gatsby-node.js

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

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

  return graphql(`
    {
      allMarkdownRemark(
        sort: { order: ASC, fields: [frontmatter___date] }
        filter: { frontmatter: { templateKey: { eq: "blog-post" } } }
      ) {
        edges {
          previous {
            id
          }
          node {
            id
            fields {
              slug
            }
            frontmatter {
              tags
              templateKey
            }
          }
          next {
            id
          }
        }
      }
    }
  `).then(result => {
    if (result.errors) {
      result.errors.forEach(e => console.error(e.toString()));
      return Promise.reject(result.errors);
    }

    const posts = result.data.allMarkdownRemark.edges;

    posts.forEach(edge => {
      const { node, previous, next } = edge;
      const id = node.id;

      createPage({
        path: node.fields.slug,
        tags: node.frontmatter.tags,
        component: path.resolve(
          `src/templates/${String(node.frontmatter.templateKey)}.tsx`
        ),
        // additional data can be passed via context
        context: {
          id,
          previousId: previous != null ? previous.id : null,
          hasPrevious: previous != null,
          nextId: next != null ? next.id : null,
          hasNext: next != null
        }
      });
    });

    // Tag pages:
    let tags = [];
    // Iterate through each post, putting all found tags into `tags`
    posts.forEach(edge => {
      if (edge.node.frontmatter.tags) {
        tags = tags.concat(edge.node.frontmatter.tags);
      }
    });
    // Eliminate duplicate tags
    tags = [...new Set(tags)];

    // Make tag pages
    tags.forEach(tag => {
      const tagPath = `/tags/${tag}/`;

      createPage({
        path: tagPath,
        component: path.resolve(`src/templates/tags.tsx`),
        context: {
          tag
        }
      });
    });
  });
};

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

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