How to convert x,y coordinates to an angle?

If you get deltaX and deltaY from your coordinates then Math.atan2 returns the arctangent of the quotient of its arguments. The return value is in radians.

var deltaX = x2 - x1;
var deltaY = y2 - y1;
var rad = Math.atan2(deltaY, deltaX); // In radians

Then you can convert it to degrees as easy as:

var deg = rad * (180 / Math.PI)

Edit

There was some bugs in my initial answer. I believe in the updated answer all bugs are addressed. Please comment here if you think there is a problem here.