datocms-html-to-structured-text

datocms-html-to-structured-text

This package contains utilities to convert HTML (or a Hast to a DatoCMS Structured Text dast (DatoCMS Abstract Syntax Tree) document.

Please refer to the dast format docs to learn more about the syntax tree format and the available nodes.

Usage

The main utility in this package is htmlToStructuredText which takes a string of HTML and transforms it into a valid dast document.

htmlToStructuredText returns a Promise that resolves with a Structured Text document.

import { htmlToStructuredText } from 'datocms-html-to-structured-text';

const html = `
  <article>
    <h1>DatoCMS</h1>
    <p>The most complete, user-friendly and performant Headless CMS.</p>
  </article>
`;

htmlToStructuredText(html).then((structuredText) => {
  console.log(structuredText);
});

htmlToStructuredText is meant to be used in a browser environment.

In Node.js you can use the parse5ToStructuredText helper which instead takes a document generated with parse5.

import parse5 from 'parse5';
import { parse5ToStructuredText } from 'datocms-html-to-structured-text';

parse5ToStructuredText(
  parse5.parse(html, {
    sourceCodeLocationInfo: true,
  }),
).then((structuredText) => {
  console.log(structuredText);
});

Internally, both utilities work on a Hast. Should you have a hast already you can use a third utility called hastToDast.

Validate dast documents

dast is a strict format for DatoCMS' Structured Text fields. As such the resulting document is generally a simplified, content-centric version of the input HTML.

When possible, the library relies on semantic HTML to generate a valid dast document.

The datocms-structured-text-utils package provides a validate utility to validate a value to make sure that the resulting tree is compatible with DatoCMS' Structured Text field.

import { validate } from 'datocms-structured-text-utils';

// ...

htmlToStructuredText(html).then((structuredText) => {
  const { valid, message } = validate(structuredText);

  if (!valid) {
    throw new Error(message);
  }
});

We recommend to validate every dast to avoid errors later when creating records.

Advanced Usage

Options

All the *ToStructuredText utils accept an optional options object as second argument:

type Options = Partial<{
  newlines: boolean,
  // Override existing `hast` node handlers or add new ones
  handlers: Record<string, CreateNodeFunction>,
  // Allows to tweak the `hast` tree before transforming it to a `dast` document
  preprocess: (hast: HastRootNode) => HastRootNode,
  // Array of allowed block nodes
  allowedBlocks: Array<
    BlockquoteType | CodeType | HeadingType | LinkType | ListType,
  >,
  // Array of allowed marks
  allowedMarks: Mark[],
  // Array of allowed heading levels for 'heading' nodes
  allowedHeadingLevels: Array<1 | 2 | 3 | 4 | 5 | 6>,
}>;

Transforming Nodes

The utils in this library traverse a hast tree and transform supported nodes to dast nodes. The transformation is done by working on a hast node with a handler (async) function.

Handlers are associated to hast nodes by tagName or type when node.type !== 'element' and look as follow:

import { visitChildren } from 'datocms-html-to-structured-text';

// Handler for the <p> tag.
async function p(createDastNode, hastNode, context) {
  return createDastNode('paragraph', {
    children: await visitChildren(createDastNode, hastNode, context),
  });
}

Handlers can return either a promise that resolves to a dast node, an array of dast Nodes or undefined to skip the current node.

To ensure that a valid dast is generated the default handlers also check that the current hastNode is a valid dast node for its parent and, if not, they ignore the current node and continue visiting its children.

Information about the parent dast node name is available in context.parentNodeType.

Please take a look at the default handlers implementation for examples.

The default handlers are available on context.defaultHandlers.

context

Every handler receives a context object that includes the following information:

export interface GlobalContext {
  // Whether the library has found a <base> tag or should not look further.
  // See https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base
  baseUrlFound?: boolean;
  // <base> tag url. This is used for resolving relative URLs.
  baseUrl?: string;
}

export interface Context {
  // The current parent `dast` node type.
  parentNodeType: NodeType;
  // The parent `hast` node.
  parentNode: HastNode;
  // A reference to the current handlers - merged default + user handlers.
  handlers: Record<string, Handler<unknown>>;
  // A reference to the default handlers record (map).
  defaultHandlers: Record<string, Handler<unknown>>;
  // true if the content can include newlines, and false if not (such as in headings).
  wrapText: boolean;
  // Marks for span nodes.
  marks?: Mark[];
  // Prefix for language detection in code blocks.
  // Detection is done on a class name eg class="language-html"
  // Default is `language-`
  codePrefix?: string;
  // Array of allowed Block types.
  allowedBlocks: Array<
    BlockquoteType | CodeType | HeadingType | LinkType | ListType,
  >;
  // Array of allowed marks.
  allowedMarks: Mark[];
  // Properties in this object are available to every handler as Context
  // is not deeply cloned.
  global: GlobalContext;
}

Custom Handlers

It is possible to register custom handlers and override the default behavior via options:

import { paragraphHandler } from './customHandlers';

htmlToStructuredText(html, {
  handlers: {
    p: paragraphHandler,
  },
}).then((structuredText) => {
  console.log(structuredText);
});

It is highly encouraged to validate the dast when using custom handlers because handlers are responsible for dictating valid parent-children relationships and therefore generating a tree that is compliant with DatoCMS' Structured Text.

preprocessing

Because of the strictness of the dast spec it is possible that some semantic or elements might be lost during the transformation.

To improve the final result, you might want to modify the hast before it is transformed to dast with the preprocess hook.

import { findAll } from 'unist-utils-core';
const html = `
  <p>convert this to an h1</p>
`;

htmlToStructuredText(html, {
  preprocess: (tree) => {
    // Transform <p> to <h1>
    findAll(tree, (node) => {
      if (node.type === 'element' && node.tagName === 'p') {
        node.tagName = 'h1';
      }
    });
  },
}).then((structuredText) => {
  console.log(structuredText);
});

Examples

Split a node that contains an image.

In dast images can be presented as Block nodes but these are not allowed inside of ListItem nodes (ul/ol lists). In this example we will split the list in 3 pieces and lift up the image.

The same approach can be used to split other types of branches and lift up nodes to become root nodes.

import { visit } from 'unist-utils-core';

const html = `
  <ul>
    <li>item 1</li>
    <li><div><img src="./img.png" alt></div></li>
    <li>item 2</li>
  </ul>
`;

const dast = await htmlToStructuredText(html, {
  preprocess: (tree) => {
    const liftedImages = new WeakSet();
    const body = find(tree, (node) => node.tagName === 'body');

    visit(body, (node, index, parents) => {
      if (
        !node ||
        node.tagName !== 'img' ||
        liftedImages.has(node) ||
        parents.length === 1 // is a top level img
      ) {
        return;
      }
      // remove image

      const imgParent = parents[parents.length - 1];
      imgParent.children.splice(index, 1);

      let i = parents.length;
      let splitChildrenIndex = index;
      let childrenAfterSplitPoint = [];

      while (--i > 0) {
        // Example: i == 2
        // [ 'body', 'div', 'h1' ]
        const /* h1 */ parent = parents[i];
        const /* div */ parentsParent = parents[i - 1];

        // Delete the siblings after the image and save them in a variable
        childrenAfterSplitPoint /* [ 'h1.2' ] */ = parent.children.splice(
          splitChildrenIndex,
        );
        // parent.children is now == [ 'h1.1' ]

        // parentsParent.children = [ 'h1' ]
        splitChildrenIndex = parentsParent.children.indexOf(parent);
        // splitChildrenIndex = 0

        let nodeInserted = false;

        // If we reached the 'div' add the image's node
        if (i === 1) {
          splitChildrenIndex += 1;
          parentsParent.children.splice(splitChildrenIndex, 0, node);
          liftedImages.add(node);

          nodeInserted = true;
        }

        splitChildrenIndex += 1;
        // Create a new branch with childrenAfterSplitPoint if we have any i.e.
        // <h1>h1.2</h1>
        if (childrenAfterSplitPoint.length > 0) {
          parentsParent.children.splice(splitChildrenIndex, 0, {
            ...parent,
            children: childrenAfterSplitPoint,
          });
        }
        // Remove the parent if empty
        if (parent.children.length === 0) {
          splitChildrenIndex -= 1;
          parentsParent.children.splice(
            nodeInserted ? splitChildrenIndex - 1 : splitChildrenIndex,
            1,
          );
        }
      }
    });
  },
  handlers: {
    img: async (createNode, node, context) => {
      // In a real scenario you would upload the image to Dato and get back an id.
      const item = '123';
      return createNode('block', {
        item,
      });
    },
  },
});

Lift up an image node

import { visit, CONTINUE } from 'unist-utils-core';

const html = `
  <ul>
    <li>item 1</li>
    <li><div><img src="./img.png" alt>item 2</div></li>
    <li>item 3</li>
  </ul>
`;

const dast = await htmlToStructuredText(html, {
  preprocess: (tree) => {
    visit(tree, (node, index, parents) => {
      if (node.tagName === 'img' && parents.length > 1) {
        const parent = parents[parents.length - 1];
        tree.children.push(node);
        parent.children.splice(index, 1);
        return [CONTINUE, index];
      }
    });
  },
  handlers: {
    img: async (createNode, node, context) => {
      // In a real scenario you would upload the image to Dato and get back an id.
      const item = '123';
      return createNode('block', { item });
    },
  },
});

Utilities

To work with hast and dast trees we recommend using the unist-utils-core library.

License

MIT