gatsby-node.js

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

exports.onCreateNode = ({ node, getNode, actions }) => {
  const { createNodeField } = actions;
  if (node.internal.type === `MarkdownRemark`) {
    const slug = createFilePath({ node, getNode, basePath: `posts` });
    createNodeField({
      node,
      name: `slug`,
      value: slug,
    });
  }
};

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

  const blogPostTemplate = path.resolve('src/templates/blogTemplate.js');
  const tagTemplate = path.resolve('src/templates/tagTemplate.js');

  const result = await graphql(`
    {
      postsRemark: allMarkdownRemark(
        sort: { order: DESC, fields: [frontmatter___date] }
        limit: 2000
      ) {
        edges {
          node {
            fields {
              slug
            }
            frontmatter {
              path
              tags
            }
          }
        }
      }
      tagsGroup: allMarkdownRemark(limit: 2000) {
        group(field: frontmatter___tags) {
          fieldValue
        }
      }
    }
  `);

  if (result.errors) {
    reporter.panicOnBuild(`Error while running GraphQL query.`);
    return;
  }

  const posts = result.data.postsRemark.edges;

  // Create posts pages
  posts.forEach(({ node }) => {
    const {
      fields: { slug },
      frontmatter: { path },
    } = node;
    actions.createPage({
      path: path,
      component: blogPostTemplate,
      context: { slug },
    });
  });

  // Extract tag data from query
  const tags = result.data.tagsGroup.group;

  // Create tag pages
  tags.forEach((tag) => {
    createPage({
      path: `/tags/${_.kebabCase(tag.fieldValue)}/`,
      component: tagTemplate,
      context: {
        tag: tag.fieldValue,
      },
    });
  });
};