docs: ✏️ docs revamp

Author: kaisermann

README.md

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+# Getting Started
+
+<!-- @import "[TOC]" {cmd="toc" depthFrom=2 depthTo=6 orderedList=false} -->
+
+<!-- code_chunk_output -->
+
+- [1. Installation](#1-installation)
+- [2. Adding `svelte-preprocess` to our build workflow](#2-adding-svelte-preprocess-to-our-build-workflow)
+- [3. Configuring preprocessors](#3-configuring-preprocessors)
+  - [3.1. Setting default languages](#31-setting-default-languages)
+  - [3.2 Prepending content](#32-prepending-content)
+
+<!-- /code_chunk_output -->
+
+_Note: The examples below are going to be using hypothetical `rollup.config.js` as if we were configuring a simple Svelte application, but `svelte-preprocess` can be used in multiple scenarios. See [Usage](docs/usage.md)._
+
+## 1. Installation
+
+```shell
+$ npm install -D svelte-preprocess
+```
+
+`svelte-preprocess` doesn't have any language specific dependency, so it's up to us to install the rest of tools we are going to use:
+
+- `babel`: `npm install -D @babel/core @babel/preset-...`
+- `coffeescript`: `npm install -D coffeescript`
+- `typescript`: `npm install -D typescript`
+- `postcss`: `npm install -D postcss postcss-load-config`
+- `less`: `npm install -D less`
+- `sass`: `npm install -D node-sass` or `npm install -D sass`
+- `pug`: `npm install -D pug`
+- `stylus`: `npm install -D stylus`
+
+## 2. Adding `svelte-preprocess` to our build workflow
+
+Let's add `svelte-preprocess` in [auto-preprocessing mode](/docs/preprocessing##auto-preprocessing) to our `rollup.config.js`:
+
+```diff
+import svelte from 'rollup-plugin-svelte'
+
+
+
++ import autoPreprocess from 'svelte-preprocess';
+
+const production = !process.env.ROLLUP_WATCH
+
+export default {
+  input: 'src/main.js',
+  output: {
+    sourcemap: true,
+    format: 'iife',
+    name: 'app',
+    file: 'public/bundle.js',
+  },
+  plugins: [
+    svelte({
++      //
++      preprocess: autoPreprocess(),
+      // enable run-time checks when not in production
+      dev: !production,
+      // we'll extract any component CSS out into
+      // a separate file — better for performance
+      css: css => {
+        css.write('public/bundle.css')
+      },
+    }),
+  ],
+}
+```
+
+Now our app's code can be written in any of the syntaxes supported by `svelte-preprocess`: `SCSS`, `Stylus`, `Less`, `Coffeescript`, `TypeScript`, `Pug`, `PostCSS`, `Babel`.
+
+_**Note:** If you use VS Code, check [its usage guide](/docs/usage.md#with-svelte-vs-code) to make the Svelte VS Code extension understand the content of your components._
+
+## 3. Configuring preprocessors
+
+Now let's assume our app markup is going to be written in `pug`, our scripts in `Typescript`, and our styles in `SCSS`. We also want our styles to be auto-prefixed, so we're also going to need `postcss`. Let's install these dependencies:
+
+```shell
+$ npm i -D typescript sass postcss autoprefixer pug
+```
+
+After the installation is complete, `pug`, `typescript` and `scss` are ready-to-go, but we still need to configure our `postcss` options. Let's add them to our `rollup.config.js`:
+
+```diff
+import svelte from 'rollup-plugin-svelte'
+import autoPreprocess from 'svelte-preprocess';
+
+const production = !process.env.ROLLUP_WATCH
+
+export default {
+  input: 'src/main.js',
+  output: {
+    sourcemap: true,
+    format: 'iife',
+    name: 'app',
+    file: 'public/bundle.js',
+  },
+  plugins: [
+    svelte({
++      preprocess: autoPreprocess({
++         postcss: {
++           plugins: [require('autoprefixer')()]
++         }
++      }),
+      // enable run-time checks when not in production
+      dev: !production,
+      // we'll extract any component CSS out into
+      // a separate file — better for performance
+      css: css => {
+        css.write('public/bundle.css')
+      },
+    }),
+  ],
+}
+```
+
+And we're done! Our components can now be written as:
+
+```html
+<template lang="pug">
+  h1 {world}
+</template>
+
+<script lang="ts">
+  export let name: string = 'world';
+</script>
+
+<style lang="scss">
+  h1 {
+    color: red;
+  }
+</style>
+```
+
+### 3.1. Setting default languages
+
+Ok, we now can write our entire app with `pug`, `typescript` and `scss`, but typing `lang="..."` in every file can become an obnoxious process. In [auto-preprocessing mode](/docs/preprocessing.md#auto-preprocessing), `svelte-preprocess` [lets us define the default languages](/docs/preprocessing.md#auto-preprocessing-options) of our components. It's setted by default to `html`, `javascript` and `css`. Let's change that so we don't need those `lang` attributes.
+
+_**Disclaimer**: The Svelte VS Code extension uses the `lang` or `type` attribute to correctly highlight your code. At the time of writing, the extension doesn't support default languages. Doing this can lead to errors on your IDE._
+
+```diff
+import svelte from 'rollup-plugin-svelte'
+import autoPreprocess from 'svelte-preprocess';
+
+export default {
+  input: 'src/main.js',
+  output: {
+    sourcemap: true,
+    format: 'iife',
+    name: 'app',
+    file: 'public/bundle.js',
+  },
+  plugins: [
+    svelte({
+      preprocess: autoPreprocess({
++        defaults: {
++          markup: 'pug',
++          script: 'typescript',
++          style: 'scss'
++        },
+         postcss: {
+           plugins: [require('autoprefixer')()]
+         }
+      }),
+      // enable run-time checks when not in production
+      dev: !production,
+      // we'll extract any component CSS out into
+      // a separate file — better for performance
+      css: css => {
+        css.write('public/bundle.css')
+      },
+    }),
+  ],
+}
+```
+
+Now our components are a bit leaner!
+
+```html
+<template>
+  h1 {world}
+</template>
+
+<script>
+  export let name: string = 'world';
+</script>
+
+<style>
+  h1 {
+    color: red;
+  }
+</style>
+```
+
+_**Note**: If the `<template>` tag is not found and the default language is not `html`, `svelte-preprocess` expects the whole markup to be written in that language. In example, for `pug`, this means the `script` and `style` tags must be written following pug's syntax._
+
+### 3.2 Prepending content
+
+Now we're in need of a `scss` file to hold some variables. Let's assume it's created at `src/styles/variables.scss`.
+
+```scss
+// src/styles/variables.scss
+$primary-color: red;
+```
+
+As in any `scss` project, we could just `@use './path/to/variables.scss`, but that can also become boring. `svelte-preprocess` [accepts a `prependData`](/docs/preprocessing.md#preprocessors) for almost every processor. Let's use it to prepend our import!
+
+```diff
++ import path from 'path'
+import svelte from 'rollup-plugin-svelte'
+import autoPreprocess from 'svelte-preprocess';
+
+export default {
+  input: 'src/main.js',
+  output: {
+    sourcemap: true,
+    format: 'iife',
+    name: 'app',
+    file: 'public/bundle.js',
+  },
+  plugins: [
+    svelte({
+      preprocess: autoPreprocess({
+         defaults: {
+           markup: 'pug',
+           script: 'typescript',
+           style: 'scss'
+         },
++        scss: {
++          // we use path.resolve because we need a absolute path
++          // because every component style is preprocessed from its directory
++          // so relative paths won't work.
++          prependData: `@use '${path.resolve(process.cwd(), 'src/styles/variables.scss')}';`
++        },
+         postcss: {
+           plugins: [require('autoprefixer')()]
+         }
+      }),
+      // enable run-time checks when not in production
+      dev: !production,
+      // we'll extract any component CSS out into
+      // a separate file — better for performance
+      css: css => {
+        css.write('public/bundle.css')
+      },
+    }),
+  ],
+}
+```
+
+Voila! We can now reference a variable from our file without having to explicitly import it.
+
+```html
+<template>
+  h1 {world}
+</template>
+
+<script>
+  export let name: string = 'world';
+</script>
+
+<style>
+  h1 {
+    color: $primary-color;
+  }
+</style>
+```