gatsby-node.js

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

exports.createPages = async ({ graphql, actions: { createPage } }) => {
  const Post = path.resolve('./src/components/templates/Post.js');
  const result = await graphql(`
    {
      allMarkdownRemark(
        sort: { fields: [frontmatter___date], order: ASC }
        limit: 1000
      ) {
        edges {
          node {
            fields {
              slug
            }
            frontmatter {
              title
            }
          }
        }
      }
    }
  `);

  if (result.errors) {
    throw result.errors;
  }

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

  posts.forEach((post, index) => {
    const next = index === posts.length - 1 ? null : posts[index + 1].node;
    const previous = index === 0 ? null : posts[index - 1].node;

    createPage({
      path: post.node.fields.slug,
      component: Post,
      context: {
        slug: post.node.fields.slug,
        previous,
        next,
      }
    });
  });
};

exports.onCreateNode = ({ node, actions: { createNodeField }, getNode }) => {
  if (node.internal.type === 'MarkdownRemark') {
    const value = createFilePath({ node, getNode });
    
    createNodeField({
      name: 'slug',
      node,
      value,
    });
  }
};

exports.onCreateWebpackConfig = ({ actions: { setWebpackConfig } }) =>
  setWebpackConfig({
    resolve: {
      modules: [path.resolve(__dirname, 'src'), 'node_modules'],
    },
  });