gatsby-node.js

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

exports.onCreateNode = ({ node, getNode, actions }) => {
   if (node.internal.type === 'MarkdownRemark') {
      const { createNodeField } = actions
      const slug = createFilePath({ node, getNode, basePath: 'markdown' })
      createNodeField({
         node,
         name: 'slug',
         value: slug,
      })
   }
}

// Create post pages programmatically
exports.createPages = ({ graphql, actions }) => {
   const { createPage } = actions

   return new Promise(resolve => {
      graphql(`
      {
         allMarkdownRemark {
            edges {
               node {
                  fields {
                     slug
                  }
               }
            }
         }
      }`
      ).then(result => {
         result.data.allMarkdownRemark.edges.forEach(({ node }) => {
            createPage({
               path: node.fields.slug,
               component: path.resolve('./src/templates/post.js'),
               context: {
                 slug: node.fields.slug,
               },
             })
         })
         resolve()
      })
   })
}