jQuery: Test if checkbox is NOT checked

if (!$("#checkSurfaceEnvironment-1").is(":checked")) {
    // do something if the checkbox is NOT checked
}

You can also use either jQuery .not() method or :not() selector:

if ($('#checkSurfaceEnvironment').not(':checked').length) {
    // do stuff for not selected
}

JSFiddle Example

Additional Notes

From the jQuery API documentation for the :not() selector:

The .not() method will end up providing you with more readable selections than pushing complex selectors or variables into a :not() selector filter. In most cases, it is a better choice.

An alternative way:

Here is a working example and here is the code, you should also use prop.

$('input[type="checkbox"]').mouseenter(function() { 
    if ($(this).is(':checked')) {
        //$(this).prop('checked',false);
        alert("is checked");
    } else {
         //$(this).prop('checked',true);
        alert("not checked");
    }
});​

I commented out the way to toggle the checked attribute.

I think the easiest way (with jQuery) to check if checkbox is checked or NOT is:

if 'checked':

if ($(this).is(':checked')) {
// I'm checked let's do something
}

if NOT 'checked':

if (!$(this).is(':checked')) {
// I'm NOT checked let's do something
}

if ( $("#checkSurfaceEnvironment-1").is(":checked") && $("#checkSurfaceEnvironment-2").not(":checked") )

I was looking for a more direct implementation like avijendr suggested.

I had a little trouble with his/her syntax, but I got this to work:

if ($('.user-forms input[id*="chkPrint"]:not(:checked)').length > 0)

In my case, I had a table with a class user-forms, and I was checking if any of the checkboxes that had the string checkPrint in their id were unchecked.

Here I have a snippet for this question.

$(function(){
   $("#buttoncheck").click(function(){
        if($('[type="checkbox"]').is(":checked")){
            $('.checkboxStatus').html("Congratulations! "+$('[type="checkbox"]:checked').length+" checkbox checked");
        }else{
            $('.checkboxStatus').html("Sorry! Checkbox is not checked");
         }
         return false;
   })
    
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<form>
  <p>
    <label>
      <input type="checkbox" name="CheckboxGroup1" value="checkbox" id="CheckboxGroup1_0">
      Checkbox</label>
    <br>
    <label>
      <input type="checkbox" name="CheckboxGroup1_" value="checkbox" id="CheckboxGroup1_1">
      Checkbox</label>
    <br>
  </p>
  <p>
    <input type="reset" value="Reset">
    <input type="submit" id="buttoncheck" value="Check">
  </p>
  <p class="checkboxStatus"></p>
</form>

To do it with .attr() like you have, to see if it's checked it would be .attr("checked", "checked"), and if it isn't it would be .attr("checked") == undefined

Return true if all checbox are checked in a div

function all_checked (id_div){
 all_checked = true;

 $(id_div+' input[type="checkbox"]').each(function() { 
    all_checked = all_checked && $('this').prop('checked');
 }

 return all_checked;
}

Simple and easy to check or unchecked condition

<input type="checkbox" id="ev-click" name="" value="" >

<script>
    $( "#ev-click" ).click(function() {
        if(this.checked){
            alert('checked');
        }
        if(!this.checked){
            alert('Unchecked');
        }
    });
</script>

try this one

if ($("#checkSurfaceEnvironment-1:checked").length>0) {
    //your code in case of NOT checked
}

In Above code selecting the element by Id (#checkSurfaceEnvironment-1) then filter out it's checked state by (:checked) filter.

It will return array of checked element object. If there any object exists in the array then if condition will be satisfied.

There are two way you can check condition.

if ($("#checkSurfaceEnvironment-1").is(":checked")) {
    // Put your code here if checkbox is checked
}

OR you can use this one also

if($("#checkSurfaceEnvironment-1").prop('checked') == true){
    // Put your code here if checkbox is checked
}

I hope this is useful for you.

Here is the simplest way given

 <script type="text/javascript">

        $(document).ready(function () {
            $("#chk_selall").change("click", function () {

                if (this.checked)
                {
                    //do something
                }
                if (!this.checked)
                {
                    //do something

                }

            });

        });

    </script>

I used this and in worked for me!

$("checkbox selector").click(function() {
    if($(this).prop('checked')==true){
       do what you need!
    }
});

I know this has already been answered, but still, this is a good way to do it:

if ($("#checkbox").is(":checked")==false) {
    //Do stuff here like: $(".span").html("<span>Lorem</span>");
}

if($("#checkbox1").prop('checked') == false){
    alert('checkbox is not checked');
    //do something
}
else
{ 
    alert('checkbox is checked');
}

var getVal=$('#checkbox_id').is(":checked"); // jQuery

var getVal=document.getElementById("checkbox_id").checked //JavaScript

if (getVal==true) {
// perform task
} else {
// perform task
}