sveld

sveld generates TypeScript definitions and component documentation (JSON and Markdown) for Svelte component libraries. It statically analyzes props, events, slots, snippets, context, module exports, and rest props, and reads JSDoc where inference isn't enough.

The generated .d.ts files give consumers autocomplete and type checking through the Svelte Language Server and TypeScript, with little effort from the library author. Carbon Components Svelte uses sveld to generate its component types and API metadata.

sveld supports Svelte 3, Svelte 4, Svelte 5 without runes (export let, <slot>, $$restProps), and Svelte 5 runes ($props(), $bindable(), {@render}, callback props).

When to use sveld

If your library is built with SvelteKit's svelte-package, it already emits .d.ts files through svelte2tsx; start there.

sveld is for:

• JavaScript-first libraries. Components written in plain JavaScript with JSDoc, where you want full editor types without converting to lang="ts".
• Docs from the same source as the types. JSON and Markdown API docs, generated in the same run.
• API drift checks. --check diffs the component API against a committed snapshot and fails CI on breaking changes.
• Richer types from JSDoc. @typedef, @callback, @slot, @event, and context types that plain inference can't produce.

lang="ts" components work too: sveld keeps their type annotations as written.

Install

npm i -D sveld
# or: pnpm i -D sveld / bun i -D sveld / yarn add -D sveld

Quick start

sveld reads the "svelte" field of your package.json as the entry point: the file that re-exports your components.

{
  "svelte": "./src/index.js"
}

CLI

npx sveld                     # writes types/*.d.ts
npx sveld --json --markdown   # also writes COMPONENT_API.json and COMPONENT_INDEX.md

Run npx sveld --help for every flag. Options can also live in a sveld.config.js:

// sveld.config.js
import { defineConfig } from "sveld";

export default defineConfig({
  json: true,
  markdown: true,
});

Vite plugin

// vite.config.ts
import { svelte } from "@sveltejs/vite-plugin-svelte";
import sveld from "sveld";
import { defineConfig } from "vite";

export default defineConfig({
  plugins: [svelte(), sveld({ json: true })],
});

The plugin runs during vite build (and during vite dev with watch: true). It also works in Rollup configs.

Node

import { sveld } from "sveld";

const { document, diagnostics, exitCode } = await sveld({
  entry: "./src/index.js",
  json: true,
});

document is the same component API document json: true writes, so you can render your own formats from it. See Options for everything sveld() accepts and returns.

Example

Given this component:

<!-- Button.svelte -->
<script>
  export let type = "button";
  export let primary = false;
</script>

<button {...$$restProps} {type} class:primary on:click>
  <slot>Click me</slot>
</button>

sveld infers prop types and defaults, the forwarded click event, the default slot, and the rest props spread onto <button>:

// types/Button.svelte.d.ts
import { SvelteComponentTyped } from "svelte";
import type { SvelteHTMLElements } from "svelte/elements";

type $RestProps = SvelteHTMLElements["button"];

type $Props = {
  /**
   * @default "button"
   */
  type?: string;

  /**
   * @default false
   */
  primary?: boolean;

  children?: (this: void) => void;

  [key: `data-${string}`]: unknown;
};

export type ButtonProps = Omit<$RestProps, keyof $Props> & $Props;

export default class Button extends SvelteComponentTyped<
  ButtonProps,
  { click: WindowEventMap["click"] },
  { default: Record<string, never> }
> {}

The default slot is also typed as a children snippet prop for Svelte 5 consumers, and the data-* index signature lets callers pass data attributes through $$restProps.

Add JSDoc to tighten types and add descriptions:

<script>
  /** @type {"button" | "submit" | "reset"} */
  export let type = "button";

  /**
   * Set to `true` to use the primary variant
   */
  export let primary = false;
</script>

type $Props = {
  /**
   * @default "button"
   */
  type?: "button" | "submit" | "reset";

  /**
   * Set to `true` to use the primary variant
   * @default false
   */
  primary?: boolean;

  children?: (this: void) => void;

  [key: `data-${string}`]: unknown;
};

The runes version (let { type = "button", primary = false, children, ...rest } = $props(), spreading rest and rendering children) produces the same $Props. It has no click event, since runes components take onclick as a prop through rest.

The .d.ts extends SvelteComponentTyped by default, which works for consumers on Svelte 3, 4, and 5. Set typesOptions.format: "component" (--types-format=component) to emit the Svelte 5 Component type instead. See Output.

JSON and Markdown

npx sveld --markdown documents the same component in COMPONENT_INDEX.md:

## `Button`

### Props

| Prop name | Required | Kind | Reactive | Binding | Type | Default value | Description |
| :- | :- | :- | :- | :- | :- | :- | :- |
| type | No | <code>let</code> | No | -- | <code>"button" &#124; "submit" &#124; "reset"</code> | <code>"button"</code> | -- |
| primary | No | <code>let</code> | No | -- | <code>boolean</code> | <code>false</code> | Set to `true` to use the primary variant |

### Slots

| Slot name | Default | Props | Fallback | Description |
| :- | :- | :- | :- | :- |
| -- | Yes | <code>Record&lt;string, never> </code> | <code>Click me</code> | -- |

### Events

| Event name | Type | Detail | Description |
| :- | :- | :- | :- |
| click | forwarded | -- | -- |

npx sveld --json writes the same data to COMPONENT_API.json, validated by a JSON Schema that ships with the package. An excerpt of the prop entry:

{
  "name": "type",
  "kind": "let",
  "type": "\"button\" | \"submit\" | \"reset\"",
  "typeSource": "jsdoc",
  "value": "\"button\"",
  "isRequired": false,
  "reactive": false
}

See Output for the full shape.

In CI

Commit COMPONENT_API.json, then gate pull requests on API changes and inference gaps:

npx sveld --check    # exit 3 on a breaking API change against the snapshot
npx sveld --strict   # exit 4 when a prop fell back to `any`, a tag was dropped, ...

sveld --check: 2 API changes detected against "COMPONENT_API.json".
Suggested semver bump: major.

  Button
    [BREAKING] prop "target" added (required)
    [BREAKING] prop "href" removed

See CI for exit codes, the change classification table, diagnostic codes, and how to ignore a diagnostic.

JSDoc tags

Tag	Use it to	Example
@type	Type a prop	@type {"sm" | "md" | "lg"}
@default	Override the inferred default shown in docs	@default () => true
@typedef	Declare a shared, exported type	@typedef {{ id: string }} Item
@property	Document a field of a @typedef {object} or event detail	@property {string} [label] - Label text
@callback	Declare a function type	@callback OnChange + @param/@returns
@slot / @snippet	Type slot or snippet props	@slot {{ item: Item }} row
@event	Type a dispatched event's detail	@event {{ id: string }} save
@param / @returns	Type an exported function	@param {string} id
@restProps	Name the element rest props are spread onto	@restProps {button | a}
@extendProps	Extend another component's props	@extendProps {"./Button.svelte"} ButtonProps
@template / @generics	Declare generics in a JS component	@template {Item} [T=Item]
@bindable	Document a prop's bind: contract	@bindable writable
@deprecated	Mark a prop, event, slot, or export deprecated	@deprecated Use "size" instead.
@ignore / @internal	Hide from every output	@internal
@since / @see / @example	Pass structured tags through to all outputs	@since 1.2.0
{@link}	Link in a description	{@link https://example.com|docs}
@csspart / @cssprop	Document shadow-DOM styling hooks	@cssprop {Color} [--bg=white]
@sveld-ignore	Suppress a diagnostic	@sveld-ignore sveld/prop-unknown-type
<!-- @component -->	Document the component itself	<!-- @component Renders a button. -->

Context types come from setContext calls with no tag needed. See Context.

Docs

• Options: every option and CLI flag, the config file, the Vite plugin, sveld() and its result, and sveld/browser.
• JSDoc tags: how types are inferred and every tag sveld reads.
• Output: .d.ts formats, JSON output and schema, Markdown, entry exports, custom element metadata, and custom output formats.
• CI: exit codes, --check for API drift, --strict and diagnostic codes, and @example checking.
• Troubleshooting

Try sveld in the browser at sveld.onrender.com (source in playground/).

Requirements

• Node 22 or later, or Bun. sveld is ESM-only.
• A sveld.config.ts needs a runtime that strips TypeScript: Bun, or Node 22.18+ / 23.6+.
• The optional checkExamples check for TS/JS examples needs typescript 7+ and a tsconfig.json. Nothing else loads TypeScript.
• Your installed Svelte version doesn't affect parsing. sveld parses with sveast, kept in parity with svelte/compiler.

Contributing

See the contributing guidelines.

License

Apache-2.0