> For the complete documentation index, see [llms.txt](https://docs.heisenware.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.heisenware.com/app-builder/build-backend/extension-nodes/filter.md).
# Filter
The filter acts as a conditional gate for your flow. It branches logic or halts it based on a condition and is the primary way to implement `if/else` scenarios in Heisenware.
To add a filter:
1. Click the + icon on the right side of a function output, modifier, or existing filter.
2. Select Filter from the list.
3. Click the new filter box (*Click to edit…*) to open the code editor and write your condition.
randomInteger function with filter extension node
## How filters work
A filter evaluates a JavaScript expression that must return a boolean value (`true` or `false`). The input value from the preceding output is available as the reserved variable `x`.
Click the filter icon on the left to evaluate the filter manually during development. The last result appears below the expression.
### Logical gate
If the result is `true`, the data passes on to the next node following the filter. If `false`, the flow halts at this point.
### Branching logic
Use the `true` and `false` states of the filter to trigger separate logic paths. For example, use the `true` state to trigger another function.
{% hint style="warning" %}
#### Falsy values halt the flow
A filter condition that evaluates to a falsy value (e.g., `false`, `0`, `null`, `""`) stops the execution of the flow.
{% endhint %}
## Filter examples
### Example 1: Threshold monitoring
Checks if a sensor value exceeds a critical limit.
Data:
```json
{
"temperature": 92.5,
"unit": "°C"
}
```
Filter content:
```javascript
x.temperature > 90
```
Return of the filter: `true`
### Example 2: Industrial error detection
Inspects a status message for specific keywords like "Error".
Data:
```json
"System Error: PLC Communication Timeout"
```
Filter content:
```javascript
x.includes('Error')
```
Return of the filter: `true`
### Example 3: Validating array data
Ensures that an array contains data before the flow processes it.
Data:
```json
[]
```
Filter content:
```javascript
x.length > 0
```
Return of the filter: `false`
## Using AI for filters
Use AI chatbots like ChatGPT, Claude, or Gemini to generate complex filter logic. Filters use standard JavaScript, so provide the AI with your data structure to get an immediate result.
For best results, copy this article as context for the AI. Use the Copy button at the top of the page or the Open in ChatGPT / Open in Claude buttons in the top navigation bar.
#### Recommended AI prompt
Copy and paste this prompt into your AI so it understands the Heisenware environment and its variable references.
```
I am working in Heisenware, a node-based visual programming tool for industrial applications.
I need a "Filter" expression that acts as a conditional gate in a flow.
Full documentation: https://docs.heisenware.com/app-builder/build-backend/extension-nodes/filter.md
Rules:
* A filter is a single JavaScript expression that must return true or false. JSONata is not supported here.
* The input data is referenced as x.
* If the result is falsy, the flow halts at the filter.
* No statements or variable declarations. For multi-step logic, use an IIFE: (() => { ... })().
* Reply with the expression only, no explanation, no markdown fences.
My input data (sample):
[Paste a sample of the data arriving at the filter here.]
My task:
[Describe your condition, e.g., "Only continue if pressure is between 2.0 and 4.5 bar."]
```
---
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