gatsby-node.js

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

exports.createPages = async ({ graphql, actions }) => {
  const { createPage } = actions
  const extractedPost = path.resolve(`./src/templates/blog-post.js`)
  const result = await graphql(`{
    allMarkdownRemark(sort: { fields: [frontmatter___date], order: DESC } limit: 1000 )
    {
      edges {
        node {
          fields {
            slug 
          }
          frontmatter {
            title
          }
        }
      }
    }
  }`)
  if (result.errors) {
    throw result.errors
  }


const posts = result.data.allMarkdownRemark.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: post.node.fields.slug,
    component: extractedPost,
    context: {
      slug: post.node.fields.slug,
      previous,
      next,
    }
  })
})

}
// Slug Generation
exports.onCreateNode = ({ node, actions, getNode }) => {
  const { createNodeField } = actions;
  if (node.internal.type === `MarkdownRemark`) {
    const value = createFilePath({ node, getNode })
    createNodeField({
      name: `slug`,
      node,
      value
    })
  }
}