a constraint based layout system

Usage no npm install needed!

<script type="module">
  import uzi from '';


An Element positioning system inspired by SICP's "Propagation of Constraints".

Quick Start

npm install
python -mSimpleHTTPServer

Browse the /examples directory.

Feature Overview

<div uz-rect id="A"></div>
<div uz-rect id="B"></div>

Constrain properties of an element.

#B {

  /*  Anchor elements to eachother */
  left: A.right;

  /* Center in another element. */
  center-in: A;

  /* Basic expressions can be used. */
  right: A.right + B.width / 2;


/* Classes are supported */
.class-name {
  height: B.width / A.height;

Interface with JavaScript.

// assign variables
engine.assign("x", 100);

// add user defined functions
engine.func("fooBar", () => 200);
engine.funcs(Math, "Math");
#B {

  /* using variable */
  height: x * A.height;

  /* using user defined functions */
  width: C.height - Math.min(A.height, fooBar());
  left:  B.right + Math.sin(x) * 100px;

Declare rules inline.

<div uz-rect id="A"></div>
<div uz-style="width: A.height * 3"></div>

Rule Attributes

  • left
  • right
  • top
  • bottom
  • width
  • height
  • center-x
  • center-y
  • center-in
  • relative-to
  • align-x
  • align-y
  • size
  • watch

Element Attributes

  • uz-rect
  • uz-style

Special Rects

  • viewport
  • body
  • document

These can be used like any other rect in the system.

#item {
  top: 10px;

Note: user's can create custom rects by extending the Rect class.


The Engine class is how the user interacts with the system.

let engine = new uzi.Engine();

// assign variables and attach functions

engine.initialize(/* options */);

By default, the engine will walk the entire DOM and mount Elements which have an uz-rect or uz-style attribute. This behaviour is configured via an options object passed into the Engine#initialize method.

interface EngineOptions {

  // Find and parse script tags where type="text/uzi"
  findStyleSheets?: boolean;

  // Walk the dom and find elements with `uz-rect` or `uz-style` attributes.
  findElements?: boolean;

  // Use the selectors in the stylesheets to lookup elements in the dom.
  lookupSelectors?: boolean;

  // Provide pre-compiled rules to the env.
  envData?: EnvData;

Default Engine Options:

  findStyleSheets: true,
  findElements:    true

Manually managing Element life-cycle

Element life-cycles can be manually managed using the Engine#mount and Engine#unmount methods.

// start managing the element.

// stop managing the element.


This is still a work in progress.

A cache of all source to compiled rules is kept during runtime. If you export this data, and initialize uzi with it, the parser doesn't need to be sent to the client.


let envData = engine.getEnv().getExportData();


let engine = new uzi.Engine({ envData });

Angular Integration

Angular integration lets you use functions and variables from the $scope in your rules.

<div uz-rect
     uz-style="width: 100px * $index"
     ng-repeat="foo in foos">


The simplest way to debug is to use the System class.

window.system = engine.getSystem();

The System#toString() method will show all relationships. toString takes an optional string parameters which is used to filter the resulting lines.

There is also a System#$ property which uses a Proxy to provide access to the underlying variables with support for tab completion.