Scroll-triggered popup on website

Implementing Scroll-triggered Popup on Website

A scroll-triggered popup appears when the user has scrolled through a certain percentage of the page. The logic: if a person has read through the middle of an article or product page, they are engaged, and a subscription offer or discount will reach them at the right moment.

Implementation with Intersection Observer

IntersectionObserver is more efficient than scroll events: it does not block the main thread and does not require requestAnimationFrame.

// scroll-popup.ts
interface ScrollPopupConfig {
  triggerPercent?: number;    // % of page scroll (0-100)
  triggerElement?: string;    // CSS selector of trigger element
  cooldownMs?: number;
  onTrigger: () => void;
}

export function initScrollPopup(config: ScrollPopupConfig) {
  const { triggerPercent, triggerElement, cooldownMs = 0, onTrigger } = config;
  let triggered = false;

  const STORAGE_KEY = 'scroll_popup_shown';

  function checkCooldown(): boolean {
    if (!cooldownMs) return true;
    const last = localStorage.getItem(STORAGE_KEY);
    if (last && Date.now() - Number(last) < cooldownMs) return false;
    return true;
  }

  function fire() {
    if (triggered || !checkCooldown()) return;
    triggered = true;
    if (cooldownMs) localStorage.setItem(STORAGE_KEY, String(Date.now()));
    onTrigger();
  }

  // Option 1: scroll percentage
  if (triggerPercent !== undefined) {
    // Create invisible marker element at the required height
    const marker = document.createElement('div');
    marker.style.cssText = 'position:absolute;top:0;left:0;width:1px;height:1px;pointer-events:none;';
    document.body.style.position = 'relative';
    document.body.appendChild(marker);

    // Position marker at triggerPercent of document height
    function updateMarker() {
      const docHeight = document.documentElement.scrollHeight;
      const viewportHeight = window.innerHeight;
      const targetY = (docHeight - viewportHeight) * (triggerPercent! / 100);
      marker.style.top = `${targetY}px`;
    }

    updateMarker();
    window.addEventListener('resize', updateMarker, { passive: true });

    const observer = new IntersectionObserver(
      ([entry]) => { if (entry.isIntersecting) fire(); },
      { threshold: 0 }
    );
    observer.observe(marker);

    return () => {
      observer.disconnect();
      marker.remove();
      window.removeEventListener('resize', updateMarker);
    };
  }

  // Option 2: specific DOM element
  if (triggerElement) {
    const el = document.querySelector(triggerElement);
    if (!el) {
      console.warn(`[scroll-popup] Element not found: ${triggerElement}`);
      return () => {};
    }

    const observer = new IntersectionObserver(
      ([entry]) => { if (entry.isIntersecting) fire(); },
      { threshold: 0.5 } // 50% of element visible
    );
    observer.observe(el);
    return () => observer.disconnect();
  }

  return () => {};
}

Usage in React

// BlogPost.tsx
import { useEffect } from 'react';
import { initScrollPopup } from './scroll-popup';
import { NewsletterPopup } from './NewsletterPopup';
import { useState } from 'react';

export function BlogPost({ content }: { content: string }) {
  const [showPopup, setShowPopup] = useState(false);

  useEffect(() => {
    const cleanup = initScrollPopup({
      triggerPercent: 60,
      cooldownMs: 7 * 24 * 60 * 60 * 1000, // once a week
      onTrigger: () => setShowPopup(true),
    });

    return cleanup;
  }, []);

  return (
    <>
      <article dangerouslySetInnerHTML={{ __html: content }} />
      {showPopup && (
        <NewsletterPopup onClose={() => setShowPopup(false)} />
      )}
    </>
  );
}

Newsletter Popup

// NewsletterPopup.tsx
import { useRef, useEffect, useState } from 'react';

export function NewsletterPopup({ onClose }: { onClose: () => void }) {
  const dialogRef = useRef<HTMLDialogElement>(null);
  const [email, setEmail] = useState('');
  const [status, setStatus] = useState<'idle' | 'loading' | 'done'>('idle');

  useEffect(() => {
    dialogRef.current?.showModal();
    return () => dialogRef.current?.close();
  }, []);

  async function subscribe() {
    if (!email || status !== 'idle') return;
    setStatus('loading');
    await fetch('/api/subscribe', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ email, source: 'scroll_popup' }),
    });
    setStatus('done');
    setTimeout(onClose, 2000);
  }

  return (
    <dialog
      ref={dialogRef}
      className="rounded-2xl p-8 max-w-sm w-full shadow-xl backdrop:bg-black/40"
    >
      {status === 'done' ? (
        <p className="text-center text-green-700 font-medium">You are subscribed!</p>
      ) : (
        <>
          <h2 className="text-lg font-bold mb-2">Liked the article?</h2>
          <p className="text-sm text-gray-600 mb-4">
            Get the best materials once a week. No spam.
          </p>
          <input
            type="email"
            value={email}
            onChange={e => setEmail(e.target.value)}
            onKeyDown={e => e.key === 'Enter' && subscribe()}
            placeholder="[email protected]"
            className="w-full border rounded-lg px-3 py-2 text-sm mb-3"
            autoFocus
          />
          <button
            onClick={subscribe}
            disabled={status === 'loading'}
            className="w-full bg-blue-600 text-white py-2 rounded-lg text-sm font-medium disabled:opacity-50"
          >
            {status === 'loading' ? 'Subscribing...' : 'Subscribe'}
          </button>
          <button
            onClick={onClose}
            className="mt-2 w-full text-xs text-gray-400 hover:text-gray-600"
          >
            No, thanks
          </button>
        </>
      )}
    </dialog>
  );
}

Timeline

One to two days accounting for responsive design, testing on real devices, and cooldown logic tuning. If you need conversion analytics (GA4 + custom tracking) — add half a day.