Edit in JSFiddle

_.mixin({
    chunk : function (array, unit) {
        if (!_.isArray(array)) return array;
        unit = Math.abs(unit);
        var results = [],
        length = Math.ceil(array.length / unit);
 
        for (var i = 0; i < length; i++) {
            results.push(array.slice( i * unit, (i + 1) * unit));
        }
        return results;
    }
});
console.log( JSON.stringify( _.chunk([1,2,3,4,5,6,7,8,9], 2) ) )