CSS in JS library, tiny yet featureful
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A lean, no hassle CSS in JS solution.

Scales from standalone use to isomorphic apps. Compatible with any framework/view library.

Supports building either inline styles of full style sheets.

In sheet mode, j2c follows a 'local by default' approach to make it easier to write components without having to worry about class and animation names clashes.

Like SASS, LESS and Stylus, j2c supports:

  • mixins
  • @extend
  • nested selectors (in sheet mode)

All standard CSS at-rules are available out of the box, most importantly:

  • @media and @supports can be nested anywhere in the sheet
  • @keyframes (with automatic generation of @-webkit-keyframes)
  • @font-face

The home page has a few interactive demos.

Table of Contents


$ npm install j2c


var j2c = require('j2c')

There are also separate builds for AMD, ES6 and a global window.j2c in the dist directory.


j2c can be used to either assemble inline declarations or full style sheets with, by default, locally unique class names.

Here's an example of locallized class names (as pioneered AFAIK by JSS):

sheet = j2c.sheet({
  ".title": {
    font_size: "3rem",
    "&:before": {
      color: "#00b",
      content: "'#'"
  ".content": {
    line_height: "1.6em",
    padding: "2rem"

Unique class names are generated automatically for title and content:

.content_j2c_fvp6zc2gdj35evhsl73ffzq_0 {
    line-height: 1.6em;
    padding: 2rem;

.title_j2c_fvp6zc2gdj35evhsl73ffzq_0 {
    font-size: 3rem;

.title_j2c_fvp6zc2gdj35evhsl73ffzq_0:before {
    content: '#';
    color: #888;

sheet is now a String object with a title and content properties that hold the unique class names. It can be used like this in your view, either on the server, in the browser of for isomorphic apps (let's say this is part of a React view):

  <h3 class="{sheet.title}">Hello</h3>
  <div class="{sheet.content}">Foo bar baz...</div>

The <style>{sheet}</style> construct works in modernish browsers (ie9+). For older IE, see below.

Animation names are also "localized" by default, font names are left untouched.

For inline declarations: j2c.inline(declarations)

The j2c function takes in JS objects and builds a property:value; list out of it.

  border: {
    top$left: {
      width: "1px",
      color: "white"

Outputs, as you could expect (white space added for readability):

background-color: red;
border-top-color: white;
border-top-width: 1px;
border-left-color: white;
border-left-width: 1px;

CamelCase and _snake_case names are turned into -dash-case, so that property names can be left unquoted in the source.

Combine (sub)properties who share the same value by using $ as a separator. It is useful to specify vendor prefixes.

Property ordering

Provided you don't delete and re-add properties to your objects, the properties will end up in the CSS sheet in the source order.

Arrays for value overloading and mixins

You can sneak in arrays anywhere in the source tree. It enables many advanced techniques, like:

Overloading properties

If you want to overload a property by using an array at the value level

    border_color: ["#33e", "rgba(64,64,255,0.8)"],




  { border_color: "#33e"},
  { border_color: "rgba(64,64,255,0.8)"}


      {color: "#33e"},
      {color: "rgba(64,64,255,0.8)"}

will give the same result.


You can mix in properties by using a function call in an array:

function mixin(color) {
  return {
    border_color: color,
    color: color


The mixin could also be a plain JS object if it doesn't need to be customized.

For building a style sheet: j2c.sheet(rules)

Everything found in the inline section applies here too, I recommend you read it first.

To give you a taste of what can be done in j2c, here's a first, rather advanced example.

s = j2c.sheet({
    "ul.foo": {
        "@media condition": {
            color: "red"
        // properties for the main ul.my_root_class elements
        font: {
            size: "2em",
            family: "sans-serif"
        // underscores in property names are converted to dashes.
        background_color: "#44f",
        // CamelCase is also automatically handled.

        // sub-selector for children element, notice the mandatory initial space
        // signifying a child element.
        " li": {
                left: "5px",
                top: "10px"
            // convenient $ shortcut.
            border: {left$right: {width: "2px"}}

Output (after indentation):

ul.foo_j2c_fgdl0s2a5fmle5g56rbuax71_0 li{
@media condition{

Were s.foo === "foo_j2c_fgdl0s2a5fmle5g56rbuax71_0 "

Global class and animation names.

You can define or refer to global names using the @global{} pseudo at-rule, and the :global() function. This will thus preserve the .foo, .bar and baz names:

s = j2c.sheet({
    "@global": {
        "ul.foo": {
            font_size: "2em",
    "p:global(.bar)" :{
        animation_name: ":global(baz)"
    "@keyframes :global(baz)": {
        // define the global "baz" animation here.

@global blocks also globalize animation names (not shown above).

Combining multiple selectors

TODO: refactor this section to mention the SASS-like & placeholder (at any arbitrary position).

Here's a excerpt from the j2c port of the PocketGrid.

j2c.sheet({"@global": {
    ",:before,:after":{          // Notice the initial comma.

Nesting ",:before,:after" inside the ".block,.blockgroup" block combines [".block", ".blockgroup"] with ["", ":before", ":after"], giving


Mathy folks call this as a Cartesian product.


j2c handles @-rules out of the box, including nested ones.

  "@media screen": {
    " p": {
      "@media (orientation: landscape)": {


@media screen {
  p {
    foo: bar;
  @media (orientation: landscape) {
    p {
      baz: qux;

For @keyframes rules, a @-webkit-keyframes block is automatically created with auto-prefixed property names.

Mixins and @extend

Mixins and @extend make j2c sheets composable. Both techniques can be combined.

Mixins and source objects composition

For mixins, arrays works the same way at the selector level as they do at the property/value one. You can therefore use the method described in the "inline" section to create mixins, that can return either at-rules, selectors, properties or a mix thereof.


j2c also supports a SASS-like @extend, more powerful in some regards, but more limited in others.

The limitation is that it can only deal with classes. Specifically:

namespace = j2c.sheet({
  '.red': {color: '#f00'}

sheet = j2c.sheet(namespace, {
  '.great': {
    fontSize: '3em'
  '.greatRed': {
    '@extend': ['.great', '.red'] // you can also pass a single class

sheet.greatRed is now defined as 'great_j2c... red_j2c... greatRed_j2c...' (class names truncated for readability).

The extra power comes from the fact that you can inherit from arbitrary classes, not just j2c-defined ones:

sheet = j2c.sheet(namespace, {
  '.myButton': {
    '@extend': ':global(.button)', // coming, say, form Bootstrap
    color: theme.highlight

Here, sheet.myButton is 'button myButton_j2c...'.

While @extend can import from arbitrary classes, it only imports into local ones.

@extend works fine with nested selectors. If there are more than one class in a selector, @extend applies to the last (right-most) one.

Invalid uses

If the last or only selector is a :global(.klass), in @global context, or in the absence of a class in the selector, @extend is turned into a at-extend property and inserted as-is in the sheet.

CSS Hacks

Since j2c.sheet only accepts property names that match /^[-_0-9A-Za-z$]+$/, it is not possible to express CSS hacks using objects. You can, however, work around the issue by using arrays and strings instead.

Here's another modified excerpt from the PocketGrid port:

  ".blockgroup": [
    "*zoom: 1; /* hackety hackery */",

Array elements are inserted in sequence, and string literals are treated as a list of properties, and inserted as is.


*zoom: 1; /* hackety hackery */

You can also pass th result of j2c.inline which is less picky about property names.

Vendor prefixes:

Note: The prefix story in j2c is currently sub-optimal. I hope at some point to port prefixfree as a plugin. It is already small, and half of it isn't needed for j2c (the half that deals with finding and updating style elements in the DOM).

Prefixing property names

You can specify the prefixes by hand using the "$" operator where needed:

  // Notice the trailing dollar, required for the unprefixed property.
  _o$_ms$_moz$_webkit$: {foo: "bar"},
  hello: "world"

Compiles to

p {

Prefixing values

/!\ This will be replaced by a plugin in a future version.

To prefix values, you can use j2c.prefix:

    "linear-gradient(90deg, #f00, #ff0)",
background-image: -moz-linear-gradient(90deg, #f00, #ff0);
background-image: -webkit-linear-gradient(90deg, #f00, #ff0);
background-image: linear-gradient(90deg, #f00, #ff0);

There's no support for prefixing a list multiple values (e.g. "linear-gradient(90deg, #f00, #ff0),linear-gradient(90deg, #f00, #ff0)").


/!\ This will be replaced by a plugin in a future version.

@keyframes blocks automatically produce their @-webkit-keyframes counterparts, even in the absence of a vendor list argument.

Inserting the stylesheet in the document

Foreword: Please note that the following is based on research on the Web, but not effectively tested in Internet explorer at the moment.


Add a text node to a new style element.

var style = document.createElement('style');
style.type = 'text/css'; // my not even be needed

In frameworks:


Sweet, innit?

ie8+ (sheets up to 32k in ie8)

As above, but with a link element and a data URI.

<link rel="stylesheet" itemprop="stylesheet" href="{'data:,' + encodeURIComponent(sheet)}" />

Note that ie8 has a 32k limit on the length of data URIs. It supports base 64 in data URIs, but doesn't provide btoa, which would not be useful in this context anyway, since base 64 encoded sheets are larger than URI encoded ones.

ie6+ (unlimited sheet size)

function stylize(element, sheet){
    element.type = 'text/css';
    if (element.styleSheet){
      element.styleSheet.cssText = sheet;
    } else {
    return element;
var el = document.createElement('style')
var sheet = j2c.sheet(...)
stylize(el, sheet);

For this to work in client-side frameworks, you need to grab a handle on the actual <style> DOM node. This means that you must create a custom component/directive.

Here are a few examples:


var j2cComponent = {
   render: function(){
        return <style />
    componentDidMount: function(){
        stylize(React.findDOMNode(this), this.prop.sheet)


var j2cComponent = {
    view: function(ctrl, args) {
        return m("style", {
            config: function(el, isinit) {
                if(!isinit) {
                    stylize(el, args.sheet);

Angular v1.3- (1.4 is ie9+, and support dynamic <style> tags).

module.directive('j2cSheet', function() {
  return {
    restrict: 'A',
    link: function link(scope, element, attrs) {
        if (element.tagName.toUpperCase() !== "STYLE") throw 'j2cSheet expects a <style> element';
        stylize(element[0], attrs.j2cSheet);

module.directive('j2cInline', function() {
  return {
    restrict: 'A',
    link: function link(scope, element, attrs) {
        element[0].style += j2c.inline(attrs.j2cInline);

Isomorphic app support

Since j2c relies on the view library/framework for DOM insertion, it supports the isomorphic scenarios the same way your view solution does (see the previous section). Caveat: local class names will be regenerated on hydration, which means that styles will have to be computed twice by the browser.

Error handling

j2c at this point does little validation. When errors are encountered, it has no way to determine where in the source code the error occurred. To make it easier to pinpoint issues, the errors are inserted in the sheet as, for example @-error-bad-at-rule "@medis"; or :bad-sub-selector-foo. This way you get the broader context of where the error occureed in the source.


Selectors and properties order

j2c relies on JS objects to define selectors and properties. The iteration order of object properties is officially undefined, but in practice it only differs in situations that do not apply to j2c. As long as we're using non-numeric keys and we don't delete then re-add object properties, the source object order is respected in the output.

Little input validation

j2c knows the bare minimum to output a valid stylesheet when provided with valid input. It will hapily accept invalid selectors, properties and values, and could in that case produce a broken stylesheet.

At this point, it will

I may get around and write a validator companion, but I'm not there yet :-).

Little pretty printing

j2c puts each selector list and properties on their own lines, but doesn't indent or add other white space.

For debugging purposes, I recommend that you pipe j2c's output through a [be au ti fier] of your choice.

Vendor prefixes corner cases

j2c doesn't provide any facility to auto-prefix a list of values. It is relevant in the context of multiple gradient backgrounds and transition/transition-property values.


  • Improve the web site. Move the docs there.
  • Test DOM insertion methods in old IE.

License: MIT