What is the general formula for radon back projection for a javascript implementation?
To implement projection the simplest way is to rotate your image then sum over a row or column. The simplest way to implement back projection is to take a line of your sinogram which is a projection from certain view (angle) then repeat that line to form an image then rotate that image regarding the projection view, then sum all of backprojected views.
There is a unique formula for continuous case but for discrete case there are many different implementation, this one is the simplest.  

Good luck.