_filter(arr, callback) {
    const ret = [];

    forEach(arr, (item) => {
      if (callback(item)) {
        ret.push(item);
      }
    });

    return ret;
  }
Internal helper function for conditionally removing items in array

Done like this because Array.filter() is not fully supported by all IE's.
@param {Array} arr
@param {Function} callback
@returns {Array}
@private