Modern jQuery

Use .prop():

$('.myCheckbox').prop('checked', true);
$('.myCheckbox').prop('checked', false);

DOM API

If you're working with just one element, you can always just access the underlying HTMLInputElement and modify its .checked property:

$('.myCheckbox')[0].checked = true;
$('.myCheckbox')[0].checked = false;

The benefit to using the .prop() and .attr() methods instead of this is that they will operate on all matched elements.

jQuery 1.5.x and below

The .prop() method is not available, so you need to use .attr().

$('.myCheckbox').attr('checked', true);
$('.myCheckbox').attr('checked', false);

Note that this is the approach used by jQuery's unit tests prior to version 1.6 and is preferable to using $('.myCheckbox').removeAttr('checked'); since the latter will, if the box was initially checked, change the behaviour of a call to .reset() on any form that contains it – a subtle but probably unwelcome behaviour change.

For more context, some incomplete discussion of the changes to the handling of the checked attribute/property in the transition from 1.5.x to 1.6 can be found in the version 1.6 release notes and the Attributes vs. Properties section of the .prop() documentation.

Use:

$(".myCheckbox").attr('checked', true); // Deprecated
$(".myCheckbox").prop('checked', true);

And if you want to check if a checkbox is checked or not:

$('.myCheckbox').is(':checked');

This is the correct way of checking and unchecking checkboxes with jQuery, as it is cross-platform standard, and will allow form reposts.

$('.myCheckBox').each(function(){ this.checked = true; });

$('.myCheckBox').each(function(){ this.checked = false; });

By doing this, you are using JavaScript standards for checking and unchecking checkboxes, so any browser that properly implements the "checked" property of the checkbox element will run this code flawlessly. This should be all major browsers, but I am unable to test previous to Internet Explorer 9.

The Problem (jQuery 1.6):

Once a user clicks on a checkbox, that checkbox stops responding to the "checked" attribute changes.

Here is an example of the checkbox attribute failing to do the job after someone has clicked the checkbox (this happens in Chrome).

Fiddle

The Solution:

By using JavaScript's "checked" property on the DOM elements, we are able to solve the problem directly, instead of trying to manipulate the DOM into doing what we want it to do.

Fiddle

This plugin will alter the checked property of any elements selected by jQuery, and successfully check and uncheck checkboxes under all circumstances. So, while this may seem like an over-bearing solution, it will make your site's user experience better, and help prevent user frustration.

(function( $ ) {
    $.fn.checked = function(value) {
        if(value === true || value === false) {
            // Set the value of the checkbox
            $(this).each(function(){ this.checked = value; });
        } 
        else if(value === undefined || value === 'toggle') {
            // Toggle the checkbox
            $(this).each(function(){ this.checked = !this.checked; });
        }

        return this;
    };
})( jQuery );

Alternatively, if you do not want to use a plugin, you can use the following code snippets:

// Check
$(':checkbox').prop('checked', true);

// Un-check
$(':checkbox').prop('checked', false);

// Toggle
$(':checkbox').prop('checked', function (i, value) {
    return !value;
});

You can do

$('.myCheckbox').attr('checked',true) //Standards compliant

or

$("form #mycheckbox").attr('checked', true)

If you have custom code in the onclick event for the checkbox that you want to fire, use this one instead:

$("#mycheckbox").click();

You can uncheck by removing the attribute entirely:

$('.myCheckbox').removeAttr('checked')

You can check all checkboxes like this:

$(".myCheckbox").each(function(){
    $("#mycheckbox").click()
});

You can also extend the $.fn object with new methods:

(function($)  {
   $.fn.extend({
      check : function()  {
         return this.filter(":radio, :checkbox").attr("checked", true);
      },
      uncheck : function()  {
         return this.filter(":radio, :checkbox").removeAttr("checked");
      }
   });
}(jQuery));

Then you can just do:

$(":checkbox").check();
$(":checkbox").uncheck();

Or you may want to give them more unique names like mycheck() and myuncheck() in case you use some other library that uses those names.

$("#mycheckbox")[0].checked = true;
$("#mycheckbox").attr('checked', true);
$("#mycheckbox").click();

The last one will fire the click event for the checkbox, the others will not. So if you have custom code in the onclick event for the checkbox that you want to fire, use the last one.

To check a checkbox you should use

 $('.myCheckbox').attr('checked',true);

or

 $('.myCheckbox').attr('checked','checked');

and to uncheck a check box you should always set it to false:

 $('.myCheckbox').attr('checked',false);

If you do

  $('.myCheckbox').removeAttr('checked')

it removes the attribute all together and therefore you will not be able to reset the form.

BAD DEMO jQuery 1.6. I think this is broken. For 1.6 I am going to make a new post on that.

NEW WORKING DEMO jQuery 1.5.2 works in Chrome.

Both demos use

$('#tc').click(function() {
    if ( $('#myCheckbox').attr('checked')) {
        $('#myCheckbox').attr('checked', false);
    } else {
        $('#myCheckbox').attr('checked', 'checked');
    }
});

This selects elements that have the specified attribute with a value containing the given substring "ckbItem":

$('input[name *= ckbItem]').prop('checked', true);

It will select all elements that contain ckbItem in its name attribute.

Assuming that the question is...

How do I check a checkbox-set BY VALUE?

Remember that in a typical checkbox set, all input tags have the same name, they differ by the attribute value: there are no ID for each input of the set.

Xian's answer can be extended with a more specific selector, using the following line of code:

$("input.myclass[name='myname'][value='the_value']").prop("checked", true);

I'm missing the solution. I'll always use:

if ($('#myCheckBox:checked').val() !== undefined)
{
    //Checked
}
else
{
    //Not checked
}

To check a checkbox using jQuery 1.6 or higher just do this:

checkbox.prop('checked', true);

To uncheck, use:

checkbox.prop('checked', false);

Here' s what I like to use to toggle a checkbox using jQuery:

checkbox.prop('checked', !checkbox.prop('checked'));

If you're using jQuery 1.5 or lower:

checkbox.attr('checked', true);

To uncheck, use:

checkbox.attr('checked', false);

Here is a way to do it without jQuery

function addOrAttachListener(el, type, listener, useCapture) {
  if (el.addEventListener) {
    el.addEventListener(type, listener, useCapture);
  } else if (el.attachEvent) {
    el.attachEvent("on" + type, listener);
  }
};

addOrAttachListener(window, "load", function() {
  var cbElem = document.getElementById("cb");
  var rcbElem = document.getElementById("rcb");
  addOrAttachListener(cbElem, "click", function() {
    rcbElem.checked = cbElem.checked;
  }, false);
}, false);

<label>Click Me!
  <input id="cb" type="checkbox" />
</label>
<label>Reflection:
  <input id="rcb" type="checkbox" />
</label>

Here is code for checked and unchecked with a button:

var set=1;
var unset=0;
jQuery( function() {
    $( '.checkAll' ).live('click', function() {
        $( '.cb-element' ).each(function () {
            if(set==1){ $( '.cb-element' ).attr('checked', true) unset=0; }
            if(set==0){ $( '.cb-element' ).attr('checked', false); unset=1; }
        });
        set=unset;
    });
});

Update: Here is the same code block using the newer Jquery 1.6+ prop method, which replaces attr:

var set=1;
var unset=0;
jQuery( function() {
    $( '.checkAll' ).live('click', function() {
        $( '.cb-element' ).each(function () {
            if(set==1){ $( '.cb-element' ).prop('checked', true) unset=0; }
            if(set==0){ $( '.cb-element' ).prop('checked', false); unset=1; }
        });
        set=unset;
    });
});

Try this:

$('#checkboxid').get(0).checked = true;  //For checking

$('#checkboxid').get(0).checked = false; //For unchecking

We can use elementObject with jQuery for getting the attribute checked:

$(objectElement).attr('checked');

We can use this for all jQuery versions without any error.

Update: Jquery 1.6+ has the new prop method which replaces attr, e.g.:

$(objectElement).prop('checked');

If you are using PhoneGap doing application development, and you have a value on the button that you want to show instantly, remember to do this

$('span.ui-[controlname]',$('[id]')).text("the value");

I found that without the span, the interface will not update no matter what you do.

Here is the code and demo for how to check multiple check boxes...

http://jsfiddle.net/tamilmani/z8TTt/

$("#check").on("click", function () {

    var chk = document.getElementById('check').checked;
    var arr = document.getElementsByTagName("input");

    if (chk) {
        for (var i in arr) {
            if (arr[i].name == 'check') arr[i].checked = true;
        }
    } else {
        for (var i in arr) {
            if (arr[i].name == 'check') arr[i].checked = false;
        }
    }
});

Another possible solution:

    var c = $("#checkboxid");
    if (c.is(":checked")) {
         $('#checkboxid').prop('checked', false);
    } else {
         $('#checkboxid').prop('checked', true);
    }

As @livefree75 said:

jQuery 1.5.x and below

You can also extend the $.fn object with new methods:

(function($)  {
   $.fn.extend({
      check : function()  {
         return this.filter(":radio, :checkbox").attr("checked", true);
      },
      uncheck : function()  {
         return this.filter(":radio, :checkbox").removeAttr("checked");
      }
   });
}(jQuery));

But in new versions of jQuery, we have to use something like this:

jQuery 1.6+

    (function($)  {
       $.fn.extend({
          check : function()  {
             return this.filter(":radio, :checkbox").prop("checked", true);
          },
          uncheck : function()  {
             return this.filter(":radio, :checkbox").prop("checked",false);
          }
       });
    }(jQuery));

Then you can just do:

    $(":checkbox").check();
    $(":checkbox").uncheck();

For jQuery 1.6+

$('.myCheckbox').prop('checked', true);
$('.myCheckbox').prop('checked', false);

For jQuery 1.5.x and below

$('.myCheckbox').attr('checked', true);
$('.myCheckbox').attr('checked', false);

To check,

$('.myCheckbox').removeAttr('checked');

If using mobile and you want the interface to update and show the checkbox as unchecked, use the following:

$("#checkbox1").prop('checked', false).checkboxradio("refresh");

To check and uncheck

$('.myCheckbox').prop('checked', true);
$('.myCheckbox').prop('checked', false);

Be aware of memory leaks in Internet Explorer prior to Internet Explorer 9, as the jQuery documentation states:

In Internet Explorer prior to version 9, using .prop() to set a DOM element property to anything other than a simple primitive value (number, string, or boolean) can cause memory leaks if the property is not removed (using .removeProp()) before the DOM element is removed from the document. To safely set values on DOM objects without memory leaks, use .data().

$('controlCheckBox').click(function(){
    var temp = $(this).prop('checked');
    $('controlledCheckBoxes').prop('checked', temp);
});

This is probably the shortest and easiest solution:

$(".myCheckBox")[0].checked = true;

or

$(".myCheckBox")[0].checked = false;

Even shorter would be:

$(".myCheckBox")[0].checked = !0;
$(".myCheckBox")[0].checked = !1;

Here is a jsFiddle as well.

Plain JavaScript is very simple and much less overhead:

var elements = document.getElementsByClassName('myCheckBox');
for(var i = 0; i < elements.length; i++)
{
    elements[i].checked = true;
}

Example here

When you checked a checkbox like;

$('.className').attr('checked', 'checked')

it might not be enough. You should also call the function below;

$('.className').prop('checked', 'true')

Especially when you removed the checkbox checked attribute.

I couldn't get it working using:

$("#cb").prop('checked', 'true');
$("#cb").prop('checked', 'false');

Both true and false would check the checkbox. What worked for me was:

$("#cb").prop('checked', 'true'); // For checking
$("#cb").prop('checked', '');     // For unchecking

Here's the complete answer using jQuery

I test it and it works 100% :D

    // when the button (select_unit_button) is clicked it returns all the checed checkboxes values 
    $("#select_unit_button").on("click", function(e){

             var arr = [];

             $(':checkbox:checked').each(function(i){
                 arr[i] = $(this).val(); // u can get id or anything else
             });

              //console.log(arr); // u can test it using this in google chrome
    });

In jQuery,

if($("#checkboxId").is(':checked')){
    alert("Checked");
}

or

if($("#checkboxId").attr('checked')==true){
    alert("Checked");
}

In JavaScript,

if (document.getElementById("checkboxID").checked){
    alert("Checked");
}