Check Line Intersection

LineIntersection.js

// http://jsfiddle.net/justin_c_rounds/Gd2S2/
function checkLineIntersection(startX1, startY1, endX1, endY1, startX2, startY2, endX2, endY2, result) {
	// if the lines intersect, the result contains the x and y of the intersection (treating the lines as infinite) and booleans for whether line segment 1 or line segment 2 contain the point
	var denominator, a, b, numerator1, numerator2, result = result || {
		x: 0,
		y: 0,
		onLine1: false,
		onLine2: false
	};
	denominator = ((endY2 - startY2) * (endX1 - startX1)) - ((endX2 - startX2) * (endY1 - startY1));
	if (denominator == 0) {
        result.x=0;
        result.y=0;
        result.onLine2=false;
        result.onLine1=false;
		return result;
	}
	a = startY1 - startY2;
	b = startX1 - startX2;
	numerator1 = ((endX2 - startX2) * a) - ((endY2 - startY2) * b);
	numerator2 = ((endX1 - startX1) * a) - ((endY1 - startY1) * b);
	a = numerator1 / denominator;
	b = numerator2 / denominator;

	result.x = startX1 + (a * (endX1 - startX1));
	result.y = startY1 + (a * (endY1 - startY1));

	result.onLine1 = a > 0 && a < 1;
	result.onLine2 = b > 0 && b < 1;

	return result;
};

I had hassles finding a routine like this one that worked, this one seems to work well.
I only changed it to let you pass in the result object so it doesnt have to create it each time, this can speed things up if your checking thousands of lines and dont need to store the result object.