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document.getElementById is a method of the Document interface in the DOM that retrieves a single element whose id attribute matches the provided string. If no element with that ID exists, it returns null. Because IDs are intended to be unique within a document, this method typically yields at most one element. It is commonly used to access a specific element for manipulation or event handling.

Related methods in the same family include document.getElementsByClassName and document.getElementsByTagName. The former returns a live HTMLCollection of all elements with the given class, while the latter returns a live collection of all elements with the specified tag name. These collections may be empty or contain multiple elements, unlike getElementById which targets a single element. Modern web development often uses document.querySelector and document.querySelectorAll, which accept CSS selectors and can locate elements more flexibly, returning a single element or a NodeList respectively.

Usage guidelines: run these lookups after the DOM is ready, such as on DOMContentLoaded, to ensure elements exist. Always check for null (or empty collections) before accessing properties or attaching listeners to avoid errors. Example: var header = document.getElementById("header"); if (header) { header.style.backgroundColor = "blue"; header.addEventListener("click", handleClick); }. These methods directly manipulate the live DOM, so changes reflect immediately in the page.