Square Waves via Fourier Series

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>fourier square wave</title>
        <style>body { margin:0; padding:0; }</style>
    </head>
    <body>
        <script>
var c = document.body.appendChild(document.createElement('canvas'));
var w = 950, h = 500;
c.width = w;
c.height = h;
var ctx = c.getContext('2d');
ctx.fillStyle = '#fff';
ctx.strokeStyle = '#000';
function wave(t, n, f) {
    var val = 0;
    for (var k = 1; k < n; k ++) {
        val += Math.sin(Math.PI * 2 * ((2 * k) - 1) * f * t) / ((2 * k) - 1);
    }
    return (2 / Math.PI) * val;
}
function draw(n, f, a) {
    ctx.fillRect(0,0,w,h);
    ctx.beginPath();
    ctx.moveTo(0,h/2);
    ctx.lineTo(w,h/2);
    ctx.strokeStyle = '#aaf';
    ctx.stroke();
    ctx.strokeStyle = '#000';
    ctx.beginPath();
    for (var t = 0; t < Math.PI * 4; t += 0.01) {
        var y = wave(t, n, f)
        ctx.lineTo(t * 100, (y * 100) + h/2);
    }
    ctx.stroke();
}
document.onmousemove = function(e) {
    draw((Math.floor(e.pageX / 20)) + 1, 0.2);
}
var it = 0, interval = window.setInterval(function() {
    if (++it < 20) draw(it + 1, 0.2);
    else window.clearInterval(it);
}, 100);
        </script>
    </body>
</html>