Q:

Vuejs Mutating Object passed as a prop

If I'm passing (a reference to) an Object as a prop is it OK to mutate values in the prop?
I'm developing a web app which will require a lot of values to be passed to a component, and I'm trying to find the best way of passing the values to the component and back to the parent.
From everything I've read mutating a prop is the wrong way to do things, because next time the component is updated the values are passed back to the child component overwriting the mutations. But only the reference to the object is passed so any mutations to the values in the object prop happen to the original object in the parent component. Also Vuejs does not complain about mutating props when this happens.
const subComponent = {
    name: "subComponent",
  template: `
    <div>
        Sub Component Input
        <input type="text" v-model="objectProp.val1"></input>
    </div>`,
  props: {
    objectProp: {
      required: false,
      default: () => {return {val1: "carrot"}}
    }
  }
}

const aComponent = {
    name: "aComponent",
  template: `
    <div>
        val1: {{mainObject.val1}}
        val2: {{mainObject.val2}}
        <sub-component :objectProp="mainObject"></sub-component>
    </div>`,
  data: function() {
      return{
        mainObject: {
        val1: "foo",
        val2: "bar"
      }
    }
  },
  components: {
    subComponent
  }
}

new Vue({
  el: "#app",
    components: {
    aComponent
  }
})

Here is a JSFiddle showing an object prop being mutated.
JSFiddle

A:

Is mutating a prop bad practice?
Yes absolutely. In more complex applications it is very easy to lose track of where/what/why is mutated
What is the right way to handle state across different components?
In small projects you can really do whatever you want, because you will most likely be able to follow the logic - even after not looking at the project for a year. The possibilities include:

Mutating Props (ugly but will work)
Using Events to mutate state in parent components; take a look at the EventBus (better)
Use global shared state; look at Vuex (best, but a little more boilerplate)

In big projects, however, you should absolutely use Vuex. It is a module for Vue that adds global shared state to your app, which you can access and mutate from all places in your app.

