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The function document.getElementById is a fundamental method in JavaScript used to access an HTML element from the Document Object Model (DOM) by its unique ID. When a web page is loaded by a browser, it creates a DOM representation of the page's structure. The getElementById method allows JavaScript code to interact with this structure, specifically by retrieving a single element.

To use getElementById, you pass a string argument to it, which is the exact value of the `id` attribute of the HTML element you want to select. For example, if you have an HTML element like `<div id="myUniqueDiv">Content</div>`, you would select it in JavaScript using `document.getElementById("myUniqueDiv")`.

If an element with the specified ID exists in the document, the method returns an object representing that element. This object can then be manipulated in various ways, such as changing its content, styles, or adding event listeners. If no element with the given ID is found, the method returns `null`. It is important to note that IDs within an HTML document should be unique; using the same ID for multiple elements can lead to unpredictable behavior when using getElementById, as it will typically return only the first element it finds with that ID.