gatsby-node.js

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

const createTagPages = (createPage, posts) => {
    const allTagsIndexTemplate = path.resolve('src/templates/allTagsIndex.jsx')
    const singleTagIndexTemplate = path.resolve('src/templates/singleTagIndex.jsx')

    const postsByTag = {}

    posts.forEach(({node}) => {
        if (node.frontmatter.tags) {
            node.frontmatter.tags.forEach(tag => {
                if (!postsByTag[tag]){
                    postsByTag[tag] = []
                }

                postsByTag[tag].push(node)
            })
        }
    })

    const tags = Object.keys(postsByTag)

    createPage({
        path: '/tags',
        component: allTagsIndexTemplate,
        context: {
            tags: tags.sort()
        }
    })

    tags.forEach(tagName => {
        const posts = postsByTag[tagName]

        createPage({
            path: `/tags/${tagName}`,
            component: singleTagIndexTemplate,
            context: {
                posts,
                tagName
            }
        })
    })
}

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

    return new Promise((resolve, reject) => {
        const blogPostTemplate = path.resolve('src/templates/blogPost.jsx')
        
        resolve(
            graphql (
                `
                    query {
                        allMarkdownRemark (
                            sort: { order: ASC, fields: [frontmatter___sortDate]}
                        ) {
                            edges {
                                node {
                                    frontmatter {
                                        path
                                        title
                                        tags
                                        sortDate
                                    }
                                }
                            }
                        }
                    }
                `
            ).then(result => {
                const posts = result.data.allMarkdownRemark.edges

                createTagPages(createPage, posts)

                posts.forEach (({node}, index) => {
                    const path = node.frontmatter.path
                    createPage({
                        path,
                        component: blogPostTemplate,
                        context: {
                            pathSlug: path,
                            prev: index === 0 ? null : posts[index - 1].node,
                            next: index === (posts.length - 1) ? null : posts[index + 1].node
                        }
                    })

                    resolve()
                })
            })
        )
    
    })
})