gatsby-node.js

const path = require(`path`)
const { createFilePath } = require(`gatsby-source-filesystem`)

exports.createPages = ({ graphql, actions }) => {
  const { createPage } = actions

  const blogPost = path.resolve(`./src/templates/episode.tsx`)
  return graphql(
    `
      {
        allMdx(
          sort: { fields: [frontmatter___number], order: DESC }
          limit: 1000
        ) {
          edges {
            node {
              id
              fields {
                slug
              }
              frontmatter {
                title
              }
              body
            }
          }
        }
      }
    `
  ).then(result => {
    if (result.errors) {
      throw result.errors
    }

    // Create blog posts pages.
    const posts = result.data.allMdx.edges

    posts.forEach((post, index) => {
      const previous = index === posts.length - 1 ? null : posts[index + 1].node
      const next = index === 0 ? null : posts[index - 1].node

      createPage({
        path: `/episodes${post.node.fields.slug}`,
        component: blogPost,
        context: {
          slug: post.node.fields.slug,
          previous,
          next,
        },
      })
    })
  })
}

function getCurrentDate() {
  const d = new Date()
  let month = (d.getMonth() + 1).toString()
  if (month.length < 2) {
    month = `0${month}`
  }
  let day = d.getDate().toString()
  if (day.length < 2) {
    day = `0${day}`
  }
  return `${d.getFullYear()}-${month}-${day}`
}

exports.onCreateNode = ({ node, actions, getNode }) => {
  const { createNodeField } = actions

  if (node.internal.type === `Mdx`) {
    const value = createFilePath({ node, getNode })
    createNodeField({
      name: `slug`,
      node,
      value,
    })
  }
}

exports.onCreatePage = ({ page, actions }) => {
  const { createPage, deletePage } = actions
  deletePage(page)
  createPage({
    ...page,
    context: {
      ...page.context,
      currentDate: getCurrentDate(),
    },
  })
}