Clean field and fdropdown onchange

Hi!
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I've a form with four dropdown: A, 1, 2 and 3 (1, 2 and 3 are hidden).
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When a user make a choice in A, one of the other dropdown becomes visible (ex: 1). When the user make a choice in this dropdown a lot of field (text and dropdown) are shown.
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If the user change his choice in 1, the visible fields change.
If the user change his choice in A (ex: 2), every fields return invisible except 2. When the user make a choice in 2, others fields are shown, but some are the same for 1 and 2 (or 2 and 3, or 3 and 1, but not for 1, 2 and 3). So i need to clean every fields when A changes.
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In CF5 i've a simple js script to do that, like:

document.getElementById("field_id").value = "";

for every fields. The script worked for the text fields and for the dropdown fields.
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In CF6 (every things is the same: name, id, etc.) it works only for the text fields.
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I've tried also

document.getElementById("field_id").selectedIndex = "0";

or

document.getElementById("field_id").selectedIndex = 0;

they work perfectly with a not CF form but not in CF.
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What am I doing wrong?
There's another way?
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Thank you all!

Hi Pigna,
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v6 has a special dropdown effect and this may not work, but you may try this:

$("#field_id").val("value here");

OR

$('#field_id').dropdown('set selected', "value here");

Best regards

Thank you Max,
i've tried

$("#field_id").val("");

$("#field_id").val(0);

$("#field_id")[0].selectedIndex = 0;

$("#field_id option").eq(0).prop("selected", true);

they all works out CF but not in CF6 (the first works also for the text fields in CF6)

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Sorry but i don't understand what is 'set selected' in your

$('#field_id').dropdown('set selected', "value here");

Hi Max!
​
I've got the solution analysing the code with the browser element inspector. I need to use

$("#field_id").val("");

AND

 $("#field_id ~ div").eq(0).text("");

If I use only the first nothing happens, but if I use only the second the dropdown field becomes empty and not works anymore.
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I hope there aren't collateral problems doing this.

I had tried before but probably I had something wrong.
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It's work perfectly!
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Thank's Max.
​
P.