Added Vue 3 migration doc to Frontend development guidelines

doc/development/fe_guide/vue.md

@@ -289,3 +289,16 @@ One should apply to be a Vue.js expert by opening an MR when the Merge Request's
 - Full understanding of testing a Vue and Vuex application
 - Vuex code follows the [documented pattern](vuex.md#actions-pattern-request-and-receive-namespaces)
 - Knowledge about the existing Vue and Vuex applications and existing reusable components
+
+## Vue 2 -> Vue 3 Migration
+
+> This section is added temporarily to support the efforts to migrate the codebase from Vue 2.x to Vue 3.x
+
+Currently, we recommend to minimize adding certain features to the codebase to prevent increasing the tech debt for the eventual migration:
+
+- filters;
+- event buses;
+- functional templated
+- `slot` attributes
+
+You can find more details on [Migration to Vue 3](vue3_migration.md)

doc/development/fe_guide/vue3_migration.md

+# Migration to Vue 3
+
+In order to prepare for the eventual migration to Vue 3.x, we should be wary about adding the following features to the codebase:
+
+## Vue filters
+
+**Why?**
+
+Filters [are removed](https://github.com/vuejs/rfcs/blob/master/active-rfcs/0015-remove-filters.md) from the Vue 3 API completely.
+
+**What to use instead**
+
+Component's computed properties / methods or external helpers.
+
+## Event bus
+
+**Why?**
+
+`$on` and `$off` methods [are removed](https://github.com/vuejs/rfcs/blob/master/active-rfcs/0020-events-api-change.md) from the Vue instance, so in Vue 3 it can't be used to create an event bus.
+
+**What to use instead**
+
+Vue docs recommend using [mitt](https://github.com/developit/mitt) library. It's relatively small (200 bytes gzipped) and has a simple API:
+
+```javascript
+import mitt from 'mitt'
+
+const emitter = mitt()
+
+// listen to an event
+emitter.on('foo', e => console.log('foo', e) )
+
+// listen to all events
+emitter.on('*', (type, e) => console.log(type, e) )
+
+// fire an event
+emitter.emit('foo', { a: 'b' })
+
+// working with handler references:
+function onFoo() {}
+
+emitter.on('foo', onFoo)   // listen
+emitter.off('foo', onFoo)  // unlisten
+```
+
+## <template functional>
+
+**Why?**
+
+In Vue 3, `{ functional: true }` option [is removed](https://github.com/vuejs/rfcs/blob/functional-async-api-change/active-rfcs/0007-functional-async-api-change.md) and `<template functional>` is no longer supported.
+
+**What to use instead**
+
+Functional components must be written as plain functions:
+
+```javascript
+import { h } from 'vue'
+
+const FunctionalComp = (props, slots) => {
+  return h('div', `Hello! ${props.name}`)
+}
+```
+
+## Old slots syntax with `slot` attribute
+
+**Why?**
+
+In Vue 2.6 `slot` attribute was already deprecated in favor of `v-slot` directive but its usage is still allowed and sometimes we prefer using them because it simplifies unit tests (with old syntax, slots are rendered on `shallowMount`). However, in Vue 3 we can't use old syntax anymore.
+
+**What to use instead**
+
+The syntax with `v-slot` directive. To fix rendering slots in `shallowMount`, we need to stub a child component with slots explicitly.
+
+```html
+<!-- MyAwesomeComponent.vue -->
+<script>
+import SomeChildComponent from './some_child_component.vue'
+
+export default {
+  components: {
+    SomeChildComponent
+  }
+}
+
+</script>
+
+<template>
+  <div>
+    <h1>Hello GitLab!</h1>
+    <some-child-component>
+      <template #header>
+        Header content
+      </template>
+    </some-child-component>
+  </div>
+</template>
+```
+
+```js
+// MyAwesomeComponent.spec.js
+
+import SomeChildComponent from '~/some_child_component.vue'
+
+shallowMount(MyAwesomeComponent, {
+  stubs: {
+    SomeChildComponent
+  }
+})
+```