Setting the Value of a Checkbox Input If Not Checked

POST unchecked HTML checkboxes

Add a hidden input for the checkbox with a different ID:

<input id='testName' type='checkbox' value='Yes' name='testName'>
<input id='testNameHidden' type='hidden' value='No' name='testName'>

Before submitting the form, disable the hidden input based on the checked condition:

form.addEventListener('submit', () => {
    if(document.getElementById("testName").checked) {
        document.getElementById('testNameHidden').disabled = true;
    }
}

How to submit 0 if checkbox is unchecked and submit 1 if checkbox is checked in HTML

Simplest one, no javascript required, just put a hidden input before the checkbox:

<input type="hidden" name="check[0]" value="0" />
<input type="checkbox" name="check[0]" value="1" />

Inputs need to have the same name. If the checkbox is checked then value 1 will be submitted, otherwise value 0 from the hidden input.

Your case javascript solution, no hidden inputs needed:

<script type="text/javascript">
    // when page is ready
    $(document).ready(function() {
         // on form submit
        $("#form").on('submit', function() {
            // to each unchecked checkbox
            $(this + 'input[type=checkbox]:not(:checked)').each(function () {
                // set value 0 and check it
                $(this).attr('checked', true).val(0);
            });
        })
    })
</script>

<form method="post" id="form">
    <input type="checkbox" name="check[0]" value="1" />
    <input type="checkbox" name="check[1]" value="1" />
    <input type="submit" value="Save Changes" />
</form>

PHP solution, no hidden inputs needed:

<?php
    // if data is posted, set value to 1, else to 0
    $check_0 = isset($_POST['check'][0]) ? 1 : 0;
    $check_1 = isset($_POST['check'][1]) ? 1 : 0;
?>

<form method="post">
    <input type="checkbox" name="check[0]" value="1" />
    <input type="checkbox" name="check[1]" value="1" />
    <input type="submit" value="Save Changes" />
</form>

EDIT: the javascript solution is not valid anymore as of jquery 1.6. Based on this, a more proper solution is the following:

<script type="text/javascript">
    // when page is ready
    $(document).ready(function() {
         // on form submit
        $("#form").on('submit', function() {
            // to each unchecked checkbox
            $(this).find('input[type=checkbox]:not(:checked)').prop('checked', true).val(0);
        })
    })
</script>

<form method="post" id="form">
    <input type="checkbox" name="check[0]" value="1" />
    <input type="checkbox" name="check[1]" value="1" />
    <input type="submit" value="Save Changes" />
</form>

Why does an unchecked checkbox have a value of 'on'?

The value of a checkbox is constant. It doesn't change (unless you change it with JavaScript). If you don't specify a value, it defaults to on.

The value has nothing to do with whether the input is checked or not.

In a traditional form submission, only successful controls will be submitted and checkboxes are not successful unless they are checked. On the server you determine if they are checked or not based on if the value is submitted or not.

You don't get different values with the input depending on its checked state.

If you want to determine if a checkbox is checked or not in client side JavaScript, look at the checked property, not the value property.

document.querySelectorAll("input").forEach(input => console.log({
  value: input.value,
  checked: input.checked
}));

<input type="checkbox" value="on">
<input type="checkbox" value="off" checked>
<input type="checkbox">
<input type="checkbox" value="foo" checked>

function uncheckOthers(ele) {  if (ele.checked && ele.value == "4") {    $('.example').prop('checked', false);   //If none is clicked. Uncheck all example  } else {    $('.termcls').prop('checked', false);   //Else, Uncheck termcls since one of the example is clicked  }};

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script><input type="checkbox" onchange="uncheckOthers(this)" class="example" name="Option 1" value="1">Option 1<br><input type="checkbox" onchange="uncheckOthers(this)" class="example" name="Option 2" value="2">Option 2<br><input type="checkbox" onchange="uncheckOthers(this)" class="example" name="Option 3" value="3">Option 3<br><input type="checkbox" onchange="uncheckOthers(this)" class='termcls' name="None" id="none" value="4">None<br>

$checkbox = isset($_POST['post_friend']) ? $_POST['post_friend'] : 0 ;

checked="false" 

<div class="form-check">
  <label class="form-check-label">
    <input onchange="inputCHKChange(this);"
           type="checkbox" 
           class="form-check-input" 
           name="undefined[]" 
           required="">A</label>
</div>

<div class="col-xs-5 drinkingLeft">
<input type="checkbox" name="beer" id="beer"class="require-one col-xs-1" value="0"/>
<input id='beerHidden'  type='hidden' value='0' name='beer'>   
<label for="beer" class="col-xs-10">Beer </label>
</div>


 $('#beer').on('change', function () {
this.value = this.checked ? 1 : 0;
 }).change();

$("#submit").click(function () {
if(document.getElementById("beer").checked){
document.getElementById('beerHidden').disabled = true;
}
});

<input type="checkbox" name = "all_post" id = "all_post" value="0"> All Posts <br/>
<input type="checkbox" name = "others_post" id = "others_post" value="0"> Other's Posts <br/>
<input type="checkbox" name = "clients_post" id = "clients_post" value="0"> Cilent's Post <br/>
<input type="checkbox" name = "assigned_post" id = "assigned_post" value="0"> Task Assigned 

$(function(){
    $(':checkbox').on('change', function() {
        if (this.checked == true)
            $(this).val("1"); 
        else
            $(this).val("0");
    });
});