check if a checkbox is checked with javascript

check_my_box('check1');

HTML

<input type="checkbox" id="check1"> check 1<br>
<input type="checkbox" id="check2"> check 2<br>

<button class='btn btn-primary' onclick="check_my_box('check1');">check check 1</button>
<button class='btn btn-primary' onclick="check_my_box('check2');">check check 2</button>

Javascript

function check_my_box(id) {
	if (document.getElementById(id).checked) {
		alert(id + " is checked");
	} else {
    	alert(id + "# is NOT checked");
	}
}