gatsby-node.js

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

exports.onCreateNode = ({ node, getNode, actions }) => {
  const { createNodeField } = actions;
  if (node.internal.type === `Mdx`) {
    const slug = `/${lodash.kebabCase(node.frontmatter.title)}`;
    createNodeField({
      node,
      name: `slug`,
      value: slug,
    });
  }
};

exports.createPages = async ({ graphql, actions }) => {
  const { createPage } = actions;
  const result = await graphql(`
    query {
      allMdx {
        edges {
          node {
            id
            fields {
              slug
            }
          }
        }
      }
    }
  `);
  result.data.allMdx.edges.forEach(({ node }) => {
    createPage({
      path: node.fields.slug,
      component: path.resolve(`./src/templates/post.js`),
      context: { slug: node.fields.slug },
    });
  });
};