There are online JavaScript engines that allow for easy testing of JS code:
(link)
Matt Kruse wrote a date library (link) that is now part of JavaScript Toolbox (link). The library allows for parsing and formatting dates similar to C# and similar typed languages.
To check if an element exists (link) use
$('element').length != 0
Knockout - data binding, Model-View-View Model (MVVM) for JavaScript and UI. (link)
Handle module dependencies (link). Read more on "Why Require.JS?" - here.
Some text editors, implemented in JavaScript, that can be used in web pages:
There is a list of JS unit testing frameworks at Wikipedia (link).
Here is a text on integrating JS testing into Visual Studio (link).