gatsby-node.js

const _ = require('lodash');

// graphql function doesn't throw an error so we have to check to check for the result.errors to throw manually
const wrapper = (promise) =>
  promise.then((result) => {
    if (result.errors) {
      throw result.errors;
    }
    return result;
  });

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

  let slug;

  if (node.internal.type === 'Mdx') {
    if (
      Object.prototype.hasOwnProperty.call(node, 'frontmatter') &&
      Object.prototype.hasOwnProperty.call(node.frontmatter, 'slug')
    ) {
      slug = `/${_.kebabCase(node.frontmatter.slug)}`;
    } else if (
      Object.prototype.hasOwnProperty.call(node, 'frontmatter') &&
      Object.prototype.hasOwnProperty.call(node.frontmatter, 'title')
    ) {
      slug = `/${_.kebabCase(node.frontmatter.title)}`;
    }
    createNodeField({ node, name: 'slug', value: slug });

    // Calculate reading time
    const parent = getNode(node.parent);
    if (parent && parent.internal && parent.internal.content) {
      const content = parent.internal.content;
      const wordsPerMinute = 200;
      const wordCount = content.split(/\s+/).length;
      const readingTime = Math.ceil(wordCount / wordsPerMinute);
      createNodeField({ node, name: 'timeToRead', value: readingTime });
    }
  }
};

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

  const postTemplate = require.resolve('./src/templates/post.js');
  const categoryTemplate = require.resolve('./src/templates/category.js');

  const result = await wrapper(
    graphql(`
      {
        allMdx(sort: { frontmatter: { date: DESC } }) {
          edges {
            node {
              fields {
                slug
              }
              frontmatter {
                title
                categories
              }
              id
              internal {
                contentFilePath
              }
            }
          }
        }
      }
    `)
  );

  const posts = result.data.allMdx.edges;

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

    createPage({
      path: edge.node.fields.slug,
      component: `${postTemplate}?__contentFilePath=${edge.node.internal.contentFilePath}`,
      context: {
        slug: edge.node.fields.slug,
        prev,
        next,
      },
    });
  });

  const categorySet = new Set();

  _.each(posts, (edge) => {
    if (_.get(edge, 'node.frontmatter.categories')) {
      edge.node.frontmatter.categories.forEach((cat) => {
        categorySet.add(cat);
      });
    }
  });

  const categories = Array.from(categorySet);

  categories.forEach((category) => {
    createPage({
      path: `/categories/${_.kebabCase(category)}`,
      component: categoryTemplate,
      context: {
        category,
      },
    });
  });
};