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facebook / css-layout


Reimplementation of CSS layout using pure JavaScript


css-layout Build Status

This project implements a subset of CSS including flexbox and the box model using pure JavaScript, then transpiled to C and Java. The goal is to have a small standalone library to layout elements. It doesn't rely on the DOM at all.

The Java, C and JavaScript version of the code is available via npm or directly from the dist folder of this repo. The JavaScript version is also available via cdnjs.

In order to make sure that the code is correct, it is developed in JavaScript using TDD where each commit adds a unit test and the associated code to make it work. All the unit tests are tested against Chrome's implementation of CSS.

The JavaScript version has been implemented in a way that can be easily transpiled to C and Java via regexes. The layout function doesn't do any allocation nor uses any of the dynamic aspect of JavaScript. The tests are also transpiled to make sure that the implementations are correct everywhere.


A single function computeLayout is exposed that

  • takes a tree of nodes: { style: { ... }, children: [ nodes ] }
  • computes the layout and writes it back to the node tree.

For example,

// create an initial tree of nodes
var nodeTree = {
    "style": {
      "padding": 50
    "children": [
        "style": {
          "padding": 10,
          "alignSelf": "stretch"

// compute the layout

// the layout information is written back to the node tree, with
// each node now having a layout property: 

// JSON.stringify(nodeTree, null, 2);
  "style": {
    "padding": 50
  "children": [
      "style": {
        "padding": 10,
        "alignSelf": "stretch"
      "layout": {
        "width": 20,
        "height": 20,
        "top": 50,
        "left": 50,
        "right": 50,
        "bottom": 50,
        "direction": "ltr"
      "children": [],
      "lineIndex": 0
  "layout": {
    "width": 120,
    "height": 120,
    "top": 0,
    "left": 0,
    "right": 0,
    "bottom": 0,
    "direction": "ltr"

Supported Attributes

Name Value
width, height positive number
minWidth, minHeight positive number
maxWidth, maxHeight positive number
left, right, top, bottom number
margin, marginLeft, marginRight, marginTop, marginBottom number
padding, paddingLeft, paddingRight, paddingTop, paddingBottom positive number
borderWidth, borderLeftWidth, borderRightWidth, borderTopWidth, borderBottomWidth positive number
flexDirection 'column', 'row'
justifyContent 'flex-start', 'center', 'flex-end', 'space-between', 'space-around'
alignItems, alignSelf 'flex-start', 'center', 'flex-end', 'stretch'
flex positive number
flexWrap 'wrap', 'nowrap'
position 'relative', 'absolute'
  • inherit value is not implemented because it's a way to disambiguate between multiple colliding rules. This should be done in a pre-processing step, not in the actual layout algorithm.

Default values

Since we are only using flexbox, we can use defaults that are much more sensible. This is the configuration to use in order to get the same behavior using the DOM and CSS. You can try those default settings with the following JSFiddle.

div, span {
  box-sizing: border-box;
  position: relative;

  display: flex;
  flex-direction: column;
  align-items: stretch;
  flex-shrink: 0;
  align-content: flex-start;

  border: 0 solid black;
  margin: 0;
  padding: 0;
  • box-sizing: border-box is the most convenient way to express the relation between width and borderWidth.
  • Everything is display: flex by default. All the behaviors of block and inline-block can be expressed in term of flex but not the opposite.
  • All the flex elements are oriented from top to bottom, left to right and do not shrink. This is how things are laid out using the default CSS settings and what you'd expect.
  • Everything is position: relative. This makes position: absolute target the direct parent and not some parent which is either relative or absolute. If you want to position an element relative to something else, you should move it in the DOM instead of relying of CSS. It also makes top, left, right, bottom do something when not specifying position: absolute.

Native Usage Notes

The C equivalent of computeLayout is layoutNode.

In order for layout to properly layout reflowable text, the measure function must be set on the css_node structure. The property can be found in css-layout.h. This function must take a void pointer to a context that will affect the size of the node and the width as computed by the layout engine, and must return a css_dim_t structure defining the actual needed size of the node. For the most part, the context field can be the text inside the node. No C implementation of this function is provided in provided - it depends on your use of css-layout. However an implementation of the function in JavaScript can be used for reference in the test utilities.


The core logic resides with Layout.js, which is transpiled into equivalent C and Java implementations.

The JavaScript build process is managed via Grunt. The build performs linting, runs the tests against Chrome, transpiles and packages the code (JavaScript and Java) into the dist folder. For JavaScript, the build output uses the Universal Module Format (UMD) so that it can be used via AMD / RequireJS, CommonJS or included directly into an HTML page.

While developing you can just run the lint / Chrome-based tests a follows:

grunt test-javascript