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 }) => {
  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 = `/${_.kebabCase(node.frontmatter.slug)}`;
    }
    if (
      Object.prototype.hasOwnProperty.call(node, "frontmatter") &&
      Object.prototype.hasOwnProperty.call(node.frontmatter, "title")
    ) {
      slug = `/${_.kebabCase(node.frontmatter.title)}`;
    }
    createNodeField({ node, name: "slug", value: slug });
  }
};