Implementing a Custom onMounted Hook in Vue (TypeScript)

The onMounted lifecycle hook in Vue 3 is crucial for executing code after a component has been mounted to the DOM. This challenge asks you to recreate this functionality from scratch, understanding the underlying mechanisms and providing a robust and type-safe implementation. This exercise will deepen your understanding of Vue's reactivity system and lifecycle management.

Problem Description

Your task is to create a custom onMounted hook that mimics the behavior of Vue's built-in onMounted. This custom hook should accept a callback function as an argument and execute that function after the component instance has been mounted. The hook should be compatible with Vue 3's Composition API and should be type-safe, ensuring that the callback function receives the component instance as its argument.

Key Requirements:

• Accepts a Callback: The hook must accept a single argument: a function to be executed.
• Execution Timing: The callback function must be executed after the component has been mounted to the DOM. This means after the component's template has been rendered and inserted into the DOM.
• Component Instance Argument: The callback function should be invoked with the component instance (this in Options API, or the component itself in Composition API) as its first argument.
• Type Safety: The hook should be written in TypeScript and provide type safety for the callback function.
• No External Dependencies: The solution should not rely on any external libraries beyond Vue itself.

Expected Behavior:

When a component uses your custom onMounted hook, the provided callback function should be executed only once, after the component is mounted. The callback should receive the component instance as its argument.

Edge Cases to Consider:

• Component Unmounting: The hook should not execute the callback function if the component is unmounted before it is mounted.
• Multiple Calls: The hook should handle multiple calls to onMounted within a component, executing each callback function in the order they were called.
• Asynchronous Operations: The callback function might contain asynchronous operations. Your hook should not block the mounting process.

Examples

Example 1:

// Component using the custom onMounted hook
<template>
  <div>Mounted: {{ message }}</div>
</template>

<script lang="ts">
import { ref, onMounted as customOnMounted } from 'vue';

export default {
  setup() {
    const message = ref('Hello from onMounted!');

    customOnMounted((component) => {
      console.log('Component instance:', component);
      message.value = 'Mounted!';
    });

    return { message };
  }
};
</script>

Output:

Mounted: Hello from onMounted! (Initially)
Mounted: Mounted! (After mounting)

Explanation: The message ref is initially set to 'Hello from onMounted!'. After the component is mounted, the callback function within customOnMounted is executed, updating message to 'Mounted!'.

Example 2:

// Component with multiple onMounted calls
<template>
  <div>Mounted: {{ message }}</div>
</template>

<script lang="ts">
import { ref, onMounted as customOnMounted } from 'vue';

export default {
  setup() {
    const message = ref('Initial Message');

    customOnMounted((component) => {
      message.value = 'First Mount';
    });

    customOnMounted((component) => {
      message.value = 'Second Mount';
    });

    return { message };
  }
};
</script>

Output:

Mounted: Initial Message (Initially)
Mounted: Second Mount (After mounting)

Explanation: Both callback functions are executed in the order they were called, updating the message ref sequentially.

Constraints

• TypeScript: The solution must be written in TypeScript.
• Vue 3 Compatibility: The solution must be compatible with Vue 3's Composition API.
• No External Libraries: Do not use any external libraries beyond Vue itself.
• Performance: The hook should not introduce significant performance overhead. Avoid unnecessary computations or DOM manipulations within the hook itself.

Notes

• Consider using Vue's onBeforeMount and onMounted lifecycle hooks as a reference for the expected behavior.
• Think about how to ensure that the callback function is executed only once and only after the component is mounted.
• The component instance can be accessed through this in Options API or directly as the first argument in Composition API.
• Focus on creating a clean, readable, and type-safe implementation. Good error handling is not required for this challenge, but clear code is.