QJavaScript · Interview preparation
What is event delegation?
Event delegation attaches a listener to an ancestor and uses event.target or closest to identify matching descendants.
The idea to remember.
Event delegation attaches a listener to an ancestor and uses event.target or closest to identify matching descendants. It can handle dynamically inserted list items without registering one listener per child.
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 what is event delegation 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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