blog/gatsby-node.js

const { createMarkdownFields } = require('./gatsby/onCreateNode')
const { createExifFields } = require('./gatsby/exif')
const {
  generatePostPages,
  generateTagPages,
  generateRedirectPages
} = require('./gatsby/createPages')
const { itemsPerPage } = require('./config')

exports.onCreateNode = ({ node, actions, getNode }) => {
  const { createNodeField } = actions

  // Markdown files
  if (node.internal.type === 'MarkdownRemark') {
    createMarkdownFields(node, createNodeField, getNode)
  }

  // Image files
  if (node.internal.mediaType === 'image/jpeg') {
    createExifFields(node, createNodeField)
  }
}

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

  return new Promise((resolve, reject) => {
    resolve(
      graphql(
        `
          {
            allMarkdownRemark(sort: { fields: [fields___date], order: DESC }) {
              edges {
                node {
                  fields {
                    slug
                    date
                  }
                  frontmatter {
                    type
                    tags
                  }
                }
              }
            }
          }
        `
      ).then(result => {
        if (result.errors) {
          /* eslint no-console: "off" */
          console.log(result.errors)
          reject(result.errors)
        }

        const posts = result.data.allMarkdownRemark.edges
        const numPages = Math.ceil(posts.length / itemsPerPage)

        // Generate posts & posts index
        generatePostPages(createPage, posts, numPages)

        // Generate Tag Pages
        generateTagPages(createPage, posts, numPages)

        // create manual redirects
        generateRedirectPages(createRedirect)

        resolve()
      })
    )
  })
}