What are the Key Differences Between `querySelector`, `querySelectorAll`, and `getElement*` Methods in JavaScript?

Differences between querySelector and getElementsByClassName

querySelector and querySelectorAll

querySelector and querySelectorAll are DOM methods used to select elements based on CSS selectors. They are more versatile than getElement* methods as they allow you to use any valid CSS selector, including complex ones like:

• document.querySelector(".myclass")
• document.querySelector("#myid")

getElementById and getElementsByClassName

getElementById and getElementsByClassName, on the other hand, are DOM methods used to select elements by specific criteria, such as:

• document.getElementById("myid")
• document.getElementsByClassName("myclass")

Key Differences

The main differences between these methods can be summarized as follows:

Feature	querySelector*	getElement*
Selector Flexibility	CSS3 selectors	Limited to id, tag, and class
Performance	O(n) complexity	O(1) complexity
Return Type	Element (querySelector) or NodeList (querySelectorAll)	Element (getElementById) or HTMLCollection (getElementsByClassName/TagName/Name)
Collection Liveness	Static (querySelectorAll)	Live (getElementsByClassName/TagName/Name)

Dynamic Ids and querySelector

In your XPages example, querySelector does not work because the ID is dynamically generated with a colon, which is not a valid character in CSS selectors. To select such elements, you must use getElementById:

document.getElementById("view:_id1:inputText1")

Additional Considerations

• querySelector is generally faster on small DOMs, while getElement is preferred on large DOMs for performance reasons.
• getElement* methods can be chained for improved performance.
• querySelector returns a static collection of elements, while getElement methods return live collections. Live collections update automatically as the DOM changes, while static collections do not.
• [HTMLCollections](https://developer.mozilla.org/en-US/docs/Web/API/HTMLCollection) are not as array-like as [NodeLists](https://developer.mozilla.org/en-US/docs/Web/API/NodeList) and do not support forEach().

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