Start Date: 2020-01-10
Target Major Version: 3.x
Implementation PR: (leave this empty)

Summary

data option accepts two types of declaration: function or object. The most common is the function declaration, because it creates a new state for each instance of the component. object declaration on the other hand shares state between all the instances and works only on a root instance. This RFC primarily focuses on removing object declaration for data.

Motivation

There are little or no use-cases for root instances to have a shared state. Even if you encounter such a case it can be achieved using function declaration. Having two types of declaration is not novice-friendly and is confusing without proper examples (which are nonexistent in the current documentation). The additional restriction that object declaration is possible only on the root instance is also confusing. With a unified declaration you could achieve the same result and eliminate the confusing part of it.

Detailed design

object declaration should no longer be valid and produce an error with an explanation that only function declaration is valid for data on the root instance. It should also contain a link to an API and migration example.

Before, using object declaration:

import { createApp, h } from 'vue'

createApp().mount({
  data: {
    counter: 1,
  },
  render() {
    return [
      h('span', this.counter),
      h('button', {
        onClick: () => { this.counter++ }
      }),
    ]
  },
}, '#app')

After, using function declaration.

import { createApp, h } from 'vue'

createApp().mount({
  data() {
    return {
      counter: 1,
    }
  },
  render() {
    return [
      h('span', this.counter),
      h('button', {
        onClick: () => { this.counter++ }
      }),
    ]
  },
}, '#app')

Drawbacks

Users that work with object declaration would require migrating to function declaration.

It would also not be possible to have top-level shared state.

Examples using object declaration should be rewritten using function declaration.

Adoption strategy

Since it's not a common pattern to have an object declaration in root instance migration should be fairly easy. The impact of this change is also minimized due to the root mounting API change in v3 (where you always use a root component).

Migration itself is straightforward:

Extract shared data into an external object and use it as a property in data.
Rewrite references to the shared data to point to a new shared object.

API page should contain an info block describing that object declaration is deprecated with a guide on how to migrate.

An adapter could be provided to retain backwards compatibility.

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