code: function normalizeList (val, name) {
    var list = [].concat(val || [])
  
    for (var i = 0; i < list.length; i++) {
      if (typeof list[i] !== 'string') {
        throw new TypeError(name + ' must be array of strings or false')
      }
    }
  
    return list
  }
docs: Normalize the index option into an array.
  
  @param {boolean|string|array} val
  @param {string} name
  @private