Either left or right. Based on Neal Ford's implementation of Either in his Functional Thinking video series.

  $ npm install either.js

	function getData() {
		data = getDataFromSomeDataStore();

		if(data)
			return either.right(data);
		else
			return either.left(new Error('The requested data could not be found.'));
	}

	getData().fold(
		function left(error) {
			console.log(error);
		},
		function right(data) {
			console.log(data);
		}
	);

Clone this repository from GitHub. Go to the directory that contains the cloned source code and install the devDependencies.

  $ npm install

Then run the tests.

  $ npm run test

The MIT License (MIT)

Copyright (c) 2013 Jan Van Ryswyck

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.