auto change side nav current

src/app/components/SideNav/SideNav.tsx

@@ -0,0 +1,86 @@
+import { useState, useEffect, SetStateAction } from 'react';
+import { SideNav as UswdsSideNav } from '@trussworks/react-uswds';
+import classNames from 'classnames';
+
+interface Section {
+  title: string;
+  id: string;
+}
+
+interface SideNavProps {
+  sections: Section[];
+}
+
+export const SideNav = ({ sections }: SideNavProps) => {
+  const [selectedHash, setSelectedHash] = useState(`#${sections[0].id}`);
+
+  useEffect(() => {
+    let currentSection = '';
+
+    const observer = new IntersectionObserver(
+      (entries) => {
+        entries.forEach((entry) => {
+          if (entry.isIntersecting) {
+            const newSection = `#${entry.target.id}`;
+            if (currentSection !== newSection) {
+              currentSection = newSection;
+              setSelectedHash(newSection);
+            }
+          }
+        });
+      },
+      {
+        rootMargin: '-20% 0px -70% 0px',
+        threshold: 0,
+      },
+    );
+
+    sections.forEach(({ id }) => {
+      const element = document.getElementById(id);
+      if (element) observer.observe(element);
+    });
+
+    return () => {
+      sections.forEach(({ id }) => {
+        const element = document.getElementById(id);
+        if (element) observer.unobserve(element);
+      });
+    };
+  }, [sections]);
+
+  return (
+    <UswdsSideNav
+      items={sections.map(({ title, id }) => (
+        <NavItem
+          title={title}
+          id={id}
+          key={id}
+          selectedHash={selectedHash}
+          setSelectedHash={setSelectedHash}
+        />
+      ))}
+    />
+  );
+};
+
+interface NavItemProps {
+  title: string;
+  id: string;
+  selectedHash: string;
+  setSelectedHash: (value: SetStateAction<string>) => void;
+}
+function NavItem({ title, id, selectedHash, setSelectedHash }: NavItemProps) {
+  const itemHash = `#${id}`;
+  return (
+    <a
+      href={itemHash}
+      key={id}
+      className={classNames({
+        'usa-current': itemHash === selectedHash,
+      })}
+      onClick={() => setSelectedHash(itemHash)}
+    >
+      {title}
+    </a>
+  );
+}

src/app/products/ecr-viewer/page.tsx

@@ -8,11 +8,9 @@ import {
   ProcessListHeading,
   Accordion,
   Link,
-  SideNav,
 } from '@trussworks/react-uswds';
-import classNames from 'classnames';
+import { SideNav } from '@/app/components/SideNav/SideNav';
 import Image from 'next/image';
-import { SetStateAction, useState } from 'react';
 import './styles.scss';
 
 export default function EcrViewer() {
@@ -554,78 +552,15 @@ export default function EcrViewer() {
 }
 
 function Navigation() {
-  const [selectedHash, setSelectedHash] = useState('#overview');
-
-  return (
-    <SideNav
-      items={[
-        <NavItem
-          title="Overview"
-          id="overview"
-          key="overview"
-          selectedHash={selectedHash}
-          setSelectedHash={setSelectedHash}
-        />,
-        <NavItem
-          title="Product features"
-          id="product-features"
-          key="product-features"
-          selectedHash={selectedHash}
-          setSelectedHash={setSelectedHash}
-        />,
-        <NavItem
-          title="How it works"
-          id="how-it-works"
-          key="how-it-works"
-          selectedHash={selectedHash}
-          setSelectedHash={setSelectedHash}
-        />,
-        <NavItem
-          title="Getting started"
-          id="getting-started"
-          key="getting-started"
-          selectedHash={selectedHash}
-          setSelectedHash={setSelectedHash}
-        />,
-        <NavItem
-          title="Technical resources"
-          id="technical-resources"
-          key="technical-resources"
-          selectedHash={selectedHash}
-          setSelectedHash={setSelectedHash}
-        />,
-        <NavItem
-          title="FAQs"
-          id="faqs"
-          key="faqs"
-          selectedHash={selectedHash}
-          setSelectedHash={setSelectedHash}
-        />,
-      ]}
-    />
-  );
-}
-
-interface NavItemProps {
-  title: string;
-  id: string;
-  selectedHash: string;
-  setSelectedHash: (value: SetStateAction<string>) => void;
-}
-function NavItem({ title, id, selectedHash, setSelectedHash }: NavItemProps) {
-  const itemHash = `#${id}`;
-  return (
-    <a
-      href={itemHash}
-      key={id}
-      className={classNames({
-        'usa-current': itemHash === selectedHash,
-      })}
-      onClick={() => setSelectedHash(itemHash)}
-    >
-      {title}
-    </a>
-  );
+  const sections = [
+    { title: 'Overview', id: 'overview' },
+    { title: 'Product features', id: 'product-features' },
+    { title: 'How it works', id: 'how-it-works' },
+    { title: 'Getting started', id: 'getting-started' },
+    { title: 'Technical resources', id: 'technical-resources' },
+    { title: 'FAQs', id: 'faqs' },
+  ];
+  return <SideNav sections={sections} />;
 }
 
 interface GithubNavProps {