How to Create a New JSFX in REAPER

1. Open REAPER and go to Track FX (click the FX button on a track).
2. In empty area of left panel of the FX window, right-click, choose: Add FX....
3. Expand the 'All Plugins' category and Right-click the JS entry
4. Click Create new JSFX...
5. Enter a name and description for your new JSFX and click OK.
6. Write your code in the editor. For example:

   // Simple gain
   slider1:gain=1<0,2,0.01>
   @sample
   spl0 *= gain;
   spl1 *= gain;
   

7. NOTE: if you're pasting in a ready-made piece of JSFX code from somewhere, be sure to delete all the existing boilerplate code first.
8. Hit Ctrl+S to save your effect. It will update live.
9. Close the editor and use your new JSFX on any track!
10. by default, your new JSFX will be saved in the REAPER User 'effects' folder.

JSFX are simple scripts for audio effects in REAPER.
You can find more info in the REAPER manual or online forums.