Refactor javascript shortcode

- Switched from XMLHttpRequest to fetch API (more modern replacement).

- Used async syntax (eliminates the need for callbacks and .then()
  chains, marginal here but still good practice).

- Removed `onreadystatechange`: with Fetch API we can simply wait for
  the fetch promise to resolve.

- Simplified error handling by using a `try/catch` block.

- Removed redundant `var` declarations for `const` and `let` variable
  declarations (modern javascript).

- Extracted a helper function `updateModifiedDateElement` to reduce code
  duplication.

- Early `return` in the `if (commits.length === 0)` block to avoid
  executing the rest of the function.

- Removed unnecessary `else` clause since we're returning early in the
  `if (commits.length === 0)` block.

layouts/shortcodes/last_modified_lineage.html

@@ -1,36 +1,41 @@
 <!-- Shortcode for GitHub Last Modified Date -->
 <span id="last_modified_github"></span>
 <script>
-  // Function to fetch the latest commit date from the GitHub repository using XMLHttpRequest
-  function fetchGitHubCommitDate() {
-    var xmlhttp = new XMLHttpRequest();
-    var githubApiUrl = "https://api.github.com/repos/MurrellGroup/lineages/commits?path=plots&page=1&per_page=1";
-
-    xmlhttp.onreadystatechange = function () {
-      if (this.readyState == 4) {
-        if (this.status == 200) {
-          var commits = JSON.parse(this.responseText);
-          if (commits.length > 0) {
-            // Extract the date of the latest commit
-            var latestCommitDate = new Date(commits[0].commit.committer.date);
-            // Format the date as YYYY-MM-DD
-            var formattedDate = latestCommitDate.toISOString().split('T')[0];
-            // Update the span with the last modified date
-            document.getElementById("last_modified_github").innerText = formattedDate;
-          } else {
-            document.getElementById("last_modified_github").innerText = "Not updated";
-          }
-        } else {
-          console.error("Error fetching GitHub commit date:", this.statusText);
-          document.getElementById("last_modified_github").innerText = "Error fetching date.";
-        }
-      }
-    };
 
-    xmlhttp.open("GET", githubApiUrl, true);
-    xmlhttp.send();
-  }
+ // Function to fetch the latest commit date from the GitHub repository using Fetch API
+ async function fetchGitHubCommitDate() {
+   const githubApiUrl = "https://api.github.com/repos/MurrellGroup/lineages/commits?path=plots&page=1&per_page=1";
+   try {
+     const response = await fetch(githubApiUrl);
+     // Check for successful response
+     if (!response.ok) {
+       throw new Error(`Error fetching GitHub commit date: ${response.statusText}`);
+     }
+     // Parse response data as JSON
+     const commits = await response.json();
+
+     // Check if there are any commits in the response data
+     if (commits.length === 0) {
+       updateModifiedDateElement("Not updated");
+       return;
+     }
+     // Extract the date of the latest commit and format as YYYY-MM-DD
+     const latestCommitDate = new Date(commits[0].commit.committer.date);
+     const formattedDate = latestCommitDate.toISOString().split('T')[0];
+     updateModifiedDateElement(formattedDate);
+
+   } catch (error) {
+     console.error(error);
+     updateModifiedDateElement("Error fetching date");
+   }
+ }
+
+ // Helper function to update the modified date element
+ function updateModifiedDateElement(dateText) {
+   document.getElementById("last_modified_github").innerText = dateText;
+ }
+
+ // Call the function to fetch the GitHub commit date
+ fetchGitHubCommitDate();
 
-  // Call the function to fetch the GitHub commit date
-  fetchGitHubCommitDate();
 </script>