Change/Get check state of CheckBox

I just want to get/change value of CheckBox with JavaScript. Not that I cannot use jQuery for this. I've tried something like this but it won't work.

JavaScript function

    function checkAddress()
    {
        if (checkAddress.checked == true)
        {
            alert("a");
        }
    }

HTML

<input type="checkbox" name="checkAddress" onchange="checkAddress()" />

Using onclick instead will work. In theory it may not catch changes made via the keyboard but all browsers do seem to fire the event anyway when checking via keyboard.

You also need to pass the checkbox into the function:

function checkAddress(checkbox)
{
    if (checkbox.checked)
    {
        alert("a");
    }
}

HTML

<input type="checkbox" name="checkAddress" onclick="checkAddress(this)" />

You need to retrieve the checkbox before using it.

Give the checkbox an id attribute to retrieve it with document.getElementById(..) and then check its current state.

For example:

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

And your HTML would then look like this:

<input type="checkbox" id="checkAddress" name="checkAddress" onclick="checkAddress()"/>

(Also changed the onchange to onclick. Doesn't work quite well in IE :).

I know this is a very late reply, but this code is a tad more flexible and should help latecomers like myself.

function copycheck(from,to) {
//retrives variables "from" (original checkbox/element) and "to" (target checkbox) you declare when you call the function on the HTML.

    if(document.getElementById(from).checked==true) 
    //checks status of "from" element. change to whatever validation you prefer.
        {
            document.getElementById(to).checked=true;
             //if validation returns true, checks target checkbox
        }
    else
        {   
            document.getElementById(to).checked=false; 
             //if validation returns true, unchecks target checkbox
        }
}

HTML being something like

<input type="radio" name="bob" onclick="copycheck('from','to');" />

where "from" and "to" are the respective ids of the elements "from" wich you wish to copy "to". As is, it would work between checkboxes but you can enter any ID you wish and any condition you desire as long as "to" (being the checkbox to be manipulated) is correctly defined when sending the variables from the html event call.

Notice, as SpYk3HH said, target you want to use is an array by default. Using the "display element information" tool from the web developer toolbar will help you find the full id of the respective checkboxes.

Hope this helps.

You need this:

window.onload = function(){
    var elCheckBox=document.getElementById("cbxTodos");
    elCheckBox.onchange =function (){
        alert("como ves");
    }
};