Following a similar pattern as the @thi.ng/hiccup package, this library generates CSS from plain nested arrays / data structures, functions, iterators.

• Uses JS arrays to define selector scopes
• Uses JS object to define selector properties
  • Multiple objects per scope are combined automatically
• Supports nested selectors and computes their cartesian products
• Configurable auto-prefixed properties & vendor prefixes (disabled by default)
• Automatically consumes embedded iterators
• Supports embeded functions, either:
  • to define entire selector branches/scopes
  • to produce single selector items
  • to produce property values
• Quoted functions to support CSS-as-JSON definitions
• Various @-rule function wrappers:
  • @import
  • @keyframes
  • @media (incl. nested media queries)
  • @namespace
  • @supports
• Attribute selector functions
• Unit formatting wrappers (no conversions yet)
• Customizable formatting (2 defaults for compact & pretty printing)

• Same as LESS / SASS etc., but enables full use of ES6 to define style rules
• CSS framework & theme generators
• Dynamic CSS generation for components

The overall approach of using S-expressions was inspired by these Clojure projects:

• hiccup
• garden

yarn add @thi.ng/hiccup-css

import * as css from "@thi.ng/hiccup-css";

This is the main function exposed by this module. It accepts a JS data structure (array, object, iterator or function) and returns a CSS string. The optional opts arg is used to control formatting, auto-prefixing and other conversion options. See api.ts for reference.

By default the generated CSS uses the css.COMPACT format, causing "minimized" outputs. Pretty printing is supported via the css.PRETTY format preset, see examples further below.

This feature is only intended for setting an element's .style attrib:

css.css({
    position: "absolute",
    border: 0,
    // function is evaluated during serialization
    top: () => css.percent((Math.random() * 100) | 0),
    // the entire properties object is passed to functions
    left: (props) => css.px(props.border),
    // arrays are joined with `,`
    // nested arrays are joined w/ ` `
    font: [["72px", "ComicSans"], "sans-serif"]
});

// "position:absolute;border:0;top:23%;left:0px;font:72px ComicSans,sans-serif;"

css.css(
    [
        ["html", "body", { margin: 0, padding: 0 }],
        ["div", { "max-width": css.rem(30)}],
        ["div.title", { color: "red" }]
    ],
    { format: css.PRETTY }
);

html, body {
    margin: 0;
    padding: 0;
}

div {
    max-width: 30rem;
}

div.title {
    color: red;
}

// re-usable property snippets
const border = { border: "1px solid black" };
const red = { color: "red" };

css.css(
    [
        ["#foo", { background: "white" }, border, red],
        ["#bar", { background: "yellow", color: "black" }, border]
    ],
    { format: css.PRETTY }
);

#foo {
    background: white;
    border: 1px solid black;
    color: red;
}

#bar {
    background: yellow;
    color: black;
    border: 1px solid black;
}

import * as tx from "@thi.ng/transducers";

// single rule generator
// e.g. `[".w25", { width: "25%" }]`
const prop = (id, key) => (x) => [id + x, {[key]: css.percent(x)}];

css.css(
    // define iterator to produce a number of width & height css classes
    // `juxt` is used to process given percentage values in parallel and produces a tuple
    // `mapcat` is used to dissolve the tuple and produce a flat stream of generated rules
    // REMEMBER: iterators are lazy and can only be consumed once (not a problem here)
    tx.iterator(
        tx.mapcat(tx.juxt(prop(".w", "width"), prop(".h", "height"))),
        tx.range(25, 101, 25)
    ),
    { format: css.PRETTY }
);

.w25 {
    width: 25%;
}

.h25 {
    height: 25%;
}

.w50 {
    width: 50%;
}

.h50 {
    height: 50%;
}

.w75 {
    width: 75%;
}

.h75 {
    height: 75%;
}

.w100 {
    width: 100%;
}

.h100 {
    height: 100%;
}

Selector nesting can be easily done via array nesting. Each new nesting level defines a child scope of the current selector. The actual CSS selectors are computed using the cartesian product of any selectors in the current scope and their previously defined parents:

css.css(
    ["header", "footer", { "font-size": css.rem(1.25) },
        ["nav", { background: "#000", color: "#666" },
            ["ul", { "list-style": "none" }],
            ["li", { padding: css.rem(0.5) },
                [css.withAttrib("selected"), { color: "#0cf" }]]]],
    { format: css.PRETTY }
)

header nav ul, footer nav ul {
    list-style: none;
}

header nav li[selected], footer nav li[selected] {
    color: #0cf;
}

header nav li, footer nav li {
    padding: 0.5rem;
}

header nav, footer nav {
    background: #000;
    color: #666;
}

header, footer {
    font-size: 1.25rem;
}

(Currently, only prefixed properties are supported. Auto-prefixing based on property values is planned, but currently low priority.)

css.css(
    ["div", {"border-radius": "4px"}],
    { autoprefix: ["border-radius"], format: css.PRETTY }
);

div {
    -moz-border-radius: 4px;
    -ms-border-radius: 4px;
    -o-border-radius: 4px;
    -webkit-border-radius: 4px;
    border-radius: 4px;
}

Media queries (optionally nested) are supported via the at_media() and at_supports() functions, both taking an object (or string) of conditionals and a body which will be based to css().

The key-value pairs of the conditional object are interpreted as follows and ALWAYS combined using and:

css.css(
    css.at_media(
        { screen: true, "min-width": css.rem(10) },
        [
            [".col", { width: css.percent(50)}],
            [
                css.at_media(
                    { "min-width": "20rem" },
                    [".col", { padding: css.rem(1) }]
                )
            ]
        ]
    ),
    { format: css.PRETTY }
);

@media screen and (min-width:10rem) {

    .col {
        width: 50%;
    }

    @media (min-width:20rem) {

        .col {
            padding: 1rem;
        }

    }

}

css.css(
    css.at_keyframes("fadein", { opacity: 0 }, { opacity: 1 }),
    { format: css.PRETTY }
);

@keyframes fadein {

    0% {
        opacity: 0;
    }

    100% {
        opacity: 1;
    }

}

css.css(
    css.at_keyframes(
        "fadein",
        {
            0: {
                color: "red"
            },
            50: {
                color: "green"
            },
            100: {
                color: "blue"
            }
        }
    ),
    { format: css.PRETTY }
);

@keyframes fadein {

    0% {
        color: red;
    }

    50% {
        color: green;
    }

    100% {
        color: blue;
    }

}

Functions are handled differently based on their position in the rule tree. Also see the section on Quoted functions below...

If a function is given as arg to css() or is in the head position (first element) of a rule scope, the function is expected to produce output directly and is called with an empty result accumulator array and the CSSOpts object passed to css(). This form is mainly used by the various at_*() functions provided (e.g. at_media() example above).

css.css(at_import("foo.css", "screen"));
// "@import url(foo.css) screen;"

The following example illustrates the head position placement, using the comment() function to emit CSS comments.

css.css([
    // comments are usually omitted with the default format (css.COMPACT)
    // pass `true` as 2nd arg to force inclusion
    css.comment("generated, please don't edit", true),
    ["div", { margin: 0 }]
]);
// "/*generated, don't edit*/div{margin:0;}"

If a function is located anywhere else in a rule scope array (2nd index or later), it will be called without arguments and the return value used in its place. Any returned functions will be eval'd recursively in the same manner.

One of this project's use cases is to support stylesheets defined as JSON. Since functions cannot be used there, an optional mechanism to map strings to functions is provided:

["function-name", ...args]

The quoted function name is looked up in a dictionary and if found, called with all remaining elements in the same array. I.e. ["@import", "foo.css"] will be the same result as at_import("foo.css").

IMPORTANT: Quoted functions are only supported in the head position of a scope.

const styles = [
    ["@comment", " CSS from JSON"],
    ["@import", "print.css", "print"],
    ["@media",
        { "screen": true, "min-width": "10em" },
        ["div", { "font-size": "80%" }]
    ],
    ["@keyframes", "fadein", { "opacity": 0}, { "opacity": 1 }]
];

css.css(styles, { format: css.PRETTY, fns: css.QUOTED_FNS });

// btw. QUOTED_FNS is simply:
const QUOTED_FNS = {
    "@comment": comment,
    "@import": at_import,
    "@keyframes": at_keyframes,
    "@media": at_media,
    "@namespace": at_namespace,
    "@supports": at_supports,
}

/*
    CSS from JSON
 */
@import url(print.css) print;
@media screen and (min-width:10em) {

    div {
        font-size: 80%;
    }

}

@keyframes fadein {

    0% {
        opacity: 0;
    }

    100% {
        opacity: 1;
    }

}

• Integration with @thi.ng/hiccup and @thi.ng/hdom
• Webpack loader (help wanted)

• Karsten Schmidt

© 2016 - 2018 Karsten Schmidt // Apache Software License 2.0