svelte

What is svelte?

Svelte is a modern JavaScript compiler that allows you to write high-performance user interfaces with significantly less boilerplate code than traditional frameworks. It shifts much of the work to compile time, producing highly optimized vanilla JavaScript at the end.

What are svelte's main functionalities?

Reactive Declarations

Svelte allows you to write reactive statements using the $: syntax. When the state changes, the reactive statements automatically update to reflect the new state.

let count = 0;

$: doubled = count * 2;

Component Definition

Components in Svelte are defined using a combination of HTML, CSS, and JavaScript, which are encapsulated within a single file with a .svelte extension.

<script>
  export let name;
</script>

<h1>Hello {name}!</h1>

Store Management

Svelte provides a simple store mechanism to manage global state. The 'writable' store is a basic store that allows reading and writing values reactively.

import { writable } from 'svelte/store';

const count = writable(0);

Transitions and Animations

Svelte makes it easy to add transitions and animations to elements when they enter or leave the DOM.

import { fade } from 'svelte/transition';

<div in:fade={{ delay: 0, duration: 200 }}>Fade In</div>

Bindings

Svelte provides a concise syntax for two-way data binding to HTML elements, allowing for easy synchronization between the DOM and component state.

<script>
  let value = '';
</script>

<input bind:value={value} />

Other packages similar to svelte

react

React is a popular JavaScript library for building user interfaces. It uses a virtual DOM for efficient updates, and it's known for its component-based architecture. Unlike Svelte, React requires a runtime library and often involves more boilerplate code.

vue

Vue is a progressive JavaScript framework used for building UIs and single-page applications. It is similar to Svelte in its component structure and reactivity model but differs in that it uses a virtual DOM and requires a runtime.

angular

Angular is a platform and framework for building single-page client applications using HTML and TypeScript. It is more prescriptive than Svelte, with a complex ecosystem and a steep learning curve, and it includes features like dependency injection and RxJS integration.

preact

Preact is a fast, 3kB alternative to React with the same modern API. It provides a similar component-based architecture but with a smaller footprint. Preact is closer to Svelte in terms of size but still operates with a virtual DOM.

README

Svelte

The magical disappearing UI framework.

• Read the introductory blog post
• Read the guide
• Try it out
• Chat on Gitter

---

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:

• svelte-cli – Command line interface for compiling components
• rollup-plugin-svelte – Rollup plugin
• sveltify - Browserify transform
• gulp-svelte - gulp plugin
• metalsmith-svelte - Metalsmith plugin
• system-svelte – System.js loader
• svelte-loader – Webpack loader
• meteor-svelte – Meteor build plugin
• sveltejs-brunch – Brunch build plugin
• More to come!

Example usage

import * as svelte from 'svelte';

const { code, map } = 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 );
	}
});

API

The Svelte compiler exposes the following API:

• compile( source [, options ] ) => { code, map, ast, css } - 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'	Whether to generate JavaScript code intended for use on the client ('dom'), or for use in server-side rendering ('ssr').	'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
store	true, false	Whether to support store integration on the compiled component.	false
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
cascade	true, false	Whether to cascade all of the component's styles to child components. If false, only selectors wrapped in :global(...) and keyframe IDs beginning with -global- are cascaded.	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'
amd.id	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.	{}
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

• charpeni/svelte-example - Some Svelte examples with configured Rollup, Babel, ESLint, directives, Two-Way binding, and nested components
• EmilTholin/svelte-test
• lukechinworth/codenames – example integration with Redux

BrowserStack

To keep Svelte's performance in check, we use BrowserStack to quickly run benchmarks for each PR that immediately give feedback to the contributor. You can see how we use BrowserStack in the svelte-bench project and check out BrowserStack's services on their website.

License

MIT