gatsby-node.js

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

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

exports.createPages = async ({graphql, actions }) => {
  const { createPage } = actions
  return graphql(`
  {
  allMarkdownRemark {
    edges {
      node {
        fields {
          slug
        }
      }
    }
}

  }
  `).then(result => {
    result.data.allMarkdownRemark.edges.forEach(({node}) => {
      createPage({
        path: node.fields.slug,
        component: path.resolve("./src/templates/blog-post-default-template.js"),
        context: {
          slug: node.fields.slug},
        defer: true,
      })

    })
  })
}