getElementsBy

getElementsBy is a family of DOM methods used to retrieve groups of elements from a document. The methods share a common purpose: return a collection of elements that match a certain criterion. The most widely used are getElementsByTagName and getElementsByClassName, with additional forms on HTML documents or Element contexts such as getElementsByName and getElementsByTagNameNS for namespace-aware contexts.

getElementsByTagName(tagName) returns a live HTMLCollection of all descendant elements whose tag name matches the provided tag. In HTML documents, tag name matching is case-insensitive; in XML it is case-sensitive. Passing '*' yields all elements in the subtree.

getElementsByClassName(classNames) returns a live HTMLCollection of elements that have all of the classes specified in the string, which can contain multiple class names separated by spaces.

getElementsByName(name) returns a NodeList of elements whose name attribute equals the given name. This is commonly used with form controls, but the name attribute is not exclusive to forms.

getElementsByTagNameNS(namespaceURI, localName) performs a tag-name-based search within a given namespace and returns a live collection of matching elements. The localName can be '*' to match any element within that namespace.

Because these collections are live, changes to the document are reflected automatically. For many use cases, modern code prefers querySelectorAll for CSS-style selectors, which returns a static NodeList and can be more predictable.