Microsoft Script Debugger
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Microsoft Script Debugger is a free lightweight debugger for JScript, which has special debugger commands and a Debug object to communicate with the Microsoft Script Debugger.
Microsoft considers it to be deprecated in favor of the more sophisticated Microsoft Script Editor, an optional tool included in Microsoft Office XP and newer versions of Office.
Usage
After installation, new options can be found in Internet Explorer's Script Debugger menu, which gets added in the View menu. Debugging can optionally be turned off from the Advanced tab in the Internet Options dialog.