gedoe.js

_ = require('underscore');

var commonPrefix = function(l) {
    return _(l).reduce(function (memo, item) {
        if (!memo) {
            return item;
        }

        var prefix = [];
        for (var i = 0; i < item.length; i++) {
            if (memo[i] == item[i]) {
                prefix.push(item[i]);
            } else {
                return (prefix.length < memo.length) ? prefix : memo;
            }
        }

        return prefix;
    });
}

var data = [['a', 'b', 'c'], ['a', 'b', 'd']];
var knippen = commonPrefix(data).length;
var result = _(data).map(function (item) {
    return item.slice(knippen);
});

console.log(result);

                var lol = [['a', 'b', 'c'], ['a', 'b', 'd']];
                var done = false;
                _.each(lol, function (tags) {
                    if (done) return;
                    var same = tags[startIdx];
                    var allSame = _.every(lol, function (tags) {
                        return tags[startIdx] === same;
                    });
                    if (allSame) {
                        startIdx += 1;
                    } else {
                        // _.each has no break
                        done = true;
                    }
                });

// ...
parts.slice(startIdx).join(' - ')