> For the complete documentation index, see [llms.txt](https://help.claritylims.illumina.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.claritylims.illumina.com/integration-toolkits/lltk/lltk-working-with-the-script/lltk-failing-a-script.md).

# Failing a Script

Use the *fail()* method to halt the script and display an error message. If the script is used on a step transition, this method also prevents the step from proceeding to the next screen.

**Example expression:**

{% code overflow="wrap" %}

```markup
-exp 'if (!step.hasValue( ::Batch ID:: )) { fail(::Batch ID step UDF is required::) }'
```

{% endcode %}

Note that this example also uses the *hasValue* function (see [Handling Non-Existent UDFs/Custom Fields](/integration-toolkits/lltk/lltk-working-with-the-script/lltk-specifying-custom-fields.md#handling-non-existent-udfscustom-fields)).


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