# Helpers

> Document engine helpers for the browser and the server: downloadExport, bindKeyboardShortcuts, validateDocument, migrateDocument and more.

Source: https://fabricjs-document-engine.jscrate.dev/docs/api/helpers
Last updated: 2026-09-28

The main entry exports a few document engine helpers next to `createDocumentEngine`. The browser helpers wire the engine into the page. The document helpers work in Node without a canvas.

## Browser helpers

```ts
function bindKeyboardShortcuts(engine: UndoRedoTarget, options?: KeyboardShortcutOptions): () => void
```

Ctrl/Cmd+Z undoes. Ctrl/Cmd+Shift+Z and Ctrl+Y redo. Ignored while the user types in an input or in Fabric text. Returns an unbind function.

```ts
function bindUnsavedChangesWarning(engine: UnsavedChangesSource, target?: EventTarget): () => void
```

Asks the browser to confirm before a page with unsaved changes closes. Returns an unbind function.

```ts
function downloadExport(result: DownloadableExport, fileName?: string): void
```

Starts a browser download of an export result.

| Name | Type | Default | Description |
| --- | --- | --- | --- |
| `target` | `EventTarget` | `window` | Where to listen for key presses. |

## State for any framework

```ts
function createDocumentStateStore(engine: DocumentEngine): DocumentStateStore
```

Framework-free `{ getSnapshot, subscribe, destroy }` with `DocumentState`. Fits Vue, Svelte and `useSyncExternalStore`.

## Document helpers

These document engine helpers run anywhere, including a server:

```ts
function validateDocument(value: unknown): DocumentIssue[]
```

Checks a value against the document format. Returns a list of issues with the path of each problem.

```ts
function migrateDocument(value: unknown, context: MigrationContext, steps?: Readonly<Record<number, Migration>>, targetVersion?: number): MigrationResult
```

Upgrades an older document or plain Fabric JSON to the current format. Useful for server-side batch upgrades.

```ts
function detectSchemaVersion(value: unknown): number | undefined
```

Returns `1` for a document, `0` for plain Fabric JSON, otherwise `undefined`.

```ts
function secureDocument(input: unknown, limits?: ContentLimits): unknown
```

Removes prototype keys and enforces the object and depth limits. Throws `UNSAFE_DOCUMENT`.

```ts
function isSafeImageUrl(url: string): boolean
```

The default image address rule: `http`, `https`, `blob`, relative addresses and `data:image/` are allowed.

## Errors

```ts
function isDocumentEngineError(value: unknown): value is DocumentEngineError
```

Type guard for `DocumentEngineError`, which carries a stable `code`.

## Example: check an upload on the server

```ts
import {
  migrateDocument,
  secureDocument,
  validateDocument,
} from "fabricjs-document-engine";

const safe = secureDocument(body, { maxObjects: 20000 });
const { document } = migrateDocument(safe, {
  canvasWidth: 1200,
  canvasHeight: 800,
});
const issues = validateDocument(document);
```

More on these document engine helpers is in [security](https://fabricjs-document-engine.jscrate.dev/docs/guides/security) and [migration](https://fabricjs-document-engine.jscrate.dev/docs/guides/migration).