gatsby-node.js

const path = require("path")

// Implement the Gatsby API “createPages”. This is called once the
// data layer is bootstrapped to let plugins create pages from data.
exports.createPages = async ({ graphql, actions, reporter }) => {
  const { createPage } = actions
  // Query for markdown nodes to use in creating pages.
  const result = await graphql(
    `
      {
        allMarkdownRemark(limit: 1000) {
          edges {
            node {
              frontmatter {
                slug
                title
                date
                description
              }
            }
          }
        }
      }
    `
  )
  // Handle errors
  if (result.errors) {
    reporter.panicOnBuild(`Error while running GraphQL query.`)
    return
  }
  // Create pages for each markdown file.
  const blogPostTemplate = path.resolve(`src/templates/BlogPost.js`)
  result.data.allMarkdownRemark.edges.forEach(({ node }) => {
    const path = `/blog${node.frontmatter.slug}`
    createPage({
      path,
      component: blogPostTemplate,
      // In your blog post template's graphql query, you can use pagePath
      // as a GraphQL variable to query for data from the markdown file.
      context: {
        pagePath: path,
      },
    })
  })
}