in-general.md

Using Preprocessors

Generic setup

If a svelte file contains some language other than html, css or javascript, svelte-vscode needs to know how to preprocess it. This can be achieved by creating a svelte.config.js file at the root of your project which exports a svelte options object (similar to svelte-loader and rollup-plugin-svelte). It's recommended to use the official svelte-preprocess package which can handle many languages.

NOTE: Prior to svelte-check 1.4.0 / svelte-language-server 0.13.0 / Svelte for VS Code 104.9.0 you cannot use the new import x from y and export const / export default syntax in svelte.config.js.

ESM-style (for everything with "type": "module" in its package.json, like SvelteKit):

import sveltePreprocess from 'svelte-preprocess';

export default {
    preprocess: sveltePreprocess()
};

CJS-style:

const sveltePreprocess = require('svelte-preprocess');

module.exports = {
    preprocess: sveltePreprocess()
};

It's also necessary to add a type="text/language-name" or lang="language-name" to your style and script tags, which defines how that code should be interpreted by the extension.

<div>
    <h1>Hello, world!</h1>
</div>

<style type="text/scss">
    div {
        h1 {
            color: red;
        }
    }
</style>

Language specific setup

• SCSS/Less
• TypeScript

Using language defaults

If you use svelte-preprocess and define the defaults inside svelte.config.js, you can in some cases omit the type/lang attributes. While these defaults get picked up by the language server, this may break your syntax highlighting and your code is no longer colored the right way, so use with caution - reason: we have to tell VSCode which part of the Svelte file is written in which language through providing static regexes, which rely on the type/lang attribute. It will also likely not work for other tooling in the ecosystem, for example eslint-plugin-svelte3 or prettier-plugin-svelte. We therefore recommend to always type the attributes.

Deduplicating your configs

Most of the preprocessor settings you write inside your svelte.config.js is likely duplicated in your build config. Here's how to deduplicate it (using rollup and CJS-style config as an example):

// svelte.config.js:
const sveltePreprocess = require('svelte-preprocess');

// using sourceMap as an example, but could be anything you need dynamically
function createPreprocessors(sourceMap) {
    return sveltePreprocess({
        sourceMap
        // ... your settings
    });
}

module.exports = {
    preprocess: createPreprocessors(true),
    createPreprocessors
};

// rollup.config.js:
// ...

const createPreprocessors = require('./svelte.config').createPreprocessors;
const production = !process.env.ROLLUP_WATCH;

export default {
    // ...

    plugins: [
        // ...
        svelte({
            // ...
            preprocess: createPreprocessors(!production)
        })
        // ...
    ]
};

Restart the svelte language server

You will need to tell svelte-vscode to restart the svelte language server in order to pick up a new configuration.

Hit ctrl-shift-p or cmd-shift-p on mac, type svelte restart, and select Svelte: Restart Language Server. Any errors you were seeing should now go away and you're now all set up!