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Cassowary JS

Cassowary is an algorithm that computes flexible, responsive layouts quickly without resorting to piles of imperative code. Just describe the preferred relationships between values, noting which constraints are more important than others, and Cassowary figures out an optimal solution based on the current inputs. When the inputs or constraints change, Cassowary is particularly efficient at computing a new answer quickly based on the last-known solution. These properties together make it ideal for use in layout systems -- indeed, it's the algorithm at the center of Apple's new Auto Layout system for iOS & OS X.

This repo hosts an improved version of Greg Badros's port of the Cassowary hierarchial constraint toolkit to JavaScript.

This version dramatically improves the performance of the original translation, removes external library dependencies, and improves hackability. The solver core can now be used inside web workers, at the command line, and directly in modern browsers.

For civil discussion of this port and constraint-based UIs, join the Overconstrained mailing list.


Cassowary JS is licensed under the [Apache 2.0 license] (

Constraint Solver? Say What?

Constraint solvers are iterative algorithms that work towards ever more ideal solutions, often using some variant of Dantzig's simplex method. They are primarily of interest in situations where it's possible to easily set up a set of rules which you would like a solution to adhere to, but when it is very difficult to consider all of the possible solutions yourself.

Cassowary and other hierarchial constraint toolkits add a unique mechanism for deciding between sets of rules that might conflict in determining which of a set of possible solutions are "better". By allowing constraint authors to specify weights for the constraints, the toolkit can decide in terms of stronger constraints over weaker ones, allowing for more optimal solutions. These sorts of situations arise all the time in UI programming; e.g.: "I'd like this to be it's natural width, but only if that's smaller than 600px, and never let it get smaller than 200px". Constraint solvers offer a way out of the primordial mess of nasty conditionals and brittle invalidations.

If all of this sounds like it's either deeply esoteric or painfully academic, you might start by boning up on what optimizers like this do and what they're good for. I recommend John W. Chinneck's "Practical Optimization: A Gentle Introduction" and the Cassowary paper that got me into all of this: "Constraint Cascading Style Sheets for the Web"

Getting Started Under Node

Cassowary is distributed as an NPM package and can be added as a dependency or used under node in the usual way. Using Cassowary under node is as simple as:

// The entire API is exported by the cassowary object
var c = require("cassowary");

var solver = new c.SimplexSolver();
var x = new c.Variable({ value: 167 });
var y = new c.Variable({ value: 2 });
var eq = new c.Equation(x, new c.Expression(y));
// ...

The current low (sub 0.1) version number reflects the instability of the API. Also, note that the NPM package includes no tests or demos. For those, clone the github repo.

To make an NPM package from sources, clone the github repo, follow the below instructions for installing dependencies, and run make dist. This is the same process the maintainers use to package NPM releases.

Getting Started From Source

This repo pulls in other Git repositories through submodules and pulls in intern for testing via npm. After cloning the repo, run:

$ git submodule update --init
$ npm install

To run the tests, point your thorougly modern browser at tests/unittests.html?config=tests/intern and view the console. You can also check out demos/quad/quaddemo.html.

Running tests from the command line requires Node. Once you've installed Node, run:

$ npm test

> cassowary@0.0.2 test /Users/bitpshr/Projects/cassowary.js
> node node_modules/intern/client.js config=tests/intern

Defaulting to "console" reporter


121/122 tests passed

If you have a working make, a Makefile is provided with a test target that does the same thing. The Makefile also provides a make build target which generates a new minified bin/c.js binary out of the files in src/. It requires Python and isn't something you should need to do manually as it's not reqired to run tests or use the solver. The checked-in binary should always be up-to-date (or at some checkpoint which is known-good), so use it in your projects instead of the source versions.

Supported Runtimes

This refactoring currently runs in:

  • Chrome
  • Firefox 9+
  • Opera 11+
  • Safari 5+
  • IE 9+
  • Command-line:
    • V8 (d8 shell)
    • JSC (built into OS X)
    • Rhino (Java) js.jar included in checkout

This is an unapologetically modern reinterpretation optimized for size, low complexity, and speed. It will not work on old versions of IE.


// Log general debugging information
c.debug = [ false || true ]; // default false
// Detailed logging
c.trace = [ false || true ]; // default false
// Verbose logging
c.verbose = [ false || true ]; // default false
// Logging of tableau additions
c.traceAdded = [ false || true ]; // default false
// Logging of ...?
c.GC = [ false || true ]; // default false

Current Build Status

Binary versions of the solver that work in both the browser and under node are available in the bin/ directory and are updated frequently. Tests are run on each commit via Travis CI:

Build Status

Pull requests that do not include tests or break the build will be denied or reverted, respectively.