Rendering Fields Conditionally

You can render Form fields conditionally by defining a hideWhenExpression inside the hide property of your question definitions.

A hideWhenExpression is a JavaScript expression that returns a boolean value (true or false). Logical operators (opens in a new tab) are used to construct these expressions. Additionally, you can incorporate injected data sources and expression helper functions into your expression definitions.

To write a hideWhenExpression, include a code block like this within the questions array of your question definition:

{
  "hide": {
    "hideWhenExpression": "isEmpty(onArt) || onArt !== 'a899b35c-1350-11df-a1f1-0026b9348838'"
  }
}

In the example above, the logic provided instructs the Form Builder to hide (not render) the question with the onArt id if the value of its backing concept does not match the supplied value.

More examples

1. Using the isEmpty helper

{
  "hide": {
    "hideWhenExpression": "isEmpty(screenedForHepatitisB) || screenedForHepatitisB == '1066AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA'"
  }
}

In this example, the question being validated gets hidden if the question with the id screenedForHepatitisB is empty or if its value is the answer backed by the concept ID 1066AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA. Most often, if your hideWhenExpression is not working, it's likely because you did not use the isEmpty helper as shown above.

2. Using the sex model from a patient data source

{
  "hide": {
    "hideWhenExpression": "sex !== 'F'"
  }
}

In this example, the question being validated gets hidden if the patient's sex is F. For this expression to work, you would need to plug in a patient object that exposes a gender property mapped to either F (for female) or M (for male). Read more about this in the data sources guide.

3. Using the age model from a patient data source

{
  "hide": {
    "hideWhenExpression": "age >= 18"
  }
}

In this example, the question being validated gets hidden if the patient's age is greater than or equal to 18. For this expression to work, you would need to plug in a patient object that exposes an age value. Read more about this in the data sources guide.