Hello I select a select in a list in javascript with
document.getElementById('age1').selectedIndex = 3;
it shows me the 4th value of the list.
I then want to add a conditional field in relation to the value of this select but the condition does not work. If I choose myself a value in the list the condition works.
Do we need to add something for the condition to execute?
THANKS

I think you need to change the display property for the field you want to hide from 'display' to 'none' or if the selected index in the age1 field is 3. Opposite way round to show it if 3 is selected,
something like
if (document.getElementById('age1').selectedIndex = 3) {

document.getElementById('conditionalFieldName').style.display = "none"; //to hide
document.getElementById('conditionalFieldName').style.display = "block"; // to show
}