gatsby-node.js

const path = require ('path');

const createTagPages = (createPage, posts) => {
  const tagPageTemplate = path.resolve (`src/templates/tags.js`);
  const allTagsTemplate = path.resolve (`src/templates/all-tags.js`);

  const postsByTags = {};

  posts.forEach (({node}) => {
    if (node.frontmatter.tags) {
      node.frontmatter.tags.forEach (tag => {
        if (!postsByTags[tag]) {
          postsByTags[tag] = [];
        }
        postsByTags[tag].push (node);
      });
    }
  });
  const tags = Object.keys (postsByTags);

  createPage ({
    path: `/tags`,
    component: allTagsTemplate,
    context: {
      tags: tags.sort (),
    },
  });
  tags.forEach (tagName => {
    const posts = postsByTags[tagName];

    createPage ({
      path: `/tags/${tagName}`,
      component: tagPageTemplate,
      context: {
        posts,
        tagName,
      },
    });
  });
};

exports.createPages = ({boundActionCreators, graphql}) => {
  const {createPage} = boundActionCreators;
  const blogPostTemplate = path.resolve (`src/templates/blog-post.js`);

  return graphql (
    `{
    allMarkdownRemark {
      edges {
        node {
          html
          id
          frontmatter {
            date
            path
            title
            excerpt
            tags
          }
        }
      }
    }
  }`
  ).then (result => {
    if (result.errors) {
      return Promise.reject (result.errors);
    }

    const posts = result.data.allMarkdownRemark.edges;

    createTagPages (createPage, posts);

    posts.forEach (({node}, index) => {
      createPage ({
        path: node.frontmatter.path,
        component: blogPostTemplate,
        context: {
          prev: index === 0 ? null : posts[index - 1].node,
          next: index === posts.length - 1 ? null : posts[index + 1].node,
        },
      });
    });
  });
};