gatsby-node.js

const path = require(`path`);
const { createFilePath } = require(`gatsby-source-filesystem`);

exports.createPages = ({ graphql, actions }) => {
  const { createPage } = actions;

  const blogPost = path.resolve(`./src/templates/blog-post.js`);
  return graphql(
    `
      {
        allMarkdownRemark(
          sort: { fields: [fields___date], order: DESC }
          limit: 1000
        ) {
          edges {
            node {
              fields {
                slug
                date
              }
              frontmatter {
                title
              }
            }
          }
        }
      }
    `,
  ).then(result => {
    if (result.errors) {
      throw result.errors;
    }

    // Create blog posts pages.
    const posts = result.data.allMarkdownRemark.edges;

    posts.forEach((post, index) => {
      createPage({
        path: post.node.fields.slug,
        component: blogPost,
        context: {
          slug: post.node.fields.slug,
          date: post.node.fields.date,
        },
      });
    });
  });
};

exports.onCreateNode = ({ node, actions, getNode }) => {
  const { createNodeField } = actions;

  if (node.internal.type === 'MarkdownRemark') {
    const fileNode = getNode(node.parent);
    const parsedFilePath = path.parse(fileNode.relativePath);

    const [year, month, day, ...title] = parsedFilePath.name.split('-');

    createNodeField({
      name: 'slug',
      node,
      value: `/${year}/${month}/${title.join('-')}`,
    });

    createNodeField({
      name: 'date',
      node,
      value: `${year}-${month}-${day}`,
    });

    createNodeField({
      name: 'year',
      node,
      value: year,
    });
  }
};