QJavaScript · Interview preparation
When is IntersectionObserver useful?
IntersectionObserver reports when elements intersect a viewport or another root.
The idea to remember.
IntersectionObserver reports when elements intersect a viewport or another root. It can replace continuous scroll calculations for lazy loading, visibility tracking, and entering-view animations.
01Learn by doing
From first example to real project.
Start with the smallest working idea, examine a more detailed example, then look at a real application pattern. Adapt dependencies, error handling and data models to your project.
Basic: delegate one listener
01 / BEGINNERA parent can handle events from matching children.
document.querySelector('#list').addEventListener('click', event => {
const button = event.target.closest('button[data-id]');
if (button) console.log(button.dataset.id);
});Intermediate: respect request lifecycles
02 / INTERMEDIATEAbort a pending request when it is no longer needed.
const controller = new AbortController();
fetch('/api/items', { signal: controller.signal });
// When the user navigates away:
controller.abort();Real scenario: safe persisted preferences
03 / REAL SCENARIOValidate stored data instead of assuming it is trustworthy.
function readTheme() {
const stored = localStorage.getItem('theme');
return stored === 'dark' ? 'dark' : 'light';
}
function saveTheme(theme) {
if (theme === 'dark' || theme === 'light') localStorage.setItem('theme', theme);
}02Check your understanding
Try it in your own words.
Explain when is IntersectionObserver useful without looking at the code. Then modify the intermediate example, describe one trade-off and identify when the real-world pattern fits.
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