slate-to-markdown.js

export default function parseToMarkdown(chunk) {
  let children = !chunk.type
    ? chunk.text
    : chunk.children.map(c => parseToMarkdown({ ...c, parentType: chunk.type })).join('');

  if (children === '') return;

  // formatting
  if (chunk.bold) {
    children = `**${children.trim()}** `;
  }
  if (chunk.italic) {
    children = `_${children.trim()}_ `;
  }
  if (chunk.strikeThrough) {
    children = `~~${children.trim()}~~ `;
  }

  // node type
  switch (chunk.type) {
    case 'heading-one':
      return `# ${children} \n`;
    case 'heading-two':
      return `## ${children} \n`;
    case 'link':
      return `[${children}](${chunk.url})`;
    case 'numbered-list':
    case 'bulleted-list':
      return `\n${children}\n`;
    case 'list-item':
      return `${chunk.parentType === 'numbered-list' ? '1.' : '-'} ${children} \n`;
    case 'paragraph':
      return `${children}\n`;
    default:
      return children;
  }
}

This was a lifesaver. Thank you for sharing it.

Also send my regards to Stephen!