Svelte

What is Svelte?

Svelte is a compiler that turns declarative .svelte components into small, efficient JavaScript that updates the DOM directly, without a virtual DOM. Source code lives in a single npm-publishable package, packages/svelte, inside this monorepo. The repo currently ships Svelte 5 (current version 5.55.5, see packages/svelte/src/version.js).

A Svelte component file contains three things — a <script> block, markup, and a <style> block. The compiler reads them, performs analysis, and emits a JavaScript module per component plus optional scoped CSS. The same input also has a server target that produces an HTML string for SSR.

Who uses it

• Application authors building web apps directly with Svelte components (typically through SvelteKit).
• Tooling authors integrating Svelte with bundlers via svelte/compiler (vite-plugin-svelte, rollup-plugin-svelte, etc.).
• Library authors publishing reusable .svelte components.

Repo layout

sveltejs/svelte/
├── packages/
│   └── svelte/                 # the published `svelte` package
│       ├── src/
│       │   ├── compiler/       # parser, analyzer, transformer
│       │   ├── internal/       # client + server runtimes
│       │   ├── reactivity/     # Map, Set, Date, URL etc. wrappers
│       │   ├── transition/, animate/, motion/, easing/
│       │   ├── attachments/, events/, store/, legacy/, server/
│       │   └── index-client.js / index-server.js
│       ├── tests/              # vitest test suites (40+ areas)
│       └── messages/           # human-readable error/warning sources
├── playgrounds/                # local dev sandboxes
├── benchmarking/               # NODE_ENV=production microbenchmarks
├── documentation/docs/         # canonical docs that sync to svelte.dev
└── pnpm-workspace.yaml         # monorepo declaration

Quick links

• Architecture — how compiler, runtime, and reactivity fit together
• Getting started — install, build, test commands
• Glossary — domain vocabulary used across this wiki
• Compiler — parse → analyze → transform pipeline
• Client runtime — DOM blocks, effects, mount/hydrate
• Reactivity engine — sources, deriveds, effects, batches
• How to contribute