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61 lines
2.1 KiB
61 lines
2.1 KiB
6 years ago
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---
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title: Preprocessing
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---
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Some developers like to use non-standard languages such as [Pug](https://pugjs.org/api/getting-started.html), [Sass](http://sass-lang.com/) or [CoffeeScript](http://coffeescript.org/).
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It's possible to use these languages, or anything else that can be converted to HTML, CSS and JavaScript, using *preprocessors*.
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### svelte.preprocess
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Svelte exports a `preprocess` function that takes some input source code and returns a Promise for a standard Svelte component, ready to be used with `svelte.compile`:
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```js
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const svelte = require('svelte');
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const input = fs.readFileSync('App.html', 'utf-8');
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svelte.preprocess(input, {
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filename: 'App.html', // this is passed to each preprocessor
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markup: ({ content, filename }) => {
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return {
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code: '<!-- some HTML -->',
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map: {...}
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};
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},
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style: ({ content, attributes, filename }) => {
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return {
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code: '/* some CSS */',
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map: {...}
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};
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},
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script: ({ content, attributes, filename }) => {
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return {
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code: '// some JavaScript',
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map: {...}
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};
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}
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}).then(preprocessed => {
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fs.writeFileSync('preprocessed/App.html', preprocessed.toString());
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const { js } = svelte.compile(preprocessed);
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fs.writeFileSync('compiled/App.js', js.code);
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});
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```
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The `markup` preprocessor, if specified, runs first. The `content` property represents the entire input string.
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The `style` and `script` preprocessors receive the contents of the `<style>` and `<script>` elements respectively, along with any `attributes` on those elements (e.g. `<style lang='scss'>`).
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All three preprocessors are optional. Each should return a `{ code, map }` object or a Promise that resolves to a `{ code, map }` object, where `code` is the resulting string and `map` is a sourcemap representing the transformation.
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> The returned `map` objects are not currently used by Svelte, but will be in future versions
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### Using build tools
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Many build tool plugins, such as [rollup-plugin-svelte](https://github.com/rollup/rollup-plugin-svelte) and [svelte-loader](https://github.com/sveltejs/svelte-loader), allow you to specify `preprocess` options, in which case the build tool will do the grunt work.
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