Changing the Default Application on Mac
In order to change the application associated with JS files on Mac, select a JS file in the Finder, execute File → Get Info or hit ⌘-i, choose a text editor under Open with and click on Change all...
If you do not have a code editor, we recommend downloading the free TextWrangler, or if you want to invest a little in a really good one, we think TextMate is more than worth it.
Changing the Default Application on Windows
Windows assumes that all JS files are scripting files for the Windows Script Host and by default assigns them with it. This leads to the scripts not opening in an editor or even displaying an error like the one below after double-clicking them.
In order to change the application associated with JS files on Windows, right-click any of the JS files in the Explorer, select Open With → Choose Program...
In the window that then appears, select Always use the selected program to open this kind of file and click Browse... to find your editor of choice. If you do not have a code editor, we recommend downloading the free Notepad++.