style.js

'use strict';

var rules = {};
var sheet;
var nextIndex = 0;

void function initStyle() {
    var el = document.createElement('style');
    document.head.appendChild(el);
    sheet = el.sheet;
}();

function insertRule(cssText, _index) {
    var index = _index;
    if(!index) {
        index = nextIndex;
        nextIndex++;
    } else {
        sheet.deleteRule(index);
    }
    sheet.insertRule(cssText, index);
    return index;
}

function buildRuleBody(declarations) {
    const body = Object.getOwnPropertyNames(declarations).map(property => {
        const value = declarations[property];
        if(value === null) {
            return '';
        }
        return `${property}:${value};`;
    }).join('');
    return '{' + body + '}';
}

function updateRule(_selector, declarations) {
    if(~_selector.indexOf(',')) {
        _selector.split(',').forEach(s => updateRule(s, declarations));
        return;
    }
    const selector = _selector.trim();
    if(rules[selector]) {
        let props = Object.getOwnPropertyNames(declarations);
        for(let i = props.length; i--;) {
            let property = props[i];
            if(declarations[property] === null) {
                delete rules[selector].declarations[property];
            } else {
                rules[selector].declarations[property] = declarations[property];
            }
        }
        let body = buildRuleBody(rules[selector].declarations);
        insertRule(selector + body, rules[selector].index);
        return;
    }
    let body = buildRuleBody(declarations);
    rules[selector] = {
        declarations,
        index: insertRule(selector + body)
    };
}

function insertStyle(styleData) {
    if(!Array.isArray(styleData)) { return; }
    styleData.forEach(rule => {
        if(rule.t !== 'r') {
            insertRule(rule.b);
            return;
        }
        rule.s.forEach(selector => updateRule(selector, rule.d));
    });
}

function getCurrentStyle(selector) {
    return rules[selector] ? rules[selector].declarations : {};
}

module.exports = {
    updateRule,
    insertStyle,
    getCurrentStyle
};