Widget Tester provides a workbench for testing JavaScript code.

Code may be entered into a multi-line text area or by dragging a text
string or a text file onto the JS button. Simple commands may be
entered directly into the command line. The Widget executes (i.e.
evaluates) the code and displays any results in the debug window. The
Widget can be used to examine and set global variables, and to execute
expressions and functions.

Widget Tester incorporates Douglas Crockford's JSLint (JavaScript
Verifier) program.