gatsby-node.js

const path = require('path');

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

    if (node.internal.type == 'MarkdownRemark') {
     const slug = path.basename(node.fileAbsolutePath,'.md');

     createNodeField({
         node,
         name: 'slug',
         value: slug
     })

    }
}

module.exports.createPages = async ({graphql, actions}) => {
    const { createPage } = actions;
     const blogTemplate = path.resolve('./src/templates/blog.js');

     const res = await graphql(`
     query{
        allMarkdownRemark{
          edges{
            node{
              fields{
                slug
              }
            }
          }
        }
      }
     `)

     res.data.allMarkdownRemark.edges.forEach((edge) => {
        createPage({
            component: blogTemplate,
            path: `/blog/${edge.node.fields.slug}`,
            context: {
               slug:  edge.node.fields.slug
            }
        })

     })

}