Javascript 计时器

<div style='font-size: 20'>
    <button id="b0" onclick='x(1)'>
        start</button>
    <button id="b1" onclick='x(0)' disabled>
        pause</button>
    <button id="b2" onclick='x(2)' disabled>
        clear</button>
    <p id="h">
        00:00:00</p>
</div>

<script>
    a = b = c = 0
    function x(n) {
        if (n) {
            if (n == 1) {
                b2.disabled = b0.disabled = true
                b1.disabled = false
                N = setInterval(function() {
                    a++
                    if (a == 60) { a = 0; b++ }
                    if (b == 60) { b = 0; c++ }
                    h.innerText = c + ':' + b + ':' + a
                }, 1)
            } else { b2.disabled = true; a = b = c = 0; h.innerText = '00:00:00' }
        }
        else { b0.disabled = b2.disabled = false; b1.disabled = true; clearInterval(N) }
    }
</script>