gatsby-node.js

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

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

	const postTemplate = path.resolve(`src/templates/postTemplate.js`);
	const tagTemplate = path.resolve(`src/templates/tagsTemplate.js`);

	return graphql(`
    {
      allMarkdownRemark(
        sort: { order: DESC, fields: [frontmatter___date] }
        limit: 1000
      ) {
        edges {
          node {
            fields {
              slug
            }
            frontmatter {
              tags
            }
          }
        }
      }
    }
  `).then(result => {
		if (result.errors) {
			return Promise.reject(result.errors);
		}

		const posts = result.data.allMarkdownRemark.edges;
		posts.forEach(({ node }) => {
			createPage({
				path: node.fields.slug,
				component: postTemplate,
				context: { slug: node.fields.slug }, // additional data can be passed via context
			});
		});

		// create Tags pages
		// pulled directly from https://www.gatsbyjs.org/docs/adding-tags-and-categories-to-blog-posts/#add-tags-to-your-markdown-files
		let tags = [];
		// Iterate through each post, putting all found tags into `tags`
		_.each(posts, edge => {
			if (_.get(edge, 'node.frontmatter.tags')) {
				tags = tags.concat(edge.node.frontmatter.tags);
			}
		});
		// Eliminate duplicate tags
		tags = _.uniq(tags);
		// Make tag pages
		tags.forEach(tag => {
			createPage({
				path: `/tags/${_.kebabCase(tag)}/`,
				component: tagTemplate,
				context: {
					tag,
				},
			});
		});

	});
};

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