gatsby-node.js

const path = require(`path`)

// CREATE SLUGS
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/markdown` })
    createNodeField({ node, name: `slug`, value: slug })
  }
}

// CREATE PAGES
exports.createPages = async ({ page, graphql, actions }) => {
  const { createPage } = actions

  return graphql(`
    {
      allMarkdownRemark {
        edges {
          node {
            fields {
              slug
            }
          }
        }
      }
    }
  `).then(result => {
    result.data.allMarkdownRemark.edges.forEach(({ node }) => {
      createPage({
        path: node.fields.slug,
        component: path.resolve(`./src/templates/markdown-page.js`),
        context: {
          // Data passed to context is available
          // in page queries as GraphQL variables.
          slug: node.fields.slug,
        },
      })
    })
    // console.log(JSON.stringify(result, null, 4))
  })
}

exports.onCreateWebpackConfig = ({ stage, loaders, actions }) => {
  if (stage === 'build-html') {
    /***
     * During the build step, `auth0-js` will break because it relies on
     * browser-specific APIs. Fortunately, we don’t need it during the build.
     * Using Webpack’s null loader, we’re able to effectively ignore `auth0-js`
     * during the build.
     ***/
    actions.setWebpackConfig({
      module: {
        rules: [
          {
            test: /auth0-js/,
            use: loaders.null(),
          },
        ],
      },
    })
  }
}