validate that the result of runCode is an uppercase string.
       assertThat(variableResult).isEqualTo('THIS IS A TEST');

   This test passes the string *`'this is a test'`* to the variable and verifies that the variable returns the expected value of *`'THIS IS A TEST'`* . The [`runCode`](https://developers.google.com/tag-platform/tag-manager/templates/api#runcode) API is used to run the template code in the **Code** tab. The argument to `runCode` is an object that is used as the data global. The [`assertThat`](https://developers.google.com/tag-platform/tag-manager/templates/api#assertthat) API returns an object that can be used to fluently make assertions about a subject's value.
8. Click **▶ Run Tests** to run the test. The output of the test will appear in the Console.

   The **▶ Run Tests** button runs all of the enabled tests in the template, in the order shown. To change the order, use the drag icon (⠿). A test can be temporarily enabled or disabled by clicking on the circle to the left of the test name. To run a single test, click the ▶ button that appears when you move the mouse over the test.

   The console should print the total number of tests run and the number of tests that failed, if any. In this case, only one test was run and it should pass.
9. Click **Add Test** again to add a second test. Change the test's name from *"Untitled test 2"* to *"Handles undefined"*.

10. Click on the test to expand it and reveal the sandboxed JavaScript editor. Enter sandboxed JavaScript in the editor:

        let variableResult = runCode({});
        assertThat(variableResult).isEqualTo(undefined);

    > [!NOTE]
    > **Note:** [`runCode`](https://developers.google.com/tag-platform/tag-manager/templates/api#runcode) is called with an empty object, which means the value of the `text1` field will be `undefined` in this test case.

11. Click **▶ Run Tests** to run all of the tests at once. The output of the test will appear in the console.

    The *Handles undefined* test should fail. Congratulations, you found a bug!
12. Click **Code** to go back and edit the template's sandboxed JavaScript code. Update the sandboxed JavaScript as follows:

        const getType = require('getType');

        let input = data.text1;
        if (getType(input) !== 'string') {
          return input;
        }
        return input.toUpperCase();

    The updated code follows the best practice of validating the `input` variable before using it.
13. Click **Tests** to go back to the list of test cases.

14. Click **▶ Run Tests** to run all of the test cases again. This time the *Handles undefined* test should pass.

15. Click **Save**, and close the Template Editor.