gatsby-node.js

const path = require('path');

exports.createPages = ({ graphql, actions }) => {
  const { createPage } = actions;
  return new Promise((resolve, reject) => {
    const blogPostTemplate = path.resolve('src/components/BlogPost/index.js');
    resolve(
      graphql(
        `
          query {
            allMarkdownRemark(
              sort: { order: DESC, fields: [frontmatter___date] }
            ) {
              edges {
                node {
                  frontmatter {
                    path
                    title
                  }
                }
              }
            }
          }
        `,
      ).then(result => {
        if (result.errors) {
          return Promise.reject(result.errors);
        }

        const posts = result.data.allMarkdownRemark.edges;

        posts.forEach((post, index) => {
          const path = post.node.frontmatter.path;
          createPage({
            path: `blog/${path}`,
            component: blogPostTemplate,
            context: {
              pathSlug: path,
              prev: index === 0 ? null : posts[index - 1].node,
              next: index === posts.length - 1 ? null : posts[index + 1].node,
            },
          });
          resolve();
        });
      }),
    );
  });
};