Why does Vue see properties that start with '_' as undefined?

Question

Vue.js

I have an option computed property in computed, but for convenience I need to make it a static duplicate of the _option I defined in data.
But for some reason, when trying to work with this._option, vue sees it as undefined, although it is an object.
Why is that?

Answer 1

https://github.com/vuejs/vue/issues/2098
https://jsfiddle.net/yarkov_aleksei/vpuq9wb4/