gatsby-node.js

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

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

    return new Promise((resolve, reject) => {
        resolve(
            graphql(`
        {
          allMarkdownRemark(
            sort: { order: DESC, fields: [frontmatter___date] }
            limit: 1000
          ) {
            edges {
              node {
                fields {
                  slug

                }
                frontmatter {
                  title
                  tags
                }
              }
            }
          }
        }
      `).then(result => {
                    if (result.errors) {
                        console.log(result.errors)
                        return reject(result.errors)
                    }

                    const blogTemplate = path.resolve('./src/templates/blog-post.js');
                    const tagsTemplate = path.resolve('./src/templates/tag-template.js');
                    const posts = result.data.allMarkdownRemark.edges

                    //All tags
                    let allTags = []
                    // Iterate through each post, putting all found tags into `allTags array`
                    _.each(posts, edge => {
                        if (_.get(edge, 'node.frontmatter.tags')) {
                            allTags = allTags.concat(edge.node.frontmatter.tags)
                        }
                    })
                    // Eliminate duplicate tags
                    allTags = _.uniq(allTags)

                    allTags.forEach((tag, index) => {
                        createPage({
                            path: `/${_.kebabCase(tag)}/`,
                            component: tagsTemplate,
                            context: {
                                tag,
                            }
                        })
                    })


                    posts.forEach(({ node }, index) => {
                        createPage({
                            path: node.fields.slug,
                            component: blogTemplate,
                            context: {
                                slug: node.fields.slug,
                                prev: index === 0 ? null : posts[index - 1],
                                next: index === result.length - 1 ? null : posts[index + 1],
                            }, // additional data can be passed via context
                        })
                    })
                    return
                })
        )
    })
}

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