gatsby-node.js

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

exports.createPages = async ({ graphql, actions, reporter }) => {
  const { createPage } = actions;

  const blogPost = path.resolve(`./src/templates/blog-post.js`);

  const result = await graphql(
    `
      {
        allMarkdownRemark(
          sort: { fields: [frontmatter___date], order: ASC }
          limit: 1000
        ) {
          nodes {
            id
            fields {
              slug
            }
          }
        }
      }
    `
  );

  if (result.errors) {
    reporter.panicOnBuild(
      `There was an error loading your blog posts`,
      result.errors
    );
    return;
  }

  const posts = result.data.allMarkdownRemark.nodes;

  if (posts.length > 0) {
    posts.forEach((post, index) => {
      const previousPostId = index === 0 ? null : posts[index - 1].id;
      const nextPostId =
        index === posts.length - 1 ? null : posts[index + 1].id;

      createPage({
        path: `/reading${post.fields.slug}`,
        component: blogPost,
        context: {
          id: post.id,
          previousPostId,
          nextPostId,
        },
      });
    });
  }
};

exports.onCreateNode = ({ node, actions, getNode }) => {
  const { createNodeField } = actions;

  if (node.internal.type === `MarkdownRemark`) {
    const value = createFilePath({ node, getNode });

    createNodeField({
      name: `slug`,
      node,
      value,
    });
  }
};