gatsby-node.js

const { createFilePath } = require(`gatsby-source-filesystem`);
const path = require(`path`);
/**
 * Implement Gatsby's Node APIs in this file.
 *
 * See: https://www.gatsbyjs.org/docs/node-apis/
 */

/**
 * Creates a slug in graphql
 */
exports.onCreateNode = ({ node, getNode, actions }) => {
  const { createNodeField } = actions;
  if (node.internal.type === `MarkdownRemark`) {
    const slug = createFilePath({ node, getNode, basePath: `data/blogs` });
    createNodeField({
      node,
      name: `slug`,
      value: slug,
    });
  }
};

/**
 * Creates a page from slug above
 */
exports.createPages = async ({ graphql, actions }) => {
  const { createPage } = actions;
  const { data } = await graphql(`
    query {
      allMarkdownRemark {
        edges {
          node {
            fields {
              slug
            }
          }
        }
      }
    }
  `);

  data.allMarkdownRemark.edges.forEach(({ node }) => {
    createPage({
      path: node.fields.slug,
      component: path.resolve(`./src/templates/BlogPage/index.jsx`),
      context: {
        // Data passed to context is available
        // in page queries as GraphQL variables.
        slug: node.fields.slug,
      },
    });
  });
};