gatsby-plugin-slug/gatsby-node.js

Add slug field to MarkdownRemark and Mdx nodes

const path = require("path")

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

  if (
    (node.internal.type === "MarkdownRemark" || node.internal.type === "Mdx") &&
    fileNode.internal.type === "File"
  ) {
    const parsedFilePath = path.parse(fileNode.relativePath)
    let slug

    if (node.frontmatter && node.frontmatter.slug) {
      slug = `/${node.frontmatter.slug}`
    } else {
      if (parsedFilePath.name !== "index" && parsedFilePath.dir !== "") {
        slug = `/${parsedFilePath.dir}/${parsedFilePath.name}`
      } else if (parsedFilePath.dir === "") {
        slug = `/${parsedFilePath.name}`
      } else {
        slug = `/${parsedFilePath.dir}`
      }
    }

    createNodeField({
      name: `slug`,
      node,
      value: slug,
    })
  }
}