contains.js

var DOCUMENT_POSITION_CONTAINED_BY = 16

module.exports = contains

function contains(container, elem) {
    if (container.contains) {
        return container.contains(elem)
    }

    var comparison = container.compareDocumentPosition(elem)

    return comparison === 0 || comparison & DOCUMENT_POSITION_CONTAINED_BY
}

getCommonAncestor.js

var contains = require("./contains")
var parents = require("ancestors")
var slice = Array.prototype.slice

module.exports = getCommonAncestor

function getCommonAncestor(node) {
    if (arguments.length < 2) {
        return node
    }

    var nodes = slice.call(arguments, 1)

    return parents(node).filter(function (parent) {
        return nodes.every(function (node) {
            return contains(parent, node)
        })
    })[0] || null
}