pass value of checkbox to variable only if it checked

I am using iCheck for checkbox and below checkbox selection html,

<input type="checkbox" name="allDay" id="allDay" class="icheck" data-checkbox="icheckbox_minimal-red" value="true"> All day event</label>

I am submitting form using Ajax, but not with .serialize() function, i am taking individual form entry as .val() and then do some manipulation on entry and then pass as variable in form ajax data,

Now i want to pass value for checkbox if its selected, otherwise it should not pass any value,

I have below code for taking value for variable,

allDay = $('#allDay').val();

But with this, it always pass value = true even if its checked or unchecked.

How can i pass value of checkbox to variable only if it checked?

Thanks,