Next and prev post

4-gatsby-node.js

posts.forEach((post, index, arr) => {
  post.node.frontmatter.category.forEach(cat => categories.push(cat))

  const prev = arr[index - 1]
  const next = arr[index + 1]

  createPage({
    path: post.node.fields.slug,
    component: blogLayout,
    context: {
      slug: post.node.fields.slug,
      prev: prev,
      next: next,
    },
  })
})