gatsbyでURLを `/YYYY/MM/DD/slug` という形式にする

gatsby-node.js

...

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

  if (node.internal.type === 'MarkdownRemark') {
    // Create slug with post date directory structure
    const createSlug = (postDate, filePath) => {
      const year = postDate.getFullYear();
      const month = `0${postDate.getMonth() + 1}`.slice(-2); // month with zero padding
      const date = `0${postDate.getDate()}`.slice(-2); // date with zero padding
      return `/${year}/${month}/${date}/${filePath.replace(/^\/posts\//, '')}`;
    };

    const date = new Date(node.frontmatter.date);
    const filePath = createFilePath({ node, getNode });
    const slug = createSlug(date, filePath);

    createNodeField({
      node,
      name: 'slug',
      value: slug,
    });
  }
};

...