gatsby-node.js

const path = require('path');

const createTagPages = (createPage, edges) => {
  const tagTemplate = path.resolve(`src/templates/tags.js`);
  const posts = {};

  edges
    .forEach(({
      node
    }) => {
      if (node.frontmatter.tags) {
        node.frontmatter.tags
          .forEach(tag => {
            if (!posts[tag]) {
              posts[tag] = [];
            }
            posts[tag].push(node);
          });
      }
    });

  createPage({
    path: '/tags',
    component: tagTemplate,
    context: {
      posts
    }
  });

  Object.keys(posts)
    .forEach(tagName => {
      const post = posts[tagName];
      createPage({
        path: `/tags/${tagName}`,
        component: tagTemplate,
        context: {
          posts,
          post,
          tag: tagName,
        }
      })
    });
};

exports.createPages = ({
  boundActionCreators,
  graphql
}) => {
  const {
    createPage
  } = boundActionCreators;

  const blogPostTemplate = path.resolve(`src/templates/blog-post.js`);
  return graphql(`{
    allMarkdownRemark(
      sort: { order: DESC, fields: [frontmatter___date] }
      limit: 1000
    ) {
      edges {
        node {
          excerpt(pruneLength: 250)
          html
          id
          timeToRead
          frontmatter {
            date(formatString: "MMMM DD, YYYY")
            path
            tags
            title
          }
        }
      }
    }
  }`)
    .then(result => {
      if (result.errors) {
        return Promise.reject(result.errors)
      }

      const posts = result.data.allMarkdownRemark.edges;

      createTagPages(createPage, posts);

      // Create pages for each markdown file.
      posts.forEach(({
        node
      }, index) => {
        const prev = index === 0 ? false : posts[index - 1].node;
        const next = index === posts.length - 1 ? false : posts[index + 1].node;
        createPage({
          path: node.frontmatter.path,
          component: blogPostTemplate,
          context: {
            prev,
            next
          }
        });
      });

      return posts;
    })
};