gatsby-node.js

const path = require(`path`)

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

  if (node.internal.type === `MarkdownRemark`) {
    createNodeField({
      node,
      name: `slug`,
      value: node.frontmatter.slug,
    })
  }
}

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

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