VueJS right way to edit prop without changing parent data

You don't have to use the JSON object.

const child = {
  props:["user"],
  data(){
    return {
      localUser: Object.assign({}, this.user)
    }
  }
}

Use localUser (or whatever you want to call it) inside your child.

Edit

I had modified a fiddle created for another answer to this question to demonstrate the above concept and @user3743266 asked

I'm coming to grips with this myself, and I'm finding this very useful. Your example works well. In the child, you've created an element in data that takes a copy of the prop, and the child works with the copy. Interesting and useful, but... it's not clear to me when the local copy gets updated if something else modifies the parent. I modified your fiddle, removing the v-ifs so everything is visible, and duplicating the edit component. If you modify name in one component, the other is orphaned and gets no changes?

The current component looks like this:

Vue.component('edit-user', {
  template: `
  <div>
    <input type="text" v-model="localUser.name">
    <button @click="$emit('save', localUser)">Save</button>
    <button @click="$emit('cancel')">Cancel</button>
  </div>
  `,
  props: ['user'],
  data() {
    return {
      localUser: Object.assign({}, this.user)
    }
  }
})

Because I made the design decision to use a local copy of the user, @user3743266 is correct, the component is not automatically updated. The property user is updated, but localUser is not. In this case, if you wanted to automatically update local data whenever the property changed, you would need a watcher.

Vue.component('edit-user', {
  template: `
  <div>
    <input type="text" v-model="localUser.name">
    <button @click="$emit('save', localUser)">Save</button>
    <button @click="$emit('cancel')">Cancel</button>
  </div>
  `,
  props: ['user'],
  data() {
    return {
      localUser: Object.assign({}, this.user)
    }
  },
  watch:{
    user(newUser){
        this.localUser = Object.assign({}, newUser)
    }
  }
})

Here is the updated fiddle.

This allows you full control over when/if the local data is updated or emitted. For example, you might want to check a condition before updating the local state.

  watch:{
    user(newUser){
      if (condition)
        this.localUser = Object.assign({}, newUser)
    }
  }

As I said elsewhere, there are times when you might want to take advantage of objects properties being mutable, but there are also times like this where you might want more control.

in the above solutions, the watcher won't trigger at first binding, only at prop change. To solve this, use immediate=true

watch: {
  test: {
    immediate: true,
    handler(newVal, oldVal) {
      console.log(newVal, oldVal)
    },
  },
}