gatsby-node.js

const path = require("path")

exports.onCreateWebpackConfig = ({ actions, stage }) => {
  if (stage === "develop-html" || stage === "build-html") {
    actions.setWebpackConfig({
      resolve: {
        mainFields: ["main"],
      },
    })
  } else {
    actions.setWebpackConfig({
      resolve: {
        mainFields: ["browser", "module", "main"],
      },
    })
  }
}

exports.createPages = ({ graphql, actions }) => {
  const { createPage } = actions
  const bookTemplate = path.resolve("src/templates/bookTemplate.js")

  return graphql(`
    {
      allBook {
        edges {
          node {
            id
          }
        }
      }
    }
  `).then(result => {
    if (result.errors) {
      throw result.errors
    }

    result.data.allBook.edges.forEach(book => {
      createPage({
        path: `/book/${book.node.id}`,
        component: bookTemplate,
        context: { bookId: book.node.id },
      })
    })
  })
}