Serialization & Deserialization

Internally, Lexical maintains the state of a given editor in memory, updating it in response to user inputs. Sometimes, it's useful to convert this state into a serialized format in order to transfer it between editors or store it for retrieval at some later time. In order to make this process easier, Lexical provides some APIs that allow Nodes to specify how they should be represented in common serialized formats.

HTML

Currently, HTML serialization is primarily used to transfer data between Lexical and non-Lexical editors (such as Google Docs or Quip) via the copy & paste functionality in @lexical/clipboard, but we also offer generic utilities for converting Lexical -> HTML and HTML -> Lexical in our @lexical/html package.

Lexical -> HTML

When generating HTML from an editor you can pass in a selection object to narrow it down to a certain section or pass in null to convert the whole editor.

import {$generateHtmlFromNodes} from '@lexical/html';

const htmlString = $generateHtmlFromNodes(editor, selection | null);

LexicalNode.exportDOM()

You can control how a LexicalNode is represented as HTML by adding an exportDOM() method.

exportDOM(editor: LexicalEditor): DOMExportOutput

When transforming an editor state into HTML, we simply traverse the current editor state (or the selected subset thereof) and call the exportDOM method for each Node in order to convert it to an HTMLElement.

Sometimes, it's necessary or useful to do some post-processing after a node has been converted to HTML. For this, we expose the "after" API on DOMExportOutput, which allows exportDOM to specify a function that should be run after the conversion to an HTMLElement has happened.

export type DOMExportOutput = {
  after?: (generatedElement: ?HTMLElement) => ?HTMLElement,
  element?: HTMLElement | null,
};

If the element property is null in the return value of exportDOM, that Node will not be represented in the serialized output.

HTML -> Lexical

import {$generateNodesFromDOM} from '@lexical/html';

editor.update(() => {
  // In the browser you can use the native DOMParser API to parse the HTML string.
  const parser = new DOMParser();
  const dom = parser.parseFromString(htmlString, textHtmlMimeType);

  // Once you have the DOM instance it's easy to generate LexicalNodes.
  const nodes = $generateNodesFromDOM(editor, dom);

  // Select the root
  $getRoot().select();

  // Insert them at a selection.
  $insertNodes(nodes);
});

If you are running in headless mode, you can do it this way using JSDOM:

import {createHeadlessEditor} from '@lexical/headless';
import {$generateNodesFromDOM} from '@lexical/html';

// Once you've generated LexicalNodes from your HTML you can now initialize an editor instance with the parsed nodes.
const editorNodes = [] // Any custom nodes you register on the editor
const editor = createHeadlessEditor({ ...config, nodes: editorNodes });

editor.update(() => {
  // In a headless environment you can use a package such as JSDom to parse the HTML string.
  const dom = new JSDOM(htmlString);

  // Once you have the DOM instance it's easy to generate LexicalNodes.
  const nodes = $generateNodesFromDOM(editor, dom.window.document);

  // Select the root
  $getRoot().select();

  // Insert them at a selection.
  const selection = $getSelection();
  selection.insertNodes(nodes);
});

LexicalNode.importDOM()

You can control how an HTMLElement is represented in Lexical by adding an importDOM() method to your LexicalNode.

static importDOM(): DOMConversionMap | null;

The return value of importDOM is a map of the lower case (DOM) Node.nodeName property to an object that specifies a conversion function and a priority for that conversion. This allows LexicalNodes to specify which type of DOM nodes they can convert and what the relative priority of their conversion should be. This is useful in cases where a DOM Node with specific attributes should be interpreted as one type of LexicalNode, and otherwise it should be represented as another type of LexicalNode.

export type DOMConversionMap = {
  [NodeName]: <T: HTMLElement>(node: T) => DOMConversion | null,
};

export type DOMConversion = {
  conversion: DOMConversionFn,
  priority: 0 | 1 | 2 | 3 | 4,
};

export type DOMConversionFn = (
  element: Node,
  parent?: Node,
  preformatted?: boolean,
) => DOMConversionOutput;

export type DOMConversionOutput = {
  after?: (childLexicalNodes: Array<LexicalNode>) => Array<LexicalNode>,
  forChild?: DOMChildConversion,
  node: LexicalNode | null,
};

export type DOMChildConversion = (
  lexicalNode: LexicalNode,
) => LexicalNode | null | void;

@lexical/code provides a good example of the usefulness of this design. GitHub uses HTML <table> elements to represent the structure of copied code in HTML. If we interpreted all HTML <table> elements as literal tables, then code pasted from GitHub would appear in Lexical as a Lexical TableNode. Instead, CodeNode specifies that it can handle <table> elements too:

class CodeNode extends ElementNode {
...
static importDOM(): DOMConversionMap | null {
  return {
    ...
    table: (node: Node) => {
      if (isGitHubCodeTable(node as HTMLTableElement)) {
        return {
          conversion: convertTableElement,
          priority: 3,
        };
      }
      return null;
    },
    ...
  };
}
...
}

If the imported <table> doesn't align with the expected GitHub code HTML, then we return null and allow the node to be handled by lower priority conversions.

Much like exportDOM, importDOM exposes APIs to allow for post-processing of converted Nodes. The conversion function returns a DOMConversionOutput which can specify a function to run for each converted child (forChild) or on all the child nodes after the conversion is complete (after). The key difference here is that forChild runs for every deeply nested child node of the current node, whereas after will run only once after the transformation of the node and all its children is complete. Finally, preformatted flag indicates that nested text content is preformatted (similar to <pre> tag) and all newlines and spaces should be preserved as is.

export type DOMConversionFn = (
  element: Node,
  parent?: Node,
  preformatted?: boolean,
) => DOMConversionOutput;

export type DOMConversionOutput = {
  after?: (childLexicalNodes: Array<LexicalNode>) => Array<LexicalNode>,
  forChild?: DOMChildConversion,
  node: LexicalNode | null,
};

export type DOMChildConversion = (
  lexicalNode: LexicalNode,
  parentLexicalNode: LexicalNode | null | undefined,
) => LexicalNode | null;

JSON

Lexical -> JSON

To generate a JSON snapshot from an EditorState, you can call the toJSON() method on the EditorState object.

const editorState = editor.getEditorState();
const json = editorState.toJSON();

Alternatively, if you are trying to generate a stringified version of the EditorState, you can simply using JSON.stringify directly:

const editorState = editor.getEditorState();
const jsonString = JSON.stringify(editorState);

LexicalNode.exportJSON()

You can control how a LexicalNode is represented as JSON by adding an exportJSON() method. It's important to ensure your serialized JSON node has a type field and a children field if it's an ElementNode.

export type SerializedLexicalNode = {
  type: string;
  version: number;
};

exportJSON(): SerializedLexicalNode

When transforming an editor state into JSON, we simply traverse the current editor state and call the exportJSON method for each Node in order to convert it to a SerializedLexicalNode object that represents the JSON object for the given node. The built-in nodes from Lexical already have a JSON representation defined, but you'll need to define ones for your own custom nodes.

Here's an example of exportJSON for the HeadingNode:

export type SerializedHeadingNode = Spread<
  {
    tag: 'h1' | 'h2' | 'h3' | 'h4' | 'h5' | 'h6';
  },
  SerializedElementNode
>;

exportJSON(): SerializedHeadingNode {
  return {
    ...super.exportJSON(),
    tag: this.getTag(),
    type: 'heading',
    version: 1,
  };
}

LexicalNode.importJSON()

You can control how a LexicalNode is serialized back into a node from JSON by adding an importJSON() method.

export type SerializedLexicalNode = {
  type: string;
  version: number;
};

importJSON(jsonNode: SerializedLexicalNode): LexicalNode

This method works in the opposite way to how exportJSON works. Lexical uses the type field on the JSON object to determine what Lexical node class it needs to map to, so keeping the type field consistent with the getType() of the LexicalNode is essential.

Here's an example of importJSON for the HeadingNode:

static importJSON(serializedNode: SerializedHeadingNode): HeadingNode {
  const node = $createHeadingNode(serializedNode.tag);
  node.setFormat(serializedNode.format);
  node.setIndent(serializedNode.indent);
  node.setDirection(serializedNode.direction);
  return node;
}

Versioning & Breaking Changes

It's important to note that you should avoid making breaking changes to existing fields in your JSON object, especially if backwards compatibility is an important part of your editor. That's why we recommend using a version field to separate the different changes in your node as you add or change functionality of custom nodes. Here's the serialized type definition for Lexical's base TextNode class:

import type {Spread} from 'lexical';

// Spread is a Typescript utility that allows us to spread the properties
// over the base SerializedLexicalNode type.
export type SerializedTextNode = Spread<
  {
    detail: number;
    format: number;
    mode: TextModeType;
    style: string;
    text: string;
  },
  SerializedLexicalNode
>;

If we wanted to make changes to the above TextNode, we should be sure to not remove or change an existing property, as this can cause data corruption. Instead, opt to add the functionality as a new property field instead, and use the version to determine how to handle the differences in your node.

export type SerializedTextNodeV1 = Spread<
  {
    detail: number;
    format: number;
    mode: TextModeType;
    style: string;
    text: string;
  },
  SerializedLexicalNode
>;

export type SerializedTextNodeV2 = Spread<
  {
    detail: number;
    format: number;
    mode: TextModeType;
    style: string;
    text: string;
    // Our new field we've added
    newField: string,
    // Notice the version is now 2
    version: 2,
  },
  SerializedLexicalNode
>;

export type SerializedTextNode = SerializedTextNodeV1 | SerializedTextNodeV2;

Handling extended HTML styling

Since the TextNode is foundational to all Lexical packages, including the plain text use case. Handling any rich text logic is undesirable. This creates the need to override the TextNode to handle serialization and deserialization of HTML/CSS styling properties to achieve full fidelity between JSON <-> HTML. Since this is a very popular use case, below we are proving a recipe to handle the most common use cases.

You need to override the base TextNode:

const initialConfig: InitialConfigType = {
    namespace: 'editor',
    theme: editorThemeClasses,
    onError: (error: any) => console.log(error),
    nodes: [
      ExtendedTextNode,
      { replace: TextNode, with: (node: TextNode) => new ExtendedTextNode(node.__text, node.__key) },
      ListNode,
      ListItemNode,   
    ]
  };

and create a new Extended Text Node plugin

import {
  $isTextNode,
  DOMConversion,
  DOMConversionMap,
  DOMConversionOutput,
  NodeKey,
  TextNode,
  SerializedTextNode
} from 'lexical';

export class ExtendedTextNode extends TextNode {
  constructor(text: string, key?: NodeKey) {
    super(text, key);
  }

  static getType(): string {
    return 'extended-text';
  }

  static clone(node: ExtendedTextNode): ExtendedTextNode {
    return new ExtendedTextNode(node.__text, node.__key);
  }

  static importDOM(): DOMConversionMap | null {
    const importers = TextNode.importDOM();
    return {
      ...importers,
      code: () => ({
        conversion: patchStyleConversion(importers?.code),
        priority: 1
      }),
      em: () => ({
        conversion: patchStyleConversion(importers?.em),
        priority: 1
      }),
      span: () => ({
        conversion: patchStyleConversion(importers?.span),
        priority: 1
      }),
      strong: () => ({
        conversion: patchStyleConversion(importers?.strong),
        priority: 1
      }),
      sub: () => ({
        conversion: patchStyleConversion(importers?.sub),
        priority: 1
      }),
      sup: () => ({
        conversion: patchStyleConversion(importers?.sup),
        priority: 1
      }),
    };
  }

  static importJSON(serializedNode: SerializedTextNode): TextNode {
    return TextNode.importJSON(serializedNode);
  }
}

function patchStyleConversion(
  originalDOMConverter?: (node: HTMLElement) => DOMConversion | null
): (node: HTMLElement) => DOMConversionOutput | null {
  return (node) => {
    const original = originalDOMConverter?.(node);
    if (!original) {    
      return null;
    }
    const originalOutput = original.conversion(node);

    if (!originalOutput) {
      return originalOutput;
    }

    const backgroundColor = node.style.backgroundColor;
    const color = node.style.color;
    const fontFamily = node.style.fontFamily;
    const fontWeight = node.style.fontWeight;
    const fontSize = node.style.fontSize;
    const textDecoration = node.style.textDecoration;

    return {
      ...originalOutput,
      forChild: (lexicalNode, parent) => {
        const originalForChild = originalOutput?.forChild ?? ((x) => x);
        const result = originalForChild(lexicalNode, parent);
        if ($isTextNode(result)) {
          const style = [
            backgroundColor ? `background-color: ${backgroundColor}` : null,
            color ? `color: ${color}` : null,
            fontFamily ? `font-family: ${fontFamily}` : null,
            fontWeight ? `font-weight: ${fontWeight}` : null,
            fontSize ? `font-size: ${fontSize}` : null,
            textDecoration ? `text-decoration: ${textDecoration}` : null,
          ]
            .filter((value) => value != null)
            .join('; ');
          if (style.length) {
            return result.setStyle(style);
          }
        }
        return result;
      }
    };
  };
}