gatsby-node.js

const path = require("path");

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

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

    createNodeField({
      node,
      name: "slug",
      value: parsedFilePath.name,
    });
  }
};

exports.createPages = async ({ graphql, actions }) => {
  const { createPage } = actions;
  const postPage = path.resolve("src/templates/Post/Post.bs.js");

  const markdownQueryResult = await graphql(
    `
      {
        allMarkdownRemark {
          edges {
            node {
              fields {
                slug
              }
              frontmatter {
                title
                tags
              }
            }
          }
        }
      }
    `
  );

  if (markdownQueryResult.errors) {
    console.error(markdownQueryResult.errors);
    throw markdownQueryResult.errors;
  }

  const tagSet = new Set();

  const postsEdges = markdownQueryResult.data.allMarkdownRemark.edges;

  postsEdges.forEach((edge, index) => {
    if (edge.node.frontmatter.tags) {
      edge.node.frontmatter.tags.forEach((tag) => {
        tagSet.add(tag);
      });
    }

    const nextID = index + 1 < postsEdges.length ? index + 1 : 0;
    const prevID = index - 1 >= 0 ? index - 1 : postsEdges.length - 1;
    const nextEdge = postsEdges[nextID];
    const prevEdge = postsEdges[prevID];

    createPage({
      path: `/posts/${edge.node.fields.slug}/`,
      component: postPage,
      context: {
        slug: edge.node.fields.slug,
        nexttitle: nextEdge.node.frontmatter.title,
        nextslug: nextEdge.node.fields.slug,
        prevtitle: prevEdge.node.frontmatter.title,
        prevslug: prevEdge.node.fields.slug,
      },
    });
  });
};