Vue Shallow Mount: Isolating Component Behavior

Vue Test Utils provides powerful tools for testing Vue components. One crucial utility is shallowMount, which renders a component without rendering its child components. This allows you to focus on testing the logic and rendering of a single component in isolation. This challenge will guide you through implementing a simplified version of this concept to understand its core principles.

Problem Description

Your task is to create a function shallowMount that simulates the behavior of Vue Test Utils' shallowMount. This function should take a Vue component definition (as an object) and its options (e.g., props, slots) and return a "wrapper" object representing the mounted component. The key requirement is that any child components defined within the component being mounted should not be rendered. Instead, they should be replaced by placeholder elements.

Key Requirements:

1. Component Rendering: Render the provided Vue component.
2. Child Component Isolation: For any component registered within the components option of the target component, render a placeholder instead of the actual child component.
3. Props and Slots: Pass down provided props and slots to the target component.
4. Wrapper Object: Return an object that mimics a Vue Test Utils wrapper, at least exposing a html() method to get the rendered HTML.

Expected Behavior:

When shallowMount is called with a component that has child components, the output HTML should contain specific placeholder elements for those children, clearly indicating their presence but not their rendered content.

Edge Cases:

• Components with no registered child components.
• Components with props and slots.
• Components that are globally registered (though for this simplified version, we'll primarily focus on locally registered components).

Examples

Example 1:

Input:

import { defineComponent, h } from 'vue';

const ChildComponent = defineComponent({
  template: '<div>I am a child</div>',
});

const ParentComponent = defineComponent({
  components: {
    ChildComponent,
  },
  template: '<div><ChildComponent /></div>',
});

const mountOptions = {};

Output:

<div><div data-v-test-shallow-mount-placeholder="ChildComponent"></div></div>

Explanation: ParentComponent has ChildComponent registered. shallowMount should replace ChildComponent with a placeholder div, identified by a data-v-test-shallow-mount-placeholder attribute indicating the component name.

Example 2:

Input:

import { defineComponent, h } from 'vue';

const GrandchildComponent = defineComponent({
  template: '<span>I am a grandchild</span>',
});

const ChildComponent = defineComponent({
  components: {
    GrandchildComponent,
  },
  template: '<div><GrandchildComponent /></div>',
});

const ParentComponent = defineComponent({
  components: {
    ChildComponent,
  },
  template: '<div><ChildComponent /></div>',
});

const mountOptions = {};

Output:

<div><div data-v-test-shallow-mount-placeholder="ChildComponent"></div></div>

Explanation: Even though ChildComponent has its own child (GrandchildComponent), shallowMount only targets the direct children of the component being mounted (ParentComponent). ChildComponent itself is replaced by a placeholder.

Example 3:

Input:

import { defineComponent, h } from 'vue';

const SimpleComponent = defineComponent({
  template: '<div>Just me</div>',
});

const mountOptions = {};

Output:

<div>Just me</div>

Explanation: SimpleComponent has no registered child components, so it's rendered as is.

Constraints

• The input component definition will be a Vue 3 ComponentOptions object (or a function returning one).
• You will be working with Vue 3's Composition API or Options API.
• The placeholder element should be a <div> with a specific attribute: data-v-test-shallow-mount-placeholder="ComponentName".
• Assume standard Vue rendering mechanisms (like h and render functions) are available implicitly. You don't need to reimplement Vue's core rendering.
• Focus on template-based components for simplicity.

Notes

This challenge requires understanding how Vue components are defined and how their templates are processed. Think about how you would intercept the rendering of a component and replace specific parts of its template. You don't need to build a full Vue renderer; focus on the logic of identifying and replacing child components. Consider how you might parse the template string to find child component tags.