Making checkboxes checked using same id using Javascript

This is my code to make check boxes checked with same id

<input type="checkbox" name="MassApprove" value="setuju2"  />
<input type="checkbox" name="MassApprove" value="setuju3"  />
<input type="checkbox" name="MassApprove" value="setuju4"  />
<input type="checkbox" name="MassApprove" value="setuju5"  />
<input type="submit" value="Next Step" name="next" />

And This is my javascript code. I need to make this checkboxes as checked when am trigger this function. Help me!..

    function doMassApprove(massApproveFlag) {
        var confirmAlert = confirm("Do You Need Mass Approve !..");
        var massApprove = document.getElementById("MassApprove").length();
        if (confirmAlert == true) {
            //alert(massApproveFlag);
            for (var i = 0; i < massApprove; i++) {
                document.getElementById("MassApprove").checked = true;
            }
        } else {
            document.getElementById("headerMassApprove").checked = false;
        }
    }