Event Listener
Code that waits for something to happen, then reacts.
Definition
An event listener is a function attached to an element that runs when a specific event occurs — a click, a keypress, a page load, a form submission. It is how JavaScript makes web pages interactive. Without event listeners, clicking a button would do nothing.
Example
button.addEventListener('click', function() { alert('Clicked!'); }) attaches a function to a button that shows an alert every time the user clicks it.
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