JavaScript: Find the angle between three points. Inspired by conorbuck :https://gist.github.com/conorbuck/2606166 and Lance Roberts : http://stackoverflow.com/questions/1211212/how-to-calculate-an-angle-from-three-points

angle-between-three-points.js

var p1={
	x:0,
	y:0
};

var p2={
	x:0,
	y:1
};

var p3={
	x:-1,
	y:1	
};

var p12 = Math.sqrt(Math.pow((p1.x - p2.x),2) + Math.pow((p1.y - p2.y),2));
var p13 = Math.sqrt(Math.pow((p1.x - p3.x),2) + Math.pow((p1.y - p3.y),2));
var p23 = Math.sqrt(Math.pow((p2.x - p3.x),2) + Math.pow((p2.y - p3.y),2));

//angle in radians
var resultRadian = Math.acos(((Math.pow(p12, 2)) + (Math.pow(p13, 2)) - (Math.pow(p23, 2))) / (2 * p12 * p13));

//angle in degrees
var resultDegree = Math.acos(((Math.pow(p12, 2)) + (Math.pow(p13, 2)) - (Math.pow(p23, 2))) / (2 * p12 * p13)) * 180 / Math.PI;