https://twitter.com/peterbe/status/1079854825880326144

gistfile1.txt

const cssTree = require("css-tree@1.0.0-alpha.29");

function getSelectorParent(selector) {
    const selectorAst = cssTree.parse(selector, { context: 'selector' });
    // solution #1
    // selectorAst.children.prevUntil(selectorAst.children.tail, (node, item, list) => {
    //     list.remove(item);
    //     return node.type === 'Combinator' || node.type === 'WhiteSpace';
    // });
    
    // solution #2
    let item;
    while (item = selectorAst.children.pop()) {
        const node = item.data;
        if (node.type === 'Combinator' || node.type === 'WhiteSpace') {
            break;
        }
    }
    
    return cssTree.generate(selectorAst);
}

[
    '.foo .bar',
    '.foo',
    'div>p'
].forEach(selector => {
    console.log(`[${selector}] -parent-> [${getSelectorParent(selector)}]`);
});