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Lazy Load Images on Intersection ​

Problem ​

An image should not receive its full source until it approaches the viewport. createIntersection() provides normalized intersection state and explicit observer ownership.

Solution ​

Observe an image, assign its source once, then release the observer immediately.

ts
import { createIntersection, SentinelUnavailableError } from '@vielzeug/sentinel';

const image = document.createElement('img');
image.alt = 'Blue placeholder';
image.dataset.src =
  'data:image/svg+xml,%3Csvg xmlns="http://www.w3.org/2000/svg" width="640" height="360"%3E%3Crect width="100%25" height="100%25" fill="%23007acc"/%3E%3C/svg%3E';
document.body.append(image);

try {
  const intersection = createIntersection(image, { rootMargin: '200px' });
  const unsubscribe = intersection.subscribe(() => {
    if (!intersection.value?.isIntersecting || !image.dataset.src) return;

    image.src = image.dataset.src;
    delete image.dataset.src;
    unsubscribe();
    intersection.dispose();
  });
} catch (error) {
  if (!(error instanceof SentinelUnavailableError)) throw error;
  image.src = image.dataset.src ?? '';
}

Pitfalls ​

  • Use native loading="lazy" when it provides enough control.
  • Reserve image dimensions to prevent layout shift.
  • Dispose after the one-time load to avoid retaining the element.