svelte/site/content/guide/17-preprocessing.md

title
Preprocessing

Some developers like to use non-standard languages such as Pug, Sass or CoffeeScript.

It's possible to use these languages, or anything else that can be converted to HTML, CSS and JavaScript, using preprocessors.

svelte.preprocess

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:

const svelte = require('svelte');

const input = fs.readFileSync('App.html', 'utf-8');

svelte.preprocess(input, {
	filename: 'App.html', // this is passed to each preprocessor

	markup: ({ content, filename }) => {
		return {
			code: '<!-- some HTML -->',
			map: {...}
		};
	},

	style: ({ content, attributes, filename }) => {
		return {
			code: '/* some CSS */',
			map: {...}
		};
	},

	script: ({ content, attributes, filename }) => {
		return {
			code: '// some JavaScript',
			map: {...}
		};
	}
}).then(preprocessed => {
	fs.writeFileSync('preprocessed/App.html', preprocessed.toString());

	const { js } = svelte.compile(preprocessed);
	fs.writeFileSync('compiled/App.js', js.code);
});

The markup preprocessor, if specified, runs first. The content property represents the entire input string.

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'>).

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.

The returned map objects are not currently used by Svelte, but will be in future versions

Using build tools

Many build tool plugins, such as rollup-plugin-svelte and svelte-loader, allow you to specify preprocess options, in which case the build tool will do the grunt work.