commondir/index.js

var path = require('path');

module.exports = function (basedir, relfiles) {
    if (relfiles) {
        var files = relfiles.map(function (r) {
            return path.resolve(basedir, r);
        });
    }
    else {
        var files = basedir;
    }
    
    var res = files.slice(1).reduce(function (ps, file) {
        if (!file.match(/^([A-Za-z]:)?\/|\\/)) {
            throw new Error('relative path without a basedir');
        }
        
        var xs = file.split(/\/+|\\+/);
        for (
            var i = 0;
            ps[i] === xs[i] && i < Math.min(ps.length, xs.length);
            i++
        );
        return ps.slice(0, i);
    }, files[0].split(/\/+|\\+/));
    
    // Windows correctly handles paths with forward-slashes
    return res.length > 1 ? res.join('/') : '/'
};