Retrieving value from input in EJS with NodeJS - node.js

Question

Here is my EJS code:
<form action="/" method="POST">
<% items.forEach((item)=>{ %>
<div class="item">
<input type="checkbox" name="checkbox" value="<%=item._id%>" onchange="this.form.submit()">
<p><%= item.name %></p>
</div>
<% }) %>
</form>
And here is the app.js:
app.post('/',(req,res)=>{
console.log(req.body.checkbox);
})
My questions are:
1) Why does it only log the value of the selected checkbox (assumed that there are 3 items) instead of 3 values, is it because of the post method only post the checkbox's value that has the property checked == true?
2) If I add another checkbox like below, why does it return undefined if I try to console.log(req.body.item_id)?
<form action="/" method="POST">
<% items.forEach((item)=>{ %>
<div class="item">
<input type="checkbox" name="checkbox" value="<%=item._id%>" onchange="this.form.submit()">
<p><%= item.name %></p>
<input type="checkbox" name="item_id" value="<%=item._id%>">
</div>
<% }) %>
</form>
3) I've tried to do the same thing above but this time it is using input type of text. Why does the console.log(req.body.item_id) show 3 values instead of 1 (which corresponds with the selected checkbox like in question 1)?
<form action="/" method="POST">
<% items.forEach((item)=>{ %>
<div class="item">
<input type="checkbox" name="checkbox" value="<%=item._id%>" onchange="this.form.submit()">
<p><%= item.name %></p>
<input name="item_id" value="<%=item._id%>">
</div>
<% }) %>
</form>
All response are appreciated. Thank you!

Answer 1

That is the default behaviour of the checkbox, it only gets
passed as data if its checked, if it doesn't get passed, we can
assume it wasn't checked
However if you still want the checkbox in data even if it was not checked, one hack would be
Create a hidden element with same name as your checkbox & set value as 'off'
Before submit, if the checkbox is checked, disable the hidden input element
this will give your checkbox value 'on' when checked & 'off' when its not
As mentioned above, it will only added to formdata if its
checked, when its not checked, the element is not added to formdata
at all, so you will get undefined if its not checked
Again this is default behaviour of input types, the reason it
does not do same for checkbox is because, maybe you are only
selecting 1 checkbox with same name before submit, if you do select
multiple checkbox with same name in checkboxes, you'll get same
result for checkboxes too, but due to the reason mentioned in first
answer, it only get's us the element selected which is 1 in your
case