Remove data object declaration

active-rfcs/0019-remove-data-object-declaration.md

@@ -0,0 +1,80 @@
+- 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:
+
+```js
+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.
+
+```js
+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.