Develop Reference

how to change required to disable by javascript?

I am doing to change 2nd select tag name by javascript.
this is my HTML
1st select
<select name="bank" id="bank" class="select" onchange="myFunction(event)">
<option value="Bank Account" a="required">Bank Account</option>
<option value="UPI" a="hidden">UPI</option>
</select>
2nd Select
<select name="bank1" class="select" id="bank1" required>
<option disabled selected>Choose Bank Name</option>
<option value="SBI">SBI</option>
<option value="PNB" >PNB</option>
</select>
this is my javascript
function myFunction(e) {
document.getElementById("bank1").name = e.target.a
but i want to change in 2nd select required to hidden

probably you are looking for setAttribute.
document.getElementById("bank1").required = false;
And for style mutation. As #Kokodoko had already written.
document.getElementById("bank1").classList.add("hidden")

Do you want to hide the second select box completely?
function myFunction(e) {
document.getElementById("bank1").classList.add("hidden")
}
CSS
#bank1.hidden {
display:none;
}

Disable/enable text field and list box as per value selected in list box

Disable/enable text field and list box as per user selected value from parent list box.
Condition:
if user don't know programming then list box and text box must disable.
However it is not working, I know I am missing something.only Javascript please
function check()
{
//if(document.drop_list.choice[1].checked){
if(document.getElementById("mt").value === 'N'){
document.getElementById("no").disabled=true;
document.getElementById("txt").disabled=true;
}else{
document.getElementById("no").disabled=false;
document.getElementById("txt").disabled=false;
}
}
<form name="drop_list" action="listbox-validation-demock.php" method="post" id='f1'>
Do you want to learn Web programming languages ?
<select name="Category" id="mt" onChnage="check()">
<option value='Y'>Y</option>
<option value="N">N</option>
</select>
<br /><br />
<select name="Category" id="no">
<option value=''>Select One</option>
<option value="PHP">PHP</option>
<option value="ASP">ASP</option>
<option value="JavaScript">JavaScript</option>
<option value="HTML with design">HTML</option>
<option value="Perl">Perl</option><option value="MySQL">MySQL</option></select>
<br /><br />
Age<input type="text" size="20" id="txt">
</form>

It should have been :
<select name="Category" id="mt" onChange="check()">

Your script works, the only problem is a typo:
<select name="Category" id="mt" onChnage="check()">
-----------------------------------^^^-------------

Need JavaScript equivalent of jQuery changing the value of a dropdown when the value of a text field changes

I am working on a Formstack form. I need to use Javascript to change the value of a dropdown box to whatever the value being typed into a text field is once a match is made.
<input type="text" id="field35497729" name="field35497729" size="50" value="" class="fsField">
<select id="field35497839" name="field35497839" size="1" class="fsField">
<option value=""> </option>
<option value="CIPSmember">CIPSmember</option>
<option value="TECHCONNEXmember">TECHCONNEXmember</option>
<option value="TCBCpreferred">TCBCpreferred</option>
<option value="TCBCcomp2015">TCBCcomp2015</option>
</select>
So as soon as someone types in CIPSmember into the text field, the dropdown should be selected with the same value. If there is no match, the dropdown has no selection.
I used the following jQuery on jsFiddle, but it is not working on Formstack:
$('#field35497729').keyup( function() {
$("#field35497839").val($('#field35497729').val());
}
);
Here is one Javascript method I am trying on jsFiddle that does not work:
document.getElementByID('field35497729').onkeyup = function() {
document.getElementById('field35497839').value = document.getElementByID('field35497729').value;
};
I checked here, here and maybe 10 other places but I can't get it to work. There are plenty of tutorials on how to get a text field to change when a dropdown selection change, but not nearly as many on the opposite.

misspelled ID in getElementById
missing end bracket on the jQuery version
simplified to use this and $(this)
I am however curious. Perhaps you want an autocomplete instead?
Here are your fixed versions
Plain JS version
window.onload=function() {
document.getElementById('field35497729').onkeyup = function() {
document.getElementById('field35497839').value = this.value;
}
}
<input type="text" id="field35497729" name="field35497729" size="50" value="" class="fsField">
<select id="field35497839" name="field35497839" size="1" class="fsField">
<option value=""> </option>
<option value="CIPSmember">CIPSmember</option>
<option value="TECHCONNEXmember">TECHCONNEXmember</option>
<option value="TCBCpreferred">TCBCpreferred</option>
<option value="TCBCcomp2015">TCBCcomp2015</option>
</select>
jQuery version
$(function() {
$('#field35497729').on("keyup",function() {
$("#field35497839").val($(this).val()); // or (this.value)
})
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<input type="text" id="field35497729" name="field35497729" size="50" value="" class="fsField">
<select id="field35497839" name="field35497839" size="1" class="fsField">
<option value=""> </option>
<option value="CIPSmember">CIPSmember</option>
<option value="TECHCONNEXmember">TECHCONNEXmember</option>
<option value="TCBCpreferred">TCBCpreferred</option>
<option value="TCBCcomp2015">TCBCcomp2015</option>
</select>

Set input text default value and select option value then disable

Using JavaScript, how can I:
set a default value (and disable it) on the textbox
disable the <select> option "Days"
? This is my HTML:
Deliver every <input type="text" maxlength="2" size="3">
<select>
<option value="Days">Days</option>
<option value="Weeks">Weeks</option>
</select>

I hope this one would work.
<input type="text" id="defaultDays" size="3">
<select id="disableOption">
<option value="Days">Days</option>
<option value="Weeks">Weeks</option>
</select>
<script type="text/javascript">
document.getElementById('defaultDays').setAttribute('value','28');
document.getElementById('defaultDays').disabled = true;
document.getElementById('disableOption').disabled = true;
</script>
Thank you guys for your previous comments.

Display other field onchange

I have here selectbox with option value USA and OTHERS. What I need is if I choose OTHERS on Selectbox1 the Selectbox2 should be display none and textbox1 comes out. Other problem also is it's name field, If I POST the value of name="cboState" . How to remove the name of a field? Any help will appreciate.
<form action="try.php" method="POST">
<select id="cboCountry">
<option value="USA">USA</option>
<option value="OTHERS">OTHERS</option>
</select>
Selectbox 2
<select id="cboState" name="cboState">
<option value="Alabama">Alabama</option>
</select>
Textbox 1
<input type="text" id="cboState" name="cboState"/>
<input type="submit">
</form>

Try this
HTML
<select id="cboCountry">
<option value="USA">USA</option>
<option value="OTHERS">OTHERS</option>
</select>
<select id="cboState" name="cboState">
<option value="Alabama">Alabama</option>
</select>
<input type="text" id="txtcboState" name="cboState" style="display:none;"/>
JS
$("#cboCountry").change(function() {
if ( $(this).val() == "OTHERS") {
$("#cboState").hide();
$("#txtcboState").show();
}
else{
$("#cboState").show();
$("#txtcboState").hide();
}
});
Note: ID Must be unique for each element.
To remove The name attribute of input, you can use the following
$("#txtcboState").removeAttr('name');
DEMO HERE

Something like this?
$("#selectbox1").onchange(function() { selectionChanged();});
function selectionChanged() {
if ( $("#selectbox1").val() == "other") {
$("#selectbox2").hide();
$("#textBox1").show();//should already be hidden
}
}
Make sure the IDs for your elements are unique.