A simple, context-aware selector engine

query.js

function query(selector, root = document) {
    let context = [root];
    const tokens = selector.trim().split(/\s*(>|\+(?!\d)|~(?!=)|\s)\s*/).filter(n => !!n);
    while (tokens.length && context.length) {
        let token = tokens.shift(), combinator;
        if (/>|\+(?!\d)|~(?!=)|\s/.test(token)) {
            combinator = token;
            token = tokens.shift();
        }
        context = context.reduce((nodes, el) => {
            let node;
            switch (combinator) {
                case '>':
                    const elements = el.children;
                    for (let i = 0, len = elements.length; i < len; i++) {
                        node = elements[i];
                        if (node.matches(token)) {
                            nodes.push(node);
                        }
                    }
                    return nodes;
                case '+':
                    node = el.nextElementSibling;
                    if (node && node.matches(token)) {
                        nodes.push(node);
                    }
                    return nodes;
                case '~':
                    node = el.nextElementSibling;
                    while (node) {
                        if (node.matches(token)) {
                            nodes.push(node);
                        }
                        node = node.nextElementSibling;
                    }
                    return nodes;
                default:
                    return nodes.concat([].slice.call(el.querySelectorAll(token)));
            }
        }, []);
    }
    return context.sort((a, b) => 3 - (a.compareDocumentPosition(b) & 6));
}