How to solve the "Property or method XXX is not defined on the instance but referenced during render" in Vue.js
One common error during my first steps with Vue was the following, that is logged in the console of your browser:
*vue.js:634 [Vue warn]: Property or method “XXX” 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.
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