Uncaught TypeError: this.fields.forEach is not a function(…) about vuetable-2 HOT 4 OPEN

  var self = this
  var obj
  this.fields.forEach(function(field, i) {
    if (typeof (field) === 'string') {
      obj = {
        name: field,
        title: self.setTitle(field),
        titleClass: '',
        dataClass: '',
        callback: null,
        visible: true,
      }
    } else {
      obj = {
        name: field.name,
        title: (field.title === undefined) ? self.setTitle(field.name) : field.title,
        sortField: field.sortField,
        titleClass: (field.titleClass === undefined) ? '' : field.titleClass,
        dataClass: (field.dataClass === undefined) ? '' : field.dataClass,
        callback: (field.callback === undefined) ? '' : field.callback,
        visible: (field.visible === undefined) ? true : field.visible,
      }
    }
    Vue.set(self.fields, i, obj)
  })
},`