gatsby-node.js

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

exports.onCreateNode = ({ node, getNode, actions }) => {
  const { createNodeField } = actions
  if (node.internal.type === "MarkdownRemark") {
    const slug = createFilePath({ node, getNode, basePath: "posts" })

    createNodeField({
      node,
      name: "slug",
      value: slug,
    })
  }
}
exports.createPages = ({ graphql, actions }) => {
    const {createPage} = actions
  return graphql(
    `
      {
        allMarkdownRemark {
          nodes {
            fields {
              slug
            }
          }
        }
      }
    `
  )
  .then( result => {
      result.data.allMarkdownRemark.nodes.forEach( node => {
          createPage({
              path: node.fields.slug,
              component: path.resolve('./src/layouts/blogpostLayout.js'),
              context: {
                  slug: node.fields.slug
              }
          })
      })
  })
}