> ## Documentation Index
> Fetch the complete documentation index at: https://docs.usehandled.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Custom Validation

> Add complex validation rules using JSONata expressions

## Enable Validation

<Steps>
  <Step title="Add Fields">
    Add your form fields
  </Step>

  <Step title="Enable">
    Toggle **Custom Validation**
  </Step>
</Steps>

## Write Validation Expression

The expression receives all form data and returns:

* `undefined` if valid
* Array of error objects if invalid

### Example: Minimum Selection

Require at least 5 projects:

```jsonata theme={null}
($exists(projects) and $count(projects) >= 5)
? undefined
: [{"field": "projects", "message": "Select at least 5 projects"}]
```

### Example: Required Together

If warehouse is selected, region must be too:

```jsonata theme={null}
($exists(warehouse_id) and not $exists(region_id))
? [{"field": "region_id", "message": "Region is required when warehouse is selected"}]
: undefined
```

## Error Object Format

```json theme={null}
{
  "field": "field_name",
  "message": "Error message shown to user"
}
```

## Testing Validation

<Steps>
  <Step title="Enter Test Data">
    Enter example input data
  </Step>

  <Step title="Write Expression">
    Write your expression
  </Step>

  <Step title="View Output">
    View the output
  </Step>

  <Step title="Verify">
    Verify errors appear for invalid data
  </Step>
</Steps>

### Example Input

```json theme={null}
{
  "warehouse_id": "wh-123",
  "projects": [
    {"value": "1"},
    {"value": "2"}
  ]
}
```

### Example Output (invalid)

```json theme={null}
[
  {"field": "projects", "message": "Select at least 5 projects"}
]
```