Paths and Expressions

Choose the right path syntax

Form Effects use several similar-looking values with different syntax. This is the most important distinction to remember:

Property	Expected value	Example
listen	A fully prefixed watched path	$value.customer.id
field, from, to	A literal form-control path	customer.id
when, set.value	A JEXL expression	${$value.customer.id != null}
scriptData, runEffect.inputs	Values or objects containing JEXL expressions	{"id": "${$value.customer.id}"}
effectId	A literal effect ID	process-customer
objectPointer	A JSON Pointer to a widget in the schema	/layout/0/items/2

Control paths use dot notation. Do not add $value, ${...}, or JSON Pointer slashes to field, from, or to. The only dynamic segment supported in an action path is the fluent-array row placeholder $row.

// Correct
{"type": "set", "field": "customer.email", "value": "${$value.contact.email}"}

// Incorrect: field is not a JEXL expression
{"type": "set", "field": "${$value.customer.email}", "value": "${$value.contact.email}"}

listen is the trigger, not automatic dependency discovery

Reading $value.customer.id in when, set.value, or scriptData does not automatically make the effect react to that field. Add every form value whose change should rerun the effect to listen. The designer warns when it detects a $value dependency that is not covered by listen.

Legacy listen entries that wrap a simple path in ${...} still work, but new effects should use a direct prefixed path.

Expression Context

Effect expressions use the same JEXL runtime as computed form properties, with an effect-specific scope:

Value	Meaning in a Form Effect
$value	Current value of the complete root form.
$context	Form context from the Fluent Forms store.
$outputContext	Output context from composed widgets and flows.
$config	Application/API configuration.
$configUi	Reserved for UI configuration. The current Form Effects runtime supplies {}, so do not depend on it yet.
$runtimeInfo	Current user and runtime metadata.
$version	Application version.
$build	Build metadata.
$row	Concrete row index for a row-scoped value effect or array widget event; 0 when no row scope is present.
dataSources	Registered datasource functions.
dataSourcesValues	Settled datasource values available to the evaluator.
$dataSourcesLoading	Whether an expression datasource is still resolving.
$event	Complete semantic event in a widget-event effect.
$payload	Shorthand for $event.payload.
$input	Evaluated input object in an invoked effect.
$response	Script result in the script's direct outcome branch.

The stage-specific values are preserved in a script outcome branch belonging to that same effect. An invoked effect receives only the values explicitly mapped into $input.

Member access

Author ordinary dot and bracket access in Form Effects expressions:

$value.customer.address.street
$response.items[0].name

The expression compiler adds safe member access. Do not add JavaScript optional access operators such as ?. to persisted Form Effects expressions. Use a fallback when the final result can be missing:

$value.customer.address.street || 'N/A'

See the JEXL language reference for operators, functions, transforms, and Public API data sources.

Datasources

Only datasources declared for effect usage can be called from an effect expression. The current runtime resolves datasource calls in when, set.value, and runEffect.inputs; it does not resolve them in scriptData:

Effect mode	Runtime behavior
plainValue	Resolve a scalar or object value on the first emission.
identifiedPack	Wait until the returned pack has loading !== true. Read the result from .data.
unsupported	Reject the datasource in an effect and omit it from effect autocomplete.

Example using an identifiedPack datasource:

{
  "id": "load-user-name",
  "listen": ["$value.userId"],
  "do": [
    {
      "type": "set",
      "field": "userName",
      "value": "${evalScriptByCode('GetUserName').data}"
    }
  ]
}

Datasource parameters are reevaluated when their dependencies change. A new value-change run supersedes an older synchronous run that has not completed.