Building a Custom Webpack Loader for Vue Components in TypeScript

Vue.js is a popular progressive framework for building user interfaces. When developing Vue applications, Webpack is commonly used as a module bundler. While Webpack comes with many built-in loaders and plugins, there are often scenarios where you need to create custom loaders to process specific file types or transform your code in unique ways. This challenge focuses on creating a custom Webpack loader that specifically targets Vue Single File Components (SFCs) and performs a simple transformation on their <script> blocks using TypeScript.

Problem Description

Your task is to create a custom Webpack loader written in TypeScript that processes Vue Single File Components (.vue files). This loader should identify the <script> block within each SFC and apply a predefined transformation to the TypeScript code contained within it. Specifically, the loader should prepend a comment string to the beginning of the <script> block's content. This exercise will help you understand the fundamental principles of creating Webpack loaders and how they interact with module bundling processes, particularly in the context of Vue and TypeScript.

Key Requirements:

1. Loader Functionality: The loader must be a Node.js module that exports a function. This function will receive the source code of the file as input.
2. Vue SFC Parsing: The loader needs to be able to parse the .vue file to isolate different sections, particularly the <script> block. You can leverage existing libraries for this purpose.
3. TypeScript Transformation: The loader should only process the <script> block and specifically target TypeScript code.
4. Comment Prepending: The loader must prepend a predefined comment (e.g., // Processed by CustomVueLoader\n) to the beginning of the content of the <script> block.
5. Reconstruction: After transformation, the loader must reconstruct the entire .vue file, ensuring that other blocks (like <template>, <style>, and other custom blocks) are preserved in their original order and form.
6. TypeScript Implementation: The loader itself must be written in TypeScript.
7. Webpack Integration: The loader should be designed to be easily integrated into a Webpack configuration.

Expected Behavior:

When a .vue file is processed by this custom loader, the TypeScript code within its <script> block should have the specified comment prepended. The rest of the .vue file's content should remain unchanged.

Edge Cases to Consider:

• No <script> block: If a .vue file does not contain a <script> block, the loader should simply return the original content without modification.
• Empty <script> block: If the <script> block exists but is empty, the comment should still be prepended.
• Non-TypeScript <script> block: While the requirement is to target TypeScript, a robust loader might gracefully handle cases where the script block is not explicitly marked as lang="ts". For this challenge, assume the <script> block contains valid TypeScript if it exists.
• Multiple <script> blocks: While less common in standard Vue SFCs, consider how your parsing approach handles such cases. For this challenge, focus on the primary <script> block.

Examples

Example 1:

Input Vue SFC Content:
<template>
  <div>Hello</div>
</template>

<script lang="ts">
import Vue from 'vue';

export default Vue.extend({
  data() {
    return {
      message: 'World'
    };
  }
});
</script>

<style scoped>
div {
  color: blue;
}
</style>

Output Vue SFC Content:
<template>
  <div>Hello</div>
</template>

<script lang="ts">
// Processed by CustomVueLoader
import Vue from 'vue';

export default Vue.extend({
  data() {
    return {
      message: 'World'
    };
  }
});
</script>

<style scoped>
div {
  color: blue;
}
</style>

Explanation: The loader identified the <script lang="ts"> block and prepended the comment // Processed by CustomVueLoader\n to its content. The <template> and <style> blocks remain untouched.

Example 2:

Input Vue SFC Content:
<template>
  <div>Greeting</div>
</template>

<style>
  h1 { font-size: 20px; }
</style>

Output Vue SFC Content:
<template>
  <div>Greeting</div>
</template>

<style>
  h1 { font-size: 20px; }
</style>

Explanation: This SFC does not have a <script> block. Therefore, the loader returns the original content without any modifications.

Example 3:

Input Vue SFC Content:
<script lang="ts"></script>

Output Vue SFC Content:
<script lang="ts">
// Processed by CustomVueLoader
</script>

Explanation: The loader handles an empty <script> block correctly by prepending the comment.

Constraints

• The loader must be implemented as a Node.js module exporting a function.
• The output of the loader must be a string representing the transformed Vue SFC content.
• The loader should be designed to be used with @vue/compiler-sfc or a similar library for parsing Vue SFCs.
• The comment to be prepended is // Processed by CustomVueLoader\n.
• The loader should handle standard Vue SFC structure.

Notes

• You will need to install necessary dependencies such as typescript, @vue/compiler-sfc, and potentially a TypeScript compiler API for more advanced transformations (though for this challenge, simple string manipulation after parsing is sufficient).
• Consider how your loader will fit into the Webpack loader chain. It's likely to be used after vue-loader or as part of its configuration.
• The core of this challenge involves parsing the Vue SFC, isolating the script content, modifying it, and then reassembling the file. Libraries like @vue/compiler-sfc are highly recommended for robust SFC parsing.
• Remember that Webpack loaders execute in a Node.js environment.