## Custom Client-Side Validator - #js_custom {}

  Attach a custom validator, written in Javascript, to an element. The
  validator will be executed on the client when a postback is
  triggered. Validation failure will prevent the postback.

  See [Live Validation's Validate Custom](http://livevalidation.com/documentation#ValidateCustom) for more information about the
  validation function and args.

### Security Note

	Note that because this is a purely Javascript-based validation, that you
	can't trust this validator in the same way you can trust that the
	server-side validators will ensure that a value is valid prior to postback.
	You will still likely have to verify your values server-side to protect
	against tampering.

	This validator is provided more to give faster response to your users, since
	Javascript doesn't have to communicate with the server to validate. It
	usually goes together with the `#custom` server-side validator.

### Usage

```erlang
   % Wire the validator against the 'my_validator' Javascript
   % function, passing in some args...
   wf:wire(ButtonID, TextBoxID, #validate { validators=[
     #js_custom {
       text="Validation failed.",
       function=my_validator,
       args="{ amount: 5 }"
     }
   ]})

```

```erlang
   wf:wire(ButtonID, TextBoxID, #validate { validators=[
     #js_custom {
       text="Validation failed.",
       function="function(value, args) { return value == 'the only correct answer'} ",
       when_empty=true }
   ]})

```

### Attributes

   * `text` (string) - The text to display if validation fails.

   * `function` (atom or string) - The name of a Javascript function of an
     anonymous javascript function that returns true if the value is valid, false
     if invalid.
     function(Value, Args) -> 'true' or 'false'

   * `args` (JSON String) - Extra arguments passed to the custom validation
     function.
   
   * `when_empty` (boolean) - Run the validator even if the specified
     field's value is empty. By default, the custom validator doesn't apply if
     the field is empty, since you would normally use a separate `#is_required`
     for that. (default: `false`)

### See Also

	*  [Custom Action (server side)](custom.md)

	*  [Validate Action](validate.md)

	*  [Validation Demos](http://nitrogenproject.com/demos/validation)