How to detect tab key is pressed in javascript?

document.onkeydown = keydown;

function keydown(event) {
        var code;
        var e;
        if (document.all) {
            if (!event) {
                var e = window.event;
                code = e.keyCode;
            }
        }
        else if (event.which) {
        code = event.which;
        e = event;
        }
        if (code == 9) {//Write some logic
        }
    }