vue报错 Property or method "***" is not defined on the instance but referenced but referenced during.

控制台报错信息

Property or method "showData" is not defined on the instance but 
referenced during render. Make sure that this property is reactive, 
either in the data option, or for class-based components, by 
initializing the property. See: 
https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties.

解决办法

1. methods 是否写成 method
2. 父组件监听子组件 $emit 事件不需要指定参数

// 错误 -> onChange(val)
<el-dropdown @change="onChange(val)"></el-dropdown>

// 正确
<el-dropdown @change="onChange"></el-dropdown>
...
methods: {
	onChange(val) { }
}