已知坐标求角度

function getAngle(x, y) {
var a = Math.atan2(y, x);
var ret = a * 180 / Math.PI; //弧度转角度，方便调试
if (ret > 360) {
ret -= 360;
}
if (ret < 0) {
ret += 360;
}
return ret;
}