gatsby-starter-deck/gatsby-node.js

// Implement the Gatsby API “onCreatePage”. This is
// called after every page is created.
exports.onCreatePage = ({ page, boundActionCreators }) => {
  const { createPage, deletePage } = boundActionCreators;

  return new Promise((resolve, reject) => {
    // Remove trailing slash
    const newPage = Object.assign({}, page, {
      path: page.path === `/` ? page.path : page.path.replace(/\/$/, ``),
    });

    if (newPage.path !== page.path) {
      // Remove the old page
      deletePage(page);
      // Add the new page
      createPage(newPage);
    }

    resolve();
  });
};