rehype-optimize-static.ts

import type { Root } from 'hast';
import { toHtml } from 'hast-util-to-html';
import type { Transformer } from 'unified';
import { walk } from 'unist-util-walker';

// accessing untyped hast and mdx types
// eslint-disable-next-line @typescript-eslint/no-explicit-any
type Node = any;

const headingRe = /h([0-6])/;

/**
 * For MDX only, collapse static subtrees of the hast into `set:html`. Subtrees
 * do not include any MDX elements or headings (for `rehypeHeadingIds` to work).
 * This optimization reduces the JS output as more content are represented as a
 * string instead, which also reduces the AST size that Rollup holds in memory.
 */
export function rehypeOptimizeStatic(): Transformer<Root, Root> {
	return (tree) => {
		// All possible elements that could be the root of a subtree
		const allPossibleElements = new Set<Node>();
		// The current collapsible element stack while traversing the tree
		const elementStack: Node[] = [];

		walk(tree, {
			enter(node) {
				// @ts-expect-error test tagName naively
				const isHeading = node.tagName && headingRe.test(node.tagName);
				// For nodes that can't be optimized, eliminate all elements in the
				// `elementStack` from the `allPossibleElements` set.
				if (node.type.startsWith('mdx') || isHeading) {
					for (const el of elementStack) {
						allPossibleElements.delete(el);
					}
				}
				// If is heading node, skip it and its children. This prevents the content
				// from being optimized, as the content is used to generate the heading text.
				if (isHeading) {
					this.skip();
					return;
				}
				// For possible subtree root nodes, record them
				if (node.type === 'element' || node.type === 'mdxJsxFlowElement') {
					elementStack.push(node);
					allPossibleElements.add(node);
				}
			},
			leave(node, parent) {
				// Similar as above, but pop the `elementStack`
				if (node.type === 'element' || node.type === 'mdxJsxFlowElement') {
					elementStack.pop();
					// Many possible elements could be part of a subtree, in order to find
					// the root, we check the parent of the element we're popping. If the
					// parent exists in `allPossibleElements`, then we're definitely not
					// the root, so remove ourselves. This will work retroactively as we
					// climb back up the tree.
					if (allPossibleElements.has(parent)) {
						allPossibleElements.delete(node);
					}
				}
			},
		});

		// For all possible subtree roots, collapse them into `set:html` and
		// strip of their children
		for (const el of allPossibleElements) {
			if (el.type === 'mdxJsxFlowElement') {
				el.attributes.push({
					type: 'mdxJsxAttribute',
					name: 'set:html',
					value: toHtml(el.children),
				});
			} else {
				el.properties['set:html'] = toHtml(el.children);
			}
			el.children = [];
		}
	};
}