gatsby-node.js

// uses require syntax not import
const path = require('path');

exports.createPages = ({graphql, actions}) => {
    const {createPage} = actions;  // same as actions.createPage
    return new Promise((resolve, reject) => {
        graphql (`
        {
            allMarkdownRemark { 
                edges {
                 node {
                  frontmatter{
                    title
                    slug
                  }
                 } 
                }
              }
            }
          `).then(results => {
            results.data.allMarkdownRemark.edges.forEach(({node}) => {
                createPage({
                    path: `/posts${node.frontmatter.slug}`,
                    component: path.resolve('./src/components/postLayout.js'),
                    context: {
                        slug: node.frontmatter.slug,
                    }
                });
            })
            resolve();
          })
    });
}