vue-loader

What is vue-loader?

vue-loader is a loader for webpack that allows you to author Vue components in a format called Single-File Components (SFCs). It provides the ability to transform and load .vue files directly into webpack. It handles the parsing of .vue files, which can contain template, script, and style elements, and it applies other webpack loaders to the respective parts, such as babel-loader for the script section and css-loader for the style section.

What are vue-loader's main functionalities?

Parsing Single-File Components

vue-loader parses .vue files, extracting the script, template, and style sections. This code snippet shows how to configure webpack to use vue-loader for .vue files.

module.exports = {
  module: {
    rules: [
      {
        test: /\.vue$/,
        loader: 'vue-loader'
      }
    ]
  },
  plugins: [
    new VueLoaderPlugin()
  ]
}

Scoped CSS

vue-loader enables scoped CSS for Vue components, ensuring styles only apply to the current component. The code sample shows a scoped style block within a .vue file.

<style scoped>
p[data-v-f3f3eg9] {
  color: red;
}
</style>

Hot Module Replacement

vue-loader supports Hot Module Replacement (HMR) for Vue components, allowing components to be updated in real-time without a full page reload. The code sample demonstrates how to set up HMR for a Vue component.

if (module.hot) {
  module.hot.accept('./components/MyComponent.vue', function () {
    // Any required update logic...
  })
}

Pre-Processors

vue-loader allows the use of pre-processors like SCSS, LESS, and Stylus within the style section of a .vue file. The code sample shows how to use SCSS within a Vue component.

<style lang="scss">
$color: red;
.style-class {
  color: $color;
}
</style>

Asset URL Handling

vue-loader can transform asset URLs found in a Vue component's template into webpack module requests. The code sample demonstrates how to require an image asset in a .vue file's template.

<template>
  <img :src="require('./assets/logo.png')">
</template>

Other packages similar to vue-loader

react-hot-loader

react-hot-loader is similar to vue-loader in that it provides hot module replacement for React components. However, it is specific to React and does not handle single-file components like vue-loader does for Vue.

angular2-template-loader

angular2-template-loader is a loader for webpack that inlines HTML and CSS into Angular components. It is similar to vue-loader in that it processes component templates and styles, but it is designed for Angular rather than Vue.

svelte-loader

svelte-loader is a webpack loader for Svelte components, which are single-file components similar to Vue's SFCs. It compiles Svelte components into JavaScript modules, much like vue-loader does for Vue components.

README

vue-loader

webpack loader for Vue Single-File Components

NOTE: The master branch now hosts the code for v15! Legacy code is now in the v14 branch.

• Documentation
• Migrating from v14

What is Vue Loader?

vue-loader is a loader for webpack that allows you to author Vue components in a format called Single-File Components (SFCs):

<template>
  <div class="example">{{ msg }}</div>
</template>

<script>
export default {
  data () {
    return {
      msg: 'Hello world!'
    }
  }
}
</script>

<style>
.example {
  color: red;
}
</style>

There are many cool features provided by vue-loader:

• Allows using other webpack loaders for each part of a Vue component, for example Sass for <style> and Pug for <template>;
• Allows custom blocks in a .vue file that can have custom loader chains applied to them;
• Treat static assets referenced in <style> and <template> as module dependencies and handle them with webpack loaders;
• Simulate scoped CSS for each component;
• State-preserving hot-reloading during development.

In a nutshell, the combination of webpack and vue-loader gives you a modern, flexible and extremely powerful front-end workflow for authoring Vue.js applications.

How It Works

The following section is for maintainers and contributors who are interested in the internal implementation details of vue-loader, and is not required knowledge for end users.

vue-loader is not a simple source transform loader. It handles each language blocks inside an SFC with its own dedicated loader chain (you can think of each block as a "virtual module"), and finally assembles the blocks together into the final module. Here's a brief overview of how the whole thing works:

1. vue-loader parses the SFC source code into an SFC Descriptor using @vue/component-compiler-utils. It then generates an import for each language block so the actual returned module code looks like this:

   // code returned from the main loader for 'source.vue'
   
   // import the <template> block
   import render from 'source.vue?vue&type=template'
   // import the <script> block
   import script from 'source.vue?vue&type=script'
   export * from 'source.vue?vue&type=script'
   // import <style> blocks
   import 'source.vue?vue&type=style&index=1'
   
   script.render = render
   export default script
   

   Notice how the code is importing source.vue itself, but with different request queries for each block.

2. We want the content in script block to be treated like .js files (and if it's <script lang="ts">, we want to to be treated like .ts files). Same for other language blocks. So we want webpack to apply any configured module rules that matches .js also to requests that look like source.vue?vue&type=script. This is what VueLoaderPlugin (src/plugins.ts) does: for each module rule in the webpack config, it creates a modified clone that targets corresponding Vue language block requests.

   Suppose we have configured babel-loader for all *.js files. That rule will be cloned and applied to Vue SFC <script> blocks as well. Internally to webpack, a request like

   import script from 'source.vue?vue&type=script'
   

   Will expand to:

   import script from 'babel-loader!vue-loader!source.vue?vue&type=script'
   

   Notice the vue-loader is also matched because vue-loader are applied to .vue files.

   Similarly, if you have configured style-loader + css-loader + sass-loader for *.scss files:

   <style scoped lang="scss">
   

   Will be returned by vue-loader as:

   import 'source.vue?vue&type=style&index=1&scoped&lang=scss'
   

   And webpack will expand it to:

   import 'style-loader!css-loader!sass-loader!vue-loader!source.vue?vue&type=style&index=1&scoped&lang=scss'
   

3. When processing the expanded requests, the main vue-loader will get invoked again. This time though, the loader notices that the request has queries and is targeting a specific block only. So it selects (src/select.ts) the inner content of the target block and passes it on to the loaders matched after it.

4. For the <script> block, this is pretty much it. For <template> and <style> blocks though, a few extra tasks need to be performed:

   • We need to compile the template using the Vue template compiler;
   • We need to post-process the CSS in <style scoped> blocks, before css-loader.

   Technically, these are additional loaders (src/templateLoader.ts and src/stylePostLoader.ts) that need to be injected into the expanded loader chain. It would be very complicated if the end users have to configure this themselves, so VueLoaderPlugin also injects a global Pitching Loader (src/pitcher.ts) that intercepts Vue <template> and <style> requests and injects the necessary loaders. The final requests look like the following:

   // <template lang="pug">
   import 'vue-loader/template-loader!pug-loader!vue-loader!source.vue?vue&type=template'
   
   // <style scoped lang="scss">
   import 'style-loader!css-loader!vue-loader/style-post-loader!sass-loader!vue-loader!source.vue?vue&type=style&index=1&scoped&lang=scss'