index.js

window.onload = function () {
  
  document.querySelectorAll('a[href^="#"]').forEach((anchor) => {
    anchor.addEventListener("click", function (e) {
      
      // Get href link value
      const targetUrl = this.getAttribute("href");
      
      // Track the event name as "anchor_click" and custom field name is "target", value is the href value.
      firebase.analytics().logEvent('anchor_click', { target: targetUrl});
      
    });
  });
};