Adding a specialized loader to vue-loader causes issues when the template contains personalized elements

My vue2 component is structured as follows:

<template>
  <p>Hello world</p>
</template>

<script>
export default { name: 'Example' };
</script>

<docs>
Some documentation...
</docs>

In addition, I'd like to incorporate a custom loader for my vue files, so my webpack configuration appears like this:

module.exports = {
  module: {
    rules: [{
      test: /\.vue$/,
      use: [{
        loader: 'vue-loader',
      }, {
        loader: path.resolve(__dirname, 'CustomLoader.js'),
      }],
    }],
  },
};

Using the following custom loader script:

// CustomLoader.js
module.exports = function(source) {
  return source;
}

When running webpack bundle, an error is thrown stating that it requires another loader for the <docs> block, despite the fact that the result from the CustomLoader remains unchanged. The error message reads:

Error in path/to/component.vue?vue&type=custom&index=0&blockType=docs
Module parse failed: Unexpected token
File was processed with these loaders:
* ./node_modules/vue-loader/lib.index.js
* ./CustomLoader.js
You may need an additional loader to handle the result of these loaders.

The issue can be resolved by removing either:

  • the CustomLoader
  • or the <docs> tag.

Could someone provide insight on what might be causing this problem?

Answer №1

After extensive investigation into the source code of vue-loader, I have successfully found the solution to my query. Essentially, vue-loader checks for the presence of another loader for Vue files. If no other loader is detected, it discards the content of the custom block. On the contrary, if another loader is identified, it returns the content with the expectation that the secondary loader will handle it accordingly. This explains why introducing an additional loader, even a redundant one, results in build failure.

For a deeper understanding of this process, I recommend examining the vue-loader source code, particularly:

To navigate around this issue, it is crucial to grasp how vue-loader interprets the template. It replaces each block with an import statement and an inline loader chain. For instance, the original code snippet:

<template>
  <!-- ... -->
</template>

<script>
// ...
</script>

<docs>
// ...
</docs>

is transformed into:

import render from 'custom-loader!vue-loader!source.vue?vue&type=template'
import script from 'custom-loader!vue-loader!source.vue?vue&type=script'
import 'custom-loader!vue-loader!source.vue?vue&type=custom&blockType=docs'

Hence, the custom loader is invoked multiple times for each .vue file - once during the initial pass, and then again for every expanded block.

To address this issue, my workaround involves checking the resourceQuery in my loader. If it indicates a custom block, I simply return an empty template as demonstrated below:

// CustomLoader.js
module.exports = function(source) {
  if (this.resourceQuery.includes('type=custom')) {
    return '<custom></custom>';
  }
  return source;
}

The key point is to provide a return value that can be parsed by vue-loader (utilizing the vue template compiler). Hence, using the <custom> tag instead of an empty string is essential.

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