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It’s your turn. Help us improve JavaScript. Take the 2017 JavaScript Ecosystem Survey » is a dialect of CoffeeScript with contract support.

Contracts allow you to write powerful runtime assertions about your code. Like types but less painful, like assert but more useful.

square :: (Num) -> Num
square = (x) -> x * x

# throws a run-time error
square "a string"   

Documentation, usage, and more examples:

Quick start

Install via npm:

npm install -g

Use the compiler:

coffee -c --contracts /path/to/

An alternative to using the --contracts flag is setting this env variable:


If you are running in the browser then you'll need to load the contracts library (found in lib/contracts/contracts.js)

<script type="text/javascript" src="contracts.js"></script>

There is also experimental support for require.js. See here for an example.

If you are running in node.js then you'll need to install the contracts.js package

npm install contracts.js

Note that requires some pretty new features of JavaScript (Proxies in particular) so it currently only works on Firefox 4+, node.js 0.8.0+, and recent versions of Chrome (though you'll need to enable the experimental JavaScript flag in about:flags).

Change Log

  • 0.3.2 (September 5, 2012)
    • disabling contracts for arrays (see issue 54)
    • various bug fixes
  • 0.3.1 (July 15th, 2012)
    • support for stable node.js (v0.8.0+)
    • some bug fixes
  • 0.3.0 (March 15th, 2012)
    • change to dependent function contracts
    • various bug fixes: #27, #41, #33, #31,
    • various contracts.js bug fixes: #9, #8, #7, #6, #5, #4
    • experimental support for require.js (see here for example use)
    • based off CoffeeScript 1.3.1
  • 0.2.0 (January 4th, 2012)
    • removed .use(), now using Contracts.exports and Contracts.use
    • various bug fixes
    • based off CoffeeScript 1.2.0
  • 0.1.0 (August 29th, 2011)
    • initial release
    • based off CoffeeScript 1.1.2