---
title: "__isVarDefined"
description: "JMeter __isVarDefined function reference: syntax, parameters, and practical examples for parameterizing test plans with dynamic values."
url: https://docs.jmeter.ai/functions/isvardefined/
lastUpdated: 2026-10-01
source: docs.jmeter.ai
---

# __isVarDefined

*Also documented in context in the [full Functions and Variables reference](/user-manual/functions/#__isVarDefined).*

The `__isVarDefined` function returns true if variable exists or false if not.

| Name | Required | Description |
| --- | --- | --- |
| Variable Name | Yes | The Variable Name to be used to check if defined |

Example:

```bash
\${__isVarDefined(JMeterThread.last_sample_ok)}
```

will return `true`

## Related

- [Functions and Variables Guide](/topics/functions-and-variables/)
- [Regex Extractor Builder](/tools/regex-tester/)
- [Full Functions Reference](/user-manual/functions/)
- [Component Reference](/user-manual/component-reference/)