Gatsby-plugin-sitemap, custom config, need to integrate pages and markdown using custom resolvePages and Serialize, what does this line of code do?

Question

just starting with javascript and react thanks to Gatsby so excuse me if this is a total newbie question. Also just starting with posting on stackoverflow, usually just consuming content, sorry about that and if my post is incomplete or unclear in anyway.

I am building a website using GatsbyJs, and want to setup a proper sitemap using gatsby-plugin-sitemap, however i am strugling to understand what the following line of code does so i can try and customize de code to do what I need, which is integrate the pages and blog posts on the sitemap, and adding a proper lastmod when applicable. I am breaking my head but cannot get the last part to work, that is, adding lastmod when it is a blog post.

on gatsby-config.js:

{
      resolve: `gatsby-plugin-sitemap`,
      options: {

        // Exclude specific pages or groups of pages using glob parameters
        // See: https://github.com/isaacs/minimatch
        // The example below will exclude the single `path/to/page` and all routes beginning with `category`
        output: '/',
        excludes: [`/legal/*`,`/gracias`],
        query: `
        {
           site {
             siteMetadata {
               siteUrl
             }
             buildTime(formatString: "YYYY-MM-DD")
           }
           allSitePage {
             nodes {
               path
             }
           }
           allMarkdownRemark(filter: {frontmatter: {path: {regex: "/blog/"}}}) {
             nodes {
               frontmatter {
                 path
                 date
               }
             }
           }
         }
        `,

        resolvePages: ({
          allSitePage: { nodes: allPages },
          allMarkdownRemark: { nodes: allPosts },
        }) => {
          const pathToDateMap = {};

          allPosts.map(post => {
            pathToDateMap [post.frontmatter.path] = { date: post.frontmatter.date };
          });

          const pages = allPages.map(page => {
            return { ...page, ...pathToDateMap [page.path] };//what does this line of code do???
          });

          return pages;
        },
        serialize: ({ path, date, buildTime }) => {
          let entry = {
            url: path,
            changefreq: 'monthly',
            priority: 0.5,
          };

          if (date) {
            entry.priority = 0.7;
            entry.lastmod = date;
          } else {

            entry.lastmod = buildtime;

          }

          return entry;
        }

      }
    }

For your knowledge both develop and build are succesful, the sitemap is generated as sitemap-index and sitemap-0.xml, and the output is there but no page has lastmod on it.

Thank you all for your help,

Answer 1

With this:

return { ...page, ...pathToDateMap [page.path] };

You are merging objects using the spread operator (...). So, you are returning an object that is the result of all properties of the page and pathToDateMap(at page.path position).

For example:

let person = {
    firstName: 'John',
    lastName: 'Doe',
    age: 25,
};


let job = {
    jobTitle: 'Web developer',
    location: 'USA'
};

let employee = {
    ...person,
    ...job
};

console.log(employee)

In the snippet above, employee is the result of merging person and job.

The lastmod parameter is not added by default unless your source has it (WordPress normally has) but you can follow this answer https://stackoverflow.com/a/70297982/5585371 to create your own.