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document.addEventListener is a method of the Document interface in the Web Platform API that registers an event listener for events that occur within the document. It enables code to respond to user interactions and other events, even when they originate from elements nested inside the page. This is commonly used to handle clicks, key presses, or custom events at the document level, often as a form of event delegation.

Syntax and parameters: document.addEventListener(type, listener, options). The type is a string such as 'click', 'keydown', or 'scroll'; listener is a function that receives an Event object; options can be a boolean for the capture phase or, more commonly, an object with properties such as capture, once, and passive. The listener runs when the specified event occurs on the document or its descendants, depending on the event flow and the capture setting.

Event flow and handling: events originate at a target element and propagate through the DOM. If capture is enabled, the listener may run during the capturing phase; otherwise it runs during the bubbling phase as the event travels back up to the document. The Event object provides details like target, currentTarget, type, and methods such as preventDefault and stopPropagation. To remove a listener, use document.removeEventListener with the same type, listener, and options used to add it.

Best practices: prefer a stable function reference for the listener to allow removal, limit document-level listeners when possible to avoid overhead, and consider using passive listeners for scroll and touch events to improve performance. Event delegation on the document can simplify handling for dynamically added elements, but it should be used judiciously to avoid unintended side effects.