Underscore: JavaScript’s “utility-belt,” it is a library with utility functions to simplify functional programming without extending built-in objects.

Lightning.js: An asynchronous embed code for third-party JavaScript to deliver third-party code safely without blocking window.onload.

Supersets and assorted libraries
TypeScript and CoffeeScript:
“Every developer should be familiar with TypeScript and CoffeeScript,” Smith said. TypeScript adds optional static typing and class-based object-oriented programming, while CoffeeScript transcompiles to JavaScript with added syntax inspired by other languages such as Haskell, Python and Ruby.

Dart: Google’s Dart programming language is a class-based language with C-style syntax for complex Web application development, which recently pivoted its strategy with the goal of compiling to JavaScript.

The list of languages that compile to JavaScript is a mile long, but a useful summary list is available on GitHub.

Phantom.js: A scripted headless WebKit and JavaScript API that “can do a lot of things,” said Smith. “It can browse, scan, and crawl sites posing as a WebKit or a browser. It’s one of those lesser-used libraries, but a subset of developers loves to play with it. Phantom is good for unit testing, good to couple with Jasmine [a behavior-driven JavaScript framework], good for screen captures and to check servers running Node.”

PDF.js: Built with HTML5 and used for parsing and rendering PDFs, “It’s a library from Mozilla that pushes the boundaries of what browsers should be able to do with PDFs,” said Smith.