gatsby-node.js

const path = require('path')

/*
module.exports.onCreateNode = ({node, actions}) => {
    const {createNodeField } = actions

    if(node.internal.type === 'MarkdownRemark') {
        const slug = path.basename(node.fileAbsolutePath, '.md');
        //console.log(JSON.stringify(node, undefined, 4));

        createNodeField({
            node,
            name: 'slug',
            value: slug
        })
        console.log('@@@@@@', slug)
    }
}
*/

module.exports.createPages = async ({ graphql, actions }) => {
    const  { createPage } = actions
    // 1. Get Path to template
    const blogTemplate = path.resolve('./src/templates/blog.js');

    //graphql return a promise
    // 2. Get markdown data
    const res = await graphql(`
        query{
            allContentfulBlogPost {
                edges {
                    node {
                        slug
                    }
                }
            }
        }
    `);
    
    // 3. Create new pages
    res.data.allContentfulBlogPost.edges.forEach((edge) => {
        createPage({
            component: blogTemplate,
            path:`/blog/${edge.node.slug}`,
            context: {
                slug: edge.node.slug
            }
        })
    })
}