gistfile1.js

tokens.map(function(el, i, arr) {
                    var nextEl = arr[i + 1]; // get next element in array
                    if (nextEl != null && nextEl.constructor === el.constructor && el.by !== undefined) {
                        nextEl.by += el.by;
                        el.toRemove = true; // mark items to remove
                    }
                    return el;
                }).filter(function(el) {
                    return !el.toRemove;
                });