Answers for "if checkbox.checked"

fill checkbox javascript

// Check
document.getElementById("checkbox").checked = true;

// Uncheck
document.getElementById("checkbox").checked = false;

oncheck checkbox javascript

<input type='checkbox' onchange='handleChange(this);'> Checkbox

function handleChange(checkbox) {
    if(checkbox.checked == true){
        document.getElementById("submit").removeAttribute("disabled");
    }else{
        document.getElementById("submit").setAttribute("disabled", "disabled");
   }
}

get the state of a checkbox

function checkAddress()
{
    var chkBox = document.getElementById('checkAddress');
    if (chkBox.checked)
    {
        // ..
    }
}