gatsby-node.js

const path = require(`path`)
const _kebabCase = require(`lodash/kebabCase`)
const { createFilePath } = require(`gatsby-source-filesystem`)

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

  let slug

  if (node.internal.type === `MarkdownRemark`) {
    // Build RSS feed
    const value = createFilePath({ node, getNode })
    createNodeField({
      name: `slug`,
      node,
      value,
    })

    const fileNode = getNode(node.parent)
    const parsedFilePath = path.parse(fileNode.relativePath)

    if (
      Object.prototype.hasOwnProperty.call(node, 'frontmatter') &&
      Object.prototype.hasOwnProperty.call(node.frontmatter, 'title')
    ) {
      slug = `/${_kebabCase(node.frontmatter.title)}`
    } else if (parsedFilePath.name !== 'index' && parsedFilePath.dir !== '') {
      slug = `/${parsedFilePath.dir}/${parsedFilePath.name}`
    } else if (parsedFilePath.dir === '') {
      slug = `/${parsedFilePath.name}`
    } else {
      slug = `/${parsedFilePath.dir}`
    }

    // If theres a slug in frontmatter, use that instead
    if (
      Object.prototype.hasOwnProperty.call(node, 'frontmatter') &&
      Object.prototype.hasOwnProperty.call(node.frontmatter, 'slug')
    ) {
      slug = `/${_kebabCase(node.frontmatter.slug)}`
    }

    createNodeField({
      node,
      name: 'slug',
      value: slug,
    })
  }
}

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

  return new Promise((resolve, reject) => {
    const postPage = path.resolve(`src/templates/post.js`)
    resolve(
      graphql(`
        {
          allMarkdownRemark {
            edges {
              node {
                fields {
                  slug
                }
              }
            }
          }
        }
      `).then(result => {
        if (result.errors) {
          /* eslint no-console: "off" */
          console.log(result.errors)
          reject(result.errors)
        }

        result.data.allMarkdownRemark.edges.forEach(edge => {
          createPage({
            path: edge.node.fields.slug,
            component: postPage,
            context: {
              slug: edge.node.fields.slug,
            },
          })
        })
      })
    )
  })
}