on-node-create.js

const grayMatter = require(`gray-matter`)
const _ = require(`lodash`)

function shouldOnCreateNode({ node }) {
  return (
    node.internal.mediaType === `text/markdown` ||
    node.internal.mediaType === `text/x-markdown`
  )
}

module.exports.onCreateNode = async function onCreateNode(
  {
    node,
    loadNodeContent,
    actions,
    createNodeId,
    reporter,
    createContentDigest,
  },
  pluginOptions
) {
  const { createNode, createParentChildLink } = actions

  const content = await loadNodeContent(node)

  try {
    const data = grayMatter(content, pluginOptions)

    if (data.data) {
      data.data = _.mapValues(data.data, value => {
        if (_.isDate(value)) {
          return value.toJSON()
        }
        return value
      })
    }

    const markdownNode = {
      id: createNodeId(`${node.id} >>> MarkdownRemark`),
      children: [],
      parent: node.id,
      internal: {
        content: data.content,
        type: `MarkdownRemark`,
      },
    }

    markdownNode.frontmatter = {
      title: ``, // always include a title
      ...data.data,
    }

    markdownNode.excerpt = data.excerpt
    markdownNode.rawMarkdownBody = data.content

    // Add path to the markdown file path
    if (node.internal.type === `File`) {
      markdownNode.fileAbsolutePath = node.absolutePath
    }

    markdownNode.internal.contentDigest = createContentDigest(markdownNode)

    createNode(markdownNode)
    createParentChildLink({ parent: node, child: markdownNode })

    return markdownNode
  } catch (err) {
    reporter.panicOnBuild(
      `Error processing Markdown ${
        node.absolutePath ? `file ${node.absolutePath}` : `in node ${node.id}`
      }:\n
      ${err.message}`
    )

    return {} // eslint
  }
}

module.exports.shouldOnCreateNode = shouldOnCreateNode