Form Effects

A Form Effect has three parts: a trigger starts it, an optional condition decides whether it applies, and actions do the work. A watched value, widget event, or another effect can start it. Effects live in the form schema's root effects[] array and can change controls, run a server-side script, or invoke another effect.

Start with one visible result

When a user changes Country, the selected City is cleared. That prevents a city from the previous country remaining in the form. This example assumes the form already has country and city controls.

1. In the Form Designer, open Effects with the lightning icon and select Add effect.
2. Name the effect clear-city-after-country-change and keep Value change.
3. Add $value.country to Listen.
4. Add a Clear action for the literal field path city and choose Empty.
5. In the preview, select a city and then change the country. City becomes blank.

The effect saved in the schema is:

{
  "id": "clear-city-after-country-change",
  "listen": ["$value.country"],
  "do": [{"type": "clear", "field": "city", "mode": "empty"}]
}

Changing City alone does not run this effect. The initial Country value is also only a baseline; use runOnInitialization when an effect must run once as the form opens. See Triggers and conditions.

Read an effect

Part	In this example	Question to ask
Trigger	listen: ["$value.country"]	What starts the rule?
Condition	No when property	Must anything else be true?
Action	clear on city	What changes when it runs?

When to Use an Effect

Choose the simplest form feature that expresses the required behavior:

Requirement	Recommended solution
Display a read-only value calculated from other fields	A JEXL default expression on that field
Hide, disable, or make a widget read-only	A dynamic widget property
Change, clear, or copy another form control	A Form Effect
React to a button, tab, step, attachment, or another widget event	A widget-event Form Effect
Execute a server-side script as part of a form workflow	A script action in a Form Effect
Dispatch page-level UI or store behavior after an operation	A Success / Error Action

Effect or computed value?

Use a computed default for a pure value derived into the same control. Use an effect when the form must change another control, clear user input, copy a value, or start an external operation.

Explore the workflow

Next page	What it explains
Triggers and conditions	Value changes, widget events, explicit calls, and when
Actions	Setting, copying, and dispatching values
Clearing and resetting	The three clear modes and array row behavior
Scripts and execution order	Server responses, waiting, cascades, and reusable effects
Paths and expressions	Literal control paths, JEXL, context, and data sources
Arrays and nested forms	Row-scoped effects and complete schemas
Form Logic Editor	Visualize and edit the same effects
Diagnostics and troubleshooting	Validation messages and runtime investigation
Form Effects Reference	Root schema and effect object contract

To inspect which effects actually ran for a field in a live form, use the Form Runtime Debugger.