gatsby-node.js

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

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

exports.createPages = async ({ graphql, actions, reporter }) => {
    const { createPage } = actions;
    const result = await graphql(`
        query {
            allMdx {
                edges {
                    node {
                        id
                        fields {
                            slug
                        }
                    }
                }
            }
        }
    `);
    if (result.errors) {
        reporter.panicOnBuild('🚨  ERROR: Loading "createPages" query');
    }
    const posts = result.data.allMdx.edges;
    posts.forEach(({ node }, index) => {
        createPage({
            path: node.fields.slug,
            component: path.resolve(`./src/components/PostTemplate.js`),
            context: { id: node.id }
        });
    });
};