The Extendscript Toolkit (ESTK) app (JavaScript development toolkit | Download Adobe ExtendScript Toolkit CC ) is your only choice to debug ExtendScript.
Getting Started:
Download the app, open Premiere Pro and ESTK. In ESTK you´ll find several different panels. Find the Javascript console and type sth. in like app.project.activeSequence.name. The result will most likely be undefined. This is because you have to connect ESTK with PPro. Therefore find the Source editor panel (the one with the play, pause, stop buttons) and in the top left corner open the app selector dropdown and select Premiere Pro. Now enter app.project.activeSequence.name into the Javascript console again. Now it should give you the current active sequence´s name (assuming you have a sequence open in PPro).
In the Source Editor panel you can write your Extendscript code and execute it by pressing the play button. This will inject your script (with all it´s variables and functions) into the PPro Extendscript environment. You can even set breakpoints next to the line number and debug your code when it´s executed.
Later, when you have written a jsx file for your panel, you can load, inject (execute) and debug it in ESTK. Since breakpoints do not always work as expected, I tend to use $.writeln(""); which is similar to Javascript´s console.log().
For new panel devs, I´ve setup a very simplistic panel in order to get started. This might help to better understand the minimum requirements and how everything works together:
GitHub - ThomasSzabo/Minimalistic-Adobe-Premiere-Pro-Panel: This Premiere extension is a minimum in order to run a Panel…
You´ll soon notice that there´s no documentation available that goes into detail about all ESTK APIs for PPro. Adobe chose a different approach: documenting by providing practical examples. Therefore I highly recommend the Adobe´s PPro Sample Panel:
Samples/PProPanel at master · Adobe-CEP/Samples · GitHub
If you haven´t already, you definitely want to make this your #1 resource:
GitHub - Adobe-CEP/CEP-Resources: Getting Started with Adobe CC 2014 Extension SDK
I hope this helps,
Thomas