gatsby-node.js

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

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

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

    createNodeField({
      name: `slug`,
      node,
      value: `/posts${value}`,
    })
  }
}

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

  const result = await graphql(`
    query {
      allMdx {
        edges {
          node {
            id
            fields {
              slug
            }
            frontmatter {
              slug
            }
          }
        }
      }
    }
  `)

  if (result.errors) {
    reporter.panicOnBuild('🚨  ERROR: Loading "createPages" query')
  }

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

  posts.forEach(({ node }, index) => {
    createPage({
      path: node.frontmatter.slug ? node.frontmatter.slug : node.fields.slug,
      component: path.resolve(`./src/components/post-page-template.tsx`),
      context: { id: node.id },
    })
  })
}