gatsby-node.js

const _ = require("lodash")
const Promise = require("bluebird")
const path = require("path")
const select = require(`unist-util-select`)
const fs = require(`fs-extra`)

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

  return new Promise((resolve, reject) => {
    const blogPostTemplate = path.resolve("./src/templates/blog-post.js")
    const tagTemplate = path.resolve("./src/templates/tags.js")
    resolve(
      graphql(
      `
      {
        allMarkdownRemark(limit: 1000) {
          edges {
            node {
              frontmatter {
                path
                tags
              }
            }
          }
        }
      }
    `
      ).then(result => {
        if (result.errors) {
          console.log(result.errors)
          reject(result.errors)
        }

        const posts = result.data.allMarkdownRemark.edges;

        // Create blog posts pages.
        posts.forEach(({ node }) => {
          createPage({
            path: node.frontmatter.path,
            component: blogPostTemplate,
          });
        });

        // Tag pages:
        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,
            },
          });
        });

      })
    )
  })
}