vue-loader/README.md

# vue-loader [![Build Status](https://circleci.com/gh/vuejs/vue-loader/tree/master.svg?style=shield)](https://circleci.com/gh/vuejs/vue-loader/tree/master) [![Windows Build status](https://ci.appveyor.com/api/projects/status/8cdonrkbg6m4k1tm/branch/master?svg=true)](https://ci.appveyor.com/project/yyx990803/vue-loader/branch/master)

> 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](https://github.com/vuejs/vue-loader/tree/v14).

- [Documentation](https://vue-loader.vuejs.org)
- [Migrating from v14](https://vue-loader.vuejs.org/migrating.html)

## What is Vue Loader?

`vue-loader` is a loader for [webpack](https://webpack.js.org/) that allows you to author Vue components in a format called [Single-File Components (SFCs)](./docs/spec.md):

``` vue
<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:

    ``` js
    // 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

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

    Will expand to:

    ``` js
    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:

    ``` html
    <style scoped lang="scss">
    ```

    Will be returned by `vue-loader` as:

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

    And webpack will expand it to:

    ``` js
    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](https://webpack.js.org/api/loaders/#pitching-loader) (`src/pitcher.ts`) that intercepts Vue `<template>` and `<style>` requests and injects the necessary loaders. The final requests look like the following:

    ``` js
    // <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'
    ```