SnapDOM

SnapDOM is a high-performance engine for capturing, modifying, and converting DOM elements into any format.

Images and canvas ship in the core. Plugins turn the same capture into self-contained HTML, PDF, editable vectors, GIF, video or context for agents. Everything runs in the page, with no core dependencies.

Documentation and demos · Technical features · Official plugins · 简体中文

Why SnapDOM

Speed. Unchanged elements reuse their last capture, and small changes rebuild only the affected subtree. Run the live benchmark in your browser.
Fidelity. The output matches what the browser painted. Every release runs a pixel suite of real demo pages before it ships.
Flexibility. Capture once, export many formats. Official plugins add outputs, and the same plugin hooks let you write your own.
Maintenance. About one release a week over the past year, MIT licensed, with a public changelog.

Quick start

npm i @zumer/snapdom

import { snapdom } from '@zumer/snapdom';
const card = document.querySelector('#card');
const image = await snapdom.toPng(card);
document.body.appendChild(image);

Capture once when you need several outputs:

const result = await snapdom(card);
const image = await result.toPng();
const canvas = await result.toCanvas();
const blob = await result.toBlob({ format: 'png' });
await result.download({ format: 'jpg', filename: 'card' });

The result keeps that capture even if the source element later changes. Call snapdom(card) again to capture its new state.

What you can build

| Use | Output | Provided by | | --- | --- | --- | | Share a card, chart, invoice or dashboard | SVG, PNG, JPG, WebP, canvas or Blob | Core | | Reuse a capture in a texture, overlay or transition | Canvas plus capture geometry | Core | | Save a page fragment for later display | HTML with captured styles and fonts | html-export plugin | | Give an agent or a log a view of page content | Text/JSON context, or an image with an element map | context-export / agent-map plugins | | Download a document or record changing content | Searchable, paginated PDF, animated GIF or browser-encoded video | pdf / gif-export / video-export plugins | | Export editable artwork or paste into Figma | Native SVG shapes and text, or a Figma clipboard payload | vector plugin |

Image, HTML and context exports use the captured state. GIF and video plugins record the live element over time.

Installation

Install the core and, when you need them, the official plugins:

npm i @zumer/snapdom@latest @zumer/snapdom-plugins@latest

Plugins 4.x require SnapDOM 3.3.0 or newer within 3.x. They include the PDF and Vector exporters, MIT licensed like the core. Their sources live in packages/plugins/.

Or load it in a browser:

<script src="https://unpkg.com/@zumer/snapdom@latest/dist/snapdom.js"></script>
<script>
  snapdom.toPng(document.querySelector('#card')).then(image => {
    document.body.appendChild(image);
  });
</script>

As an ES module from a CDN:

import { snapdom } from 'https://esm.sh/@zumer/snapdom@latest';
import { htmlExport } from 'https://esm.sh/@zumer/snapdom-plugins@latest/html-export';

https://unpkg.com/@zumer/snapdom@latest/dist/snapdom.mjs serves the same module. These examples load the latest published core and plugins.

Build outputs

| File | Use | | --- | --- | | dist/snapdom.mjs | ES module for imports and bundlers | | dist/snapdom.js | Script tag exposing window.snapdom | | types/snapdom.d.ts | TypeScript declarations |

There is no CommonJS build. @zumer/snapdom/plugins and the package root share the same runtime and plugin registry.

Usage

Choose an output

| Result method | Returns | | --- | --- | | toPng(), toJpg(), toWebp() | An HTMLImageElement | | toSvg() | An SVG-backed HTMLImageElement | | toCanvas() | An HTMLCanvasElement | | toBlob() | An SVG Blob unless a format was explicitly set on the capture or export | | toRaw() / url | The capture's SVG data URL | | download() | Downloads the chosen format | | to(name, options?) | Runs a core or plugin exporter by name |

One-step shortcuts such as snapdom.toPng(element, options) capture and export in one call. Results also have toJpeg() as an alias for toJpg(). toImg() remains available; prefer toSvg() for an SVG image.

Set size and exclude content

const result = await snapdom(card, {
  width: 800,
  dpr: 1,
  backgroundColor: '#ffffff',
  exclude: '.capture-ignore',
  excludeMode: 'remove'
});

width and height define output size. If only one is set, the aspect ratio is preserved. scale applies when neither is set, and dpr multiplies the pixel dimensions. download() always uses DPR 1, so the file matches the element's CSS size; pass scale: devicePixelRatio for a file as sharp as the screen.

| Common option | Default | Purpose | | --- | --- | --- | | scale / dpr | 1 / device pixel ratio | Output resolution | | width / height | Unset | Output dimensions | | embedFonts | 'auto' | Embed the web fonts the capture uses | | backgroundColor | Transparent; white for JPG/WebP | Output background | | exclude | None | Selectors or predicates; true means exclude | | excludeMode | 'hide' | Keep an invisible spacer, or use 'remove' | | filter | None | Predicate; true keeps a node and false filters it out | | filterMode | 'hide' | Independent layout mode for nodes rejected by filter | | clip | Unset | Capture the viewport or a page-coordinate rectangle | | captureSelection | false | Include the user's text selection | | canvas | Unset | Reuse an existing canvas | | invalidate | false | Refresh after changes such as programmatic CSSOM edits | | fast | true | false keeps the page responsive during long captures | | debug | false | Log fallbacks and errors to the console; without it SnapDOM stays quiet and result.warnings reports them |

All options include shadows, transforms, fonts, CORS, fallbacks and layout reconciliation.

Export a long page

With the default SVG engine, PNG file exports automatically handle captures beyond the browser's canvas limits. Capture once, then download one full-resolution file or get its Blob:

const capture = await snapdom(article);
await capture.download({ format: 'png', scale: 2 });
// Or upload/store the file yourself:
const blob = await capture.toBlob({ format: 'png', scale: 2, dpr: 1 });

SnapDOM renders the frozen capture in windows and encodes them into one PNG. It preserves the requested dimensions, including shadows and padding, without allocating a canvas for the whole image. download() always uses DPR 1; toBlob() uses the capture's DPR unless you override it. Ordinary-sized exports use the native encoder.

This requires CompressionStream; older browsers keep the existing downscale and warning. Very large files still take time and memory to encode, and the application opening them has its own image limits.

Export a long page in pieces

toCanvas(), toPng() and JPEG/WebP exports still use a single canvas and scale down past SnapDOM's size guards (16,384px per side in Safari, 32,767px in Chrome/Firefox, plus an area limit). When you need separate canvases, use crop:

const capture = await snapdom(article);
const { contentX, contentY, w0, h0 } = capture.meta;
const pieceHeight = 4000; // CSS px
for (let y = 0; y < h0; y += pieceHeight) {
  const canvas = await capture.toCanvas({
    scale: 2,
    dpr: 1,
    crop: { x: contentX, y: contentY + y, width: w0, height: Math.min(pieceHeight, h0 - y) }
  });
  // save or upload this piece, then move on
}

crop is a rectangle, not a switch. It is in the coordinates of capture.meta, and each call returns one canvas. The capture runs once; only the rasterization repeats. Keep pieceHeight × scale × dpr under the limit, and set dpr yourself, since it defaults to the device pixel ratio. How tiling works.

Export HTML or structured context

import { htmlExport, contextExport } from '@zumer/snapdom-plugins';
const result = await snapdom(card, {
  plugins: [htmlExport(), contextExport({ format: 'json' })]
});
const html = await result.toHtml();
const context = await result.toContext();

The same plugin system supports overlays, redaction and custom exporters. Local plugins override global plugins by name. See the official plugin reference and plugin specification.

Export PDF and editable vectors

PDF returns a Blob with searchable, selectable text, links, pagination and optional form fields. Vector returns an SVG string without , preserving supported shapes and editable text.

import { snapdom } from '@zumer/snapdom';
import { pdf } from '@zumer/snapdom-plugins/pdf';
import { vector } from '@zumer/snapdom-plugins/vector';
const documentCapture = await snapdom(element, { plugins: [pdf()] });
const blob = await documentCapture.toPdf({ page: 'a4', margin: 36 });
const artwork = await snapdom(element, { plugins: [vector()] });
const svg = await artwork.toVector();
copyButton.addEventListener('click', () => artwork.toFigma());

Pass PDF capture settings to pdf() and paper, margin and download settings to toPdf(). toFigma() returns Promise and needs the async clipboard API on HTTPS or localhost, called from a user action. Vector requires a connected source element; unsupported paint is diagnosed by the engine. See the PDF reference and Vector contract.

Capture HTML strings

const result = await snapdom.fromString('<article>Hello</article>');
const image = await result.toPng();

Sanitize untrusted HTML first, with DOMPurify or equivalent. The string is parsed and activated like markup you wrote yourself: inline handlers such as run in the caller's origin, and can keep running after the mount is removed. fromString() mounts the markup offscreen and removes it after capture.

Feed a WebGL texture

const canvas = document.createElement('canvas');
const texture = new THREE.CanvasTexture(canvas);
texture.colorSpace = THREE.SRGBColorSpace;
async function refresh(element) {
  await snapdom.toCanvas(element, { canvas, scale: 1, dpr: 1 });
  texture.needsUpdate = true;
}

result.meta contains the capture geometry needed to place an exported image over the source interface. The labs show textures, mirrors and transitions.

Limitations

SnapDOM needs a browser DOM. A server-side Node.js process needs a browser environment to run it.
Cross-origin images, fonts and stylesheets need readable resources or an appropriate proxy. crossorigin does not grant access unless the server also allows it. Cross-origin iframes use placeholders.
SVG output includes HTML inside . It is suitable for browsers; support varies in other SVG viewers and document tools.
Output depends on browser rendering and image limits; for tall PNG files see Export a long page. Safari may fall back to PNG when WebP encoding is unavailable.
Canvas, video and other changing surfaces are captured fresh. JavaScript CSSOM edits are not observable automatically; use invalidate: true after them.
Core captures visible input values. Semantic plugins redact sensitive field values in their text/map output, but their attached image needs redactInputs or exclude if you want those pixels hidden too.

Technical features and browser behavior covers the details.

What's new in v3

Eligible unchanged captures reuse the first result. Safe local changes rebuild only affected subtrees; other changes use a full capture.
Web fonts embed automatically when used. System-font captures skip that work.
The style pass avoids redundant reads, and per-capture state is isolated for concurrent captures.
Safari image decoding and drawing retain their browser-specific handling.
snapdom.preCapture() can prepare captures on user intent. It learns a control when a capture starts during its press/click event, then prepares that capture on later hover or focus.

snapdom.preCapture();
button.onclick = () => snapdom.toPng(card);

Image compression and resource caching remain automatic. They do not make rasterization or image encoding free. See performance measurements and the cache guide.

SnapDOM has two rendering engines: SVG, the default, and html-in-canvas, which paints the same captured clone through the browser's native canvas API. Select the second with engine: 'html-in-canvas'.

The second engine is experimental: it still needs a compatible browser with its canvas drawing flag enabled, plus a build compiled with SNAPDOM_CANVAS_ENGINE=1. The default build includes SVG only. Unsupported captures fall back to SVG. A successful native capture produces a bitmap, so its URL and toRaw() return PNG, not serialized SVG. See ARCHITECTURE.md for details.

Migrating from v2

The main capture pattern remains snapdom(element, options). This guide compares v3 with v2.x.x, whose source remains available. Review these changes before upgrading:

| In v2 | In v3 | What to change | | --- | --- | --- | | Web fonts were opt-in | embedFonts: 'auto' | Usually nothing; use false only if you want to omit them | | Raster width/height could be multiplied by scale | Width/height win over scale | Pass the final size: width: 400 instead of width: 200, scale: 2 | | burst opted into repeat memoization | Eligible captures memoize automatically; burst is no longer documented or supported | Remove burst (the engine still reads it for internal use, do not rely on it) and use invalidate: true for one fresh capture after unobservable changes such as sheet.insertRule() | | preCache prepared resources | Removed; preCapture() learns capture intent | Remove preCache; preCapture() is not a drop-in rename | | fast: false cloned through idle callbacks | fast: false pauses about every frame, at nearly the same total time | Keep fast: false | | filter / filterMode and exclude / excludeMode could be used together | Both controls and their independent modes remain supported; exclude also accepts predicates | Keep existing rules and modes; use the additional predicate form only when useful | | cache: 'auto' or 'full' | Both map to 'soft' | Usually omit it; 'disabled' / false is for debugging | | compress controlled embedded image downsampling | Image optimization is automatic; compress is no longer documented or supported | Remove compress; the engine still reads it for internal use, do not rely on it | | resolvePicturePlaceholders / pictureResolver configured lazy-image preparation | Responsive/lazy image resolution happens on the clone; these options are no longer documented or supported | Remove the options and handle custom loading/timeouts in your app before capture; the engine still reads resolvePicturePlaceholders for internal use, do not rely on it | | Some visible input values were redacted | Core masks passwords only | Add redactInputs() for other fields | | afterExport returns became the next hook's payload, not the caller's result | Returns are ignored; hooks receive the same export payload | Stop chaining through return values; use defineExports to produce a different output | | Plugins v2.x.x exposed @zumer/snapdom-plugins/html-in-canvas | That subpath is removed | Use the experimental core engine: 'html-in-canvas' with a compatible custom build; the default build uses SVG | | TypeScript exported PluginExportFacade | The named type is removed; ctx.exports still provides core exporters | Infer it in defineExports, or use NonNullable |

Use filter and exclude together

filter and exclude are independent controls, as in v2. filter(node) returns true to keep a node and false to filter it out; filterMode controls how a filtered node affects layout. exclude specifies additional omissions using selectors or predicates; an exclusion predicate returns true to omit the node. excludeMode controls those omissions. You can keep both controls and different modes in the same capture:

// Works in v2 and v3: hide private fields, remove the toolbar.
await snapdom(card, {
  filter: node => !node.matches('[data-private]'),
  filterMode: 'hide',
  exclude: ['.toolbar'],
  excludeMode: 'remove'
});

'hide' keeps an invisible spacer; 'remove' drops the node and allows reflow. Both omit its content. V3 additionally lets exclude mix selectors and predicates, such as exclude: ['.toolbar', node => node.dataset.export === 'omit']; any matching rule excludes the node. This is optional and does not replace filter or merge the two modes. The separate CSS-effect plugin named filter is also available.

Both modes default to 'hide'. Per node, data-capture="exclude" is checked first, then exclude, then filter. The first omission decides the mode and stops evaluation for that node: if it matches exclude, excludeMode wins even when filter would reject it with a different mode. filter uses the v2 truthiness rule: any falsy return filters the node out.

Prefer selectors to callbacks in polling loops

A function-valued filter, exclude, excludeStyleProps or fallbackURL turns off memoization and differential recapture, so a polling loop with a predicate pays a full capture on every tick. When the rule can be written as a selector, pass the selector and keep the reuse.

Captures with function-valued filter, exclude, excludeStyleProps or fallbackURL run fresh so applicable callbacks can read current application state on each new capture. They do not reuse an unchanged capture or an earlier callback's style/fallback decision. You do not need invalidate just because a callback's closure changed:

let privateMode = false;
const options = {
  exclude: node => privateMode && node.matches('[data-private]'),
  excludeMode: 'remove'
};
const before = await snapdom(card, options);
privateMode = true;
const after = await snapdom(card, options); // evaluates the current policy

before still contains its original captured state; exporting it again does not apply a new policy. Use a new capture such as after. Keep exclusion and style predicates synchronous and boolean-returning. invalidate: true remains necessary after unobservable changes such as direct CSSOM edits.

Capture-affecting plugins suspend memoization unless they declare pure: true. Declare it only for deterministic hooks; timestamps and callbacks reading external state must run again. See the v3 plugin contract.

Performance benchmarks

Recorded measurements separate first captures, repeat captures and image-heavy scenes. The live comparison runs in your browser and identifies the package version it loads.

For a useful comparison, use the same scene, output format, scale and DPR. Compare the images as well as the times.

Documentation

Plugin demos, PDF demos and Vector demos
v2 source
API and options
Framework guides and how-to examples
Official plugins, plugin specification and contributing plugins
Architecture and technical features

Development

From this checkout:

npm install
npx playwright install
npm test            # Chromium; test:firefox, test:webkit, or test:full for all three

Every test script runs npm run check (lint, types, release checks, bundle) and then the browser suite, visual demos included. Visual baselines live in __snapshots__/, which is not committed: on a fresh clone, record them once with node scripts/vitest.mjs run --browser.headless before relying on npm test. npm run site serves the docs with the local build. npm test never changes files; use npm run lint:fix to apply lint fixes. See ARCHITECTURE.md for implementation notes.