j2c

j2c

JavaScript to CSS compiler. ~850 bytes mingzipped.

Think SASS, but with JSONish syntax.

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Table of Contents

• Why?
  • But, seriously...
• Installation
• Usage
  • For inline decalrations: j2c.inline(declarations)
    • Arrays
      • Overloading properties
      • Mixins
    • Vendor prefixes:
  • For building a style sheet: j2c.sheet(rules)
    • Telling selectors and properties apart
    • Combining multiple selectors
    • At-rules
    • CSS Hacks
    • Mixins redux
  • Scoped sheet for components: j2c.scoped(...)
• Limitations
  • Selectors and properties order
  • No input validation
  • No pretty printing
• License: MIT

TOC generated with DocToc, then tweaked a bit.

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Why?

• Send small, compact, SASS-like data down the line
• Simplify your asset pipeline
• Use the full power of JavaScript for mixins, variables, macros and feature detection
• Good fit for virtual DOM frameworks like React or Mithril
• I like writing compilers :-)

But, seriously...

This is mostly intended as a client-side helper to generate styles for Virtual DOM frameworks (Mithril, React, Mercury...).

Whether or not this is useful as a general CSS replacement remains to be seen.

For that use case, it trades off file size down the line for time lost because the rendering is blocked by executing JS. Benchmarks, especially on underpowered devices are yet to be performed.

Installation

# Please send a PR if you want to see it included in other package systems.
$ npm install j2c

then

var j2c = require('j2c')

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

Usage

j2c can be used to either assemble inline declarations, full style sheets, or scoped rules, each step building on the former.

Scoped rules are especially useful for client-side frameworks, as can be seen in this simple Mithril module:

Widget = {
  styles: j2c.scoped({
    title: {
      font_size: '3rem',
      "&:before":{
        color: "#888",
        content: "#"
      }
    },
    content: {
      padding: '2rem',
      margin: '0 0 0.5rem 0'
    }
  }),

  view: function (ctrl) {
    return m('.widget', [
      m('style', Widget.styles)
      m('h3', { class: Widget.styles.title }),
      m('div', { class: Widget.styles.content })
    ])
  }
}

Unique class names are generated automatically for title and content, and assigned to the corresponding properties of the object returned by j2c.scoped().

All methods take in JS objects and return strings. It's up to you to insert the result in the DOM using your favorite method.

For inline decalrations: j2c.inline(declarations)

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

j2c.inline({
  background_color:"red",
  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;

Underscores are automatically turned into dashes so that property names can be left unquoted in the source.

You can combine (sub)properties who share the same value using $ as a separator. It is useful to specify vendor prefixes. Once again, it allows to leave property names unquoted.

Arrays

The order of iteration over the keys of a js object is undefined. If you want to ensure that properties occur in order (say, border before border-left), use an array:

j2c.inline([
  {border: "solid 1px grey"},
  {border_left: "dashed 3px green"}
])

border: solid 1px grey;
border-left: dashed 3px green;

More generally, j2c.inline([foo,bar]) is equivalent to j2c.inline(foo) + j2c.inline(bar).

This enables the following techniques:

Overloading properties

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

j2c.inline({
    border_color: ["#33e", "rgba(64,64,255,0.8)"],
})

becomes

border-color:#33e;
border-color:rgba(64,64,255,0.8);

Alternatively:

j2c.inline([
  { border_color: "#33e"},
  { border_color: "rgba(64,64,255,0.8)"}
])

and

j2c.inline({
    border:[
      {color: "#33e"},
      {color: "rgba(64,64,255,0.8)"}
    ]
})

will give the same result.

Mixins

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

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

j2c.inline([
  mixin("red"),
  {
    font_size:"2em"
  }
])

'color:red;
border-color:red;
font-size:2em;'

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

j2c.sheet({
    "ul.my_root_class": {
        "@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", 

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

Output (beautified):

@media condition {
  ul.my_root_class {
    color:red;
  }
}
ul.my_root_class  li {
  padding-left:5px;
  padding-top:10px;
  border-left-width:2px;
  border-right-width:2px;
}
ul.my_root_class {
  font-size:2em;
  font-family:sans-serif;
  background-color:#44f;
}

Telling selectors and properties apart

j2c considers that an object key matching /^[-_0-9A-Za-z$]+$/ is a property, and everything else is a (sub-)selector. Since underscores are converted to dashes, it means that property names can be left unquoted, while (sub-)selectors have to be quoted.

Selectors are concatenated as is, while properties are concatenated with hyphens. {" ul": {" li": {padding: {left:10}}}} becomes ul li{padding-left:10px;}. {" p":{".foo":{color:"red"}}}, is translated to p.foo:{color:red;}.

The properties at a given selector level are guaganteed to appear in the CSS output before the ones of sub-selectors and before those present in nested @-rules.

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({
  ".block,.blockgroup":{
    ",:before,:after":{          // Notice the initial coma.
      box_sizing:"border-box"
    }
  }
}

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

.block,.block:before,.block:after,.blockgroup,.blockgroup:before,.blockgroup:after{
    box-sizing:border-box;
}

Mathy folks call this as a Cartesian product.

At-rules

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

j2c.sheet({
  "@media screen": {
    " p": {
      foo:"bar",
      "@media (orientation: landscape)": {
        baz:"qux"
      }
    }
  }
})

becomes

@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.

At rules are guarateed to be inserted after the properties and sub-selectors at a given level. This prevents nested @media blocks to be overridden by declarations found out of them.

CSS Hacks

Since sheet.add 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:

j2c.sheet({
  ".blockgroup": [
    "*zoom: 1; /* hackety hackery */",
    {
      "list-style-type":"none",
      padding:0,
      margin:0
    }
  ]
})

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

Result:

.blockgroup{
*zoom: 1; /* hack */
}
.blockgroup{
  list-style-type:none;
  padding:0;
  margin:0;
}

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

Mixins redux

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.

Scoped sheet for components: j2c.scoped(...)

j2c.scoped offers a JSS-like functionality:

var sheet = j2c.scoped({
  foo:{color:"red"},
  bar:{margin:0}
});

console.log(sheet.bit);
// 'foo_j2c_371971407_1431849941805_0'
console.log(sheet.bat);
//  'bar_j2c_371971407_1431849941805_1'

// `sheet` is actually a String object, which can be used as a normal string.
console.log(sheet+"");

// .bar_j2c_371971407_1431849941805_1{
// margin:0;
// }
// .foo_j2c_371971407_1431849941805_0{
// color:red;
// }

Unique classes are automatically generated for each scope name. The middle part of the class names ensures that class names are unique even if several instances of j2c are used on the page.

Scoped sheets can define nested selectors and use at-rules. The full j2c.sheet() functionality is available.

Vendor prefixes:

Prefixing property names

j2c.inline() , j2c.sheet() and j2c.scoped() take a vendor prefix list as a second, optional argument. When it is present, prefixes are automatically prepended to all properties.

Most of the resulting combinations don't make any sense (-moz-color FTW), and they are simply ignored by browsers. That's the price to pay for the small code size.

j2c.inline({transition:"all 1s"}, ["moz", "webkit"])

-moz-transition:all 1s;
-webkit-transition:all 1s;
transition:all 1s;

Alternatively, you can specify the prefixes by hand using the "$" operator where needed:

j2c.inline({
  // Notice the trailing dollar, required for the unprefixed property.
  _o$_ms$_moz$_webkit$: {foo: "bar"},
  hello: "world"
});

Compiles to

p {
  -o-foo:bar;
  -ms-foo:bar;
  -moz-foo:bar;
  -webkit-foo:bar;
  foo:bar;
  hello:world;
}

Prefixing values

To prefix values, you can use j2c.prefix:

j2c.inline({
  background_image:j2c.prefix(
    "linear-gradient(90deg, #f00, #ff0)",
    ['moz','webkit']
  )
})

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)").

@-webkit-keyframes

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

Limitations

Selectors and properties order

j2c relies on JS objects to define selectors and properties. As a consequence, the source order cannot be guaranteed to be respected in the output.

j2c.sheet({
  ".hello": {
    foo:"bar",
    baz:"qux"
  }
})

This may produce either .hello{foo:bar;baz:qux;} or .hello{baz:qux;foo:bar;}.

If you need some selectors or properties to happen in order, use an array of objects.

j2c.sheet({
  ".hello":[
    {foo:"bar"},
    {baz:"qux"}
  ]
})

This will always yield .hello{foo:bar;}.hello{baz:qux;}.

No 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.

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

No 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.

@keyframes names are global, even in a scoped stylesheet.

It's up to you to pick distinctive names.

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.

License: MIT