gatsby-node.js

const path = require('path');

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,
    });
  }
};

exports.createPages = async ({ graphql, reporter, actions }) => {
  const { createPage } = actions;
  const BlogTemplate = path.resolve('./src/templates/post.js');

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

  if (result.errors) {
    reporter.panic('failed to create posts.', result.errors);
  }

  const posts = result.data.allMarkdownRemark.edges;

  posts.forEach((post) => {
    createPage({
      component: BlogTemplate,
      path: `/blog/${post.node.fields.slug}`,
      context: {
        slug: post.node.fields.slug,
      },
    });
  });
};