这样子不是很简单吗？
var oX = 300, oY=300;
var aX = 300, aY = 200;
var origin = Math.atan((aY-oY)/(aX-oX));
var oDegree = origin * 180/Math.PI;
var r = 100, degree =90;
var calcdegree = oDegree + degree;
var Radian = calcdegree * Math.PI/180;
var x = Math.cos(Radian)*r, y = Math.sin(Radian)*r;
var bX = oX + x, bY = oY +y;
console.log("bx:"+ bX + ", by:" +bY);