gatsby-node.js

const path = require('path');

exports.createPages = ({ boundActionCreators, graphql }) => {
  const { createPage } = boundActionCreators;
  return new Promise((resolve, reject) => {
    const postTemplate = path.resolve('src/layouts/post.js');
    resolve(
      graphql(`
        {
          allMarkdownRemark {
            edges {
              node {
                html
                id
                frontmatter {
                  path
                  title
                }
              }
            }
          }
        }
      `).then(res => {
        if (res.errors) {
          console.error(res.errors);
          return reject(res.errors);
        }

        res.data.allMarkdownRemark.edges.forEach(({ node }) => {
          createPage({
            path: node.frontmatter.path,
            component: postTemplate,
          });
        });
      })
    );
  });
};