gatsby-node.js

const path = require('path')
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: 'pages' })
    createNodeField({
      node,
      name: 'slug',
      value: slug
    })
  }
}

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

  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) => {
    graphql(`
      {
        allMarkdownRemark(sort: { fields: [frontmatter___date], order: DESC }) {
          edges {
            node {
              id
              excerpt
              fields {
                slug
              }
              frontmatter {
                title
                date(formatString: "DD MMMM, YYYY")
                tags
              }
            }
          }
        }
      }
    `).then(result => {
      const posts = result.data.allMarkdownRemark.edges

      createTagPages(createPage, posts)

      posts.forEach(({ node }) => {
        createPage({
          path: node.fields.slug,
          component: path.resolve(`./src/templates/blog-post.js`),
          context: {
            // Data passed to context is available
            // in page queries as GraphQL variables.
            slug: node.fields.slug
          }
        })
      })
      resolve()
    })
  })
}