Improve Testcomplete Code Editor or Work with a JScript IDE

You might look into creating a language plug-in for NotePad++.  The list of keywords, properties, and methods are all available in the TestComplete help document.  That is, of course, assuming you only wish to compose JSCRIPT code in an external editor.  If I remember correctly, you can even configure NotePad++ to autocomplete based off your keywording. You will not, however, be able to execute the scripts without TestComplete or TestExecute, but you can import the .js files directly to your current project.