The magical disappearing UI framework
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Latest commit 1deb25e Oct 20, 2018


The magical disappearing UI framework.


This is the Svelte compiler, which is primarily intended for authors of tooling that integrates Svelte with different build systems. If you just want to write Svelte components and use them in your app, you probably want one of those tools:

Build Systems

CSS Preprocessors

Additional tools

Example usage

import * as svelte from 'svelte';

const { js, css, ast } = svelte.compile(source, {
	// the target module format – defaults to 'es' (ES2015 modules), can
	// also be 'amd', 'cjs', 'umd', 'iife' or 'eval'
	format: 'umd',

	// the filename of the source file, used in e.g. generating sourcemaps
	filename: 'MyComponent.html',

	// the name of the constructor. Required for 'iife' and 'umd' output,
	// but otherwise mostly useful for debugging. Defaults to 'SvelteComponent'
	name: 'MyComponent',

	// for 'amd' and 'umd' output, you can optionally specify an AMD module ID
	amd: {
		id: 'my-component'

	// custom error/warning handlers. By default, errors will throw, and
	// warnings will be printed to the console. Where applicable, the
	// error/warning object will have `pos`, `loc` and `frame` properties
	onerror: err => {
		console.error( err.message );

	onwarn: warning => {
		console.warn( warning.message );


The Svelte compiler exposes the following API:

  • compile(source [, options]) => { js, css, ast } - Compile the component with the given options (see below). Returns an object containing the compiled JavaScript, a sourcemap, an AST and transformed CSS.
  • create(source [, options]) => function - Compile the component and return the component itself.
  • preprocess(source, options) => Promise — Preprocess a source file, e.g. to use PostCSS or CoffeeScript
  • VERSION - The version of this copy of the Svelte compiler as a string, 'x.x.x'.

Compiler options

The Svelte compiler optionally takes a second argument, an object of configuration options:

Values Description Default
generate 'dom', 'ssr', false Whether to generate JavaScript code intended for use on the client ('dom'), or for use in server-side rendering ('ssr'). If false, component will be parsed and validated but no code will be emitted 'dom'
dev true, false Whether to enable run-time checks in the compiled component. These are helpful during development, but slow your component down. false
css true, false Whether to include code to inject your component's styles into the DOM. true
hydratable true, false Whether to support hydration on the compiled component. false
customElement true, false, { tag, props } Whether to compile this component to a custom element. If tag/props are passed, compiles to a custom element and overrides the values exported by the component. false
bind boolean If false, disallows bind: directives true
shared true, false, string Whether to import various helpers from a shared external library. When you have a project with multiple components, this reduces the overall size of your JavaScript bundle, at the expense of having immediately-usable component. You can pass a string of the module path to use, or true will import from 'svelte/shared.js'. false
legacy true, false Ensures compatibility with very old browsers, at the cost of some extra code. false
format 'es', 'amd', 'cjs', 'umd', 'iife', 'eval' The format to output in the compiled component.
'es' - ES6/ES2015 module, suitable for consumption by a bundler
'amd' - AMD module
'cjs' - CommonJS module
'umd' - UMD module
'iife' - IIFE-wrapped function defining a global variable, suitable for use directly in browser
'eval' - standalone function, suitable for passing to eval()
'es' for generate: 'dom'
'cjs' for generate: 'ssr'
name string The name of the constructor in the compiled component. 'SvelteComponent'
filename string The filename to use in sourcemaps and compiler error and warning messages. 'SvelteComponent.html' string The AMD module ID to use for the 'amd' and 'umd' output formats. undefined
globals object, function When outputting to the 'umd', 'iife' or 'eval' formats, an object or function mapping the names of imported dependencies to the names of global variables. {}
preserveComments boolean Include comments in rendering. Currently, only applies to SSR rendering false
onerror function Specify a callback for when Svelte encounters an error while compiling the component. Passed two arguments: the error object, and another function that is Svelte's default onerror handling. (exception is thrown)
onwarn function Specify a callback for when Svelte encounters a non-fatal warning while compiling the component. Passed two arguments: the warning object, and another function that is Svelte's default onwarn handling. (warning is logged to console)

Preprocessor options

svelte.preprocess returns a Promise that resolves to an object with a toString method (other properties will be added in future). It takes an options object with markup, style or script properties:

const processed = await svelte.preprocess(source, {
	markup: ({ content }) => {
		// `content` is the entire component string
		return { code: '...', map: {...} };

	style: ({ content, attributes }) => {
		// `content` is what's inside the <style> element, if present
		// `attributes` is a map of attributes on the element
		if (attributes.type !== 'text/scss') return;
		return { code: '...', map: {...} };

	script: ({ content, attributes }) => {
		// `content` is what's inside the <script> element, if present
		// `attributes` is a map of attributes on the element
		if (attributes.type !== 'text/coffeescript') return;
		return { code: '...', map: {...} };

The style and script preprocessors will run after the markup preprocessor. Each preprocessor can return a) nothing (in which case no transformation takes place), b) a { code, map } object, or c) a Promise that resolves to a) or b). Note that sourcemaps are currently discarded, but will be used in future versions of Svelte.

Example/starter repos


Pull requests are encouraged and always welcome. Pick an issue and help us out!

To install and work on Svelte locally:

git clone
cd svelte
npm install
npm run dev

The compiler is written in TypeScript, but don't let that put you off — it's basically just JavaScript with type annotations. You'll pick it up in no time. If you're using an editor other than VSCode you may need to install a plugin in order to get syntax highlighting and code hints etc.

Linking to a Live Project

You can make changes locally to Svelte and test it against any Svelte project. You can also use a default template for development. Instruction on setup are found in that project repository.

From your project:

cd ~/path/to/your-svelte-project
npm install ~/path/to/svelte

And you should be good to test changes locally.

To undo this and link to the official latest Svelte release, just run:

npm install svelte@latest

Running Tests

npm run test

For running single tests, you can use pattern matching:

npm run test -- -g "includes AST in svelte.compile output"

Alternately, you can add solo: true to any given test/../_config.js file, but remember never to commit that setting.