HTML5 dashed line implementation

gistfile1.js

var CP = window.CanvasRenderingContext2D && CanvasRenderingContext2D.prototype;

if (CP && CP.lineTo) {
    CP.dashedLine = function (x, y, x2, y2, dashArray) {
        this.moveTo(x, y);
        if (!dashArray)
	    dashArray = [10,5];
        var dashCount = dashArray.length,
            dx = (x2-x),
	    dy = (y2-y),
            slope = (dx != 0) ? dy/dx : dy,
            distRemaining = Math.sqrt(dx*dx + dy*dy),
            dashIndex = 0,
	    draw = true;
        while (distRemaining >= 0.1) {
            var dashLength = dashArray[dashIndex ++ % dashCount];
            if (dashLength > distRemaining)
		dashLength = distRemaining;
            var xStep = Math.sqrt(dashLength * dashLength / (1 + slope*slope));
            x += xStep
            y += slope * xStep;
            this[draw ? 'lineTo' : 'moveTo'](x, y);
            distRemaining -= dashLength;
            draw = !draw;
        }
    }
};