Refactor based on review feedback

Place a general explaination at the top level
style guide and link to it from the vue and vuex
guides.

doc/development/fe_guide/style/vue.md

@@ -57,6 +57,66 @@ Please check this [rules](https://github.com/vuejs/eslint-plugin-vue#bulb-rules)
 
 1. Use `.vue` for Vue templates. Do not use `%template` in HAML.
 
+1. Explicitly define data being passed into the Vue app
+
+    ```javascript
+    // bad
+    return new Vue({
+      el: '#element',
+      components: {
+        componentName
+      },
+      provide: {
+        ...someDataset
+      },
+      props: {
+        ...anotherDataset
+      },
+      render: createElement => createElement('component-name'),
+    }));
+
+    // good
+    const { foobar, barfoo } = someDataset;
+    const { foo, bar } = anotherDataset;
+
+    return new Vue({
+      el: '#element',
+      components: {
+        componentName
+      },
+      provide: {
+        foobar,
+        barfoo
+      },
+      props: {
+        foo,
+        bar
+      },
+      render: createElement => createElement('component-name'),
+    }));
+    ```
+
+    We discourage the use of the spread operator in this specific case in
+    order to keep our codebase explicit, discoverable, and searchable.
+    This applies in any place where we'll benefit from the above, such as
+    when [initializing Vuex state](../vuex.md#why-not-just-spread-the-initial-state).
+    The pattern above also enables us to easily parse non scalar values during
+    instantiation.
+
+    ```javascript
+    return new Vue({
+      el: '#element',
+      components: {
+        componentName
+      },
+      props: {
+        foo,
+        bar: parseBoolean(bar)
+      },
+      render: createElement => createElement('component-name'),
+    }));
+    ```
+
 ## Naming
 
 1. **Extensions**: Use `.vue` extension for Vue components. Do not use `.js` as file extension ([#34371](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/34371)).

doc/development/fe_guide/vue.md

@@ -66,29 +66,32 @@ You should only do this while initializing the application, because the mounted
 
 The advantage of providing data from the DOM to the Vue instance through `props` in the `render` function
 instead of querying the DOM inside the main Vue component is avoiding the need to create a fixture or an HTML element in the unit test,
-which will make the tests easier. See the following example:
+which will make the tests easier.
+
+See the following example, also, please refer to our [Vue style guide](style/vue.md#basic-rules) for additional
+information on why we explicitly declare the data being passed into the Vue app;
 
 ```javascript
 // haml
 #js-vue-app{ data: { endpoint: 'foo' }}
 
 // index.js
-document.addEventListener('DOMContentLoaded', () => new Vue({
-  el: '#js-vue-app',
-  data() {
-    const dataset = this.$options.el.dataset;
-    return {
-      endpoint: dataset.endpoint,
-    };
-  },
+const el = document.getElementById('js-vue-app');
+
+if (!el) return false;
+
+const { endpoint } = el.dataset;
+
+return new Vue({
+  el,
   render(createElement) {
     return createElement('my-component', {
       props: {
-        endpoint: this.endpoint,
+        endpoint
       },
     });
   },
-}));
+}
 ```
 
 > When adding an `id` attribute to mount a Vue application, please make sure this `id` is unique across the codebase
@@ -100,7 +103,7 @@ By following this practice, we can avoid the need to mock the `gl` object, which
 It should be done while initializing our Vue instance, and the data should be provided as `props` to the main component:
 
 ```javascript
-document.addEventListener('DOMContentLoaded', () => new Vue({
+return new Vue({
   el: '.js-vue-app',
   render(createElement) {
     return createElement('my-component', {
@@ -109,7 +112,7 @@ document.addEventListener('DOMContentLoaded', () => new Vue({
       },
     });
   },
-}));
+});
 ```
 
 #### Accessing feature flags

doc/development/fe_guide/vuex.md

@@ -360,8 +360,8 @@ export default initialState => ({
 ```
 
 We've made the conscious decision to avoid this pattern to aid in the
-discoverability and searchability of our frontend codebase. The reasoning for
-this is described in [this
+discoverability and searchability of our frontend codebase. The same applies
+when [providing data to a Vue app](vue.md#providing-data-from-haml-to-javascript). The reasoning for this is described in [this
 discussion](https://gitlab.com/gitlab-org/frontend/rfcs/-/issues/56#note_302514865):
 
 > Consider a `someStateKey` is being used in the store state. You _may_ not be