code: function mergeArrays (a, b) {
      // if target is array then add b to array otherwise return undefined
      // so normal _.merge will be performed
      return Array.isArray(a) ? a.concat(b) : undefined
  }
docs: @function mergeArrays
  
  mergeWith function for lodash to merge arrays
  
  @param {any} a
  @param {any} b
  
  @returns {array|undefined}