Baseline Widely available
The HTMLCollection interface represents a generic collection (array-like object similar to arguments) of elements (in document order) and offers methods and properties for selecting from the list.
An HTMLCollection in the HTML DOM is live; it is automatically updated when the underlying document is changed. For this reason it is a good idea to make a copy (e.g., using Array.from) to iterate over if adding, moving, or removing nodes.
This interface is called HTMLCollection for historical reasons, because before the modern DOM, collections implementing this interface could only have HTML elements as their items.
This interface was an attempt to create an unmodifiable list and only continues to be supported to not break code that's already using it. Modern APIs represent list structures using types based on JavaScript arrays, thus making many array methods available, and at the same time imposing additional semantics on their usage (such as making their items read-only).
These historical reasons do not mean that you as a developer should avoid HTMLCollection. You don't create HTMLCollection objects yourself, but you get them from APIs such as Document.getElementsByClassName(), and these APIs are not deprecated. However, be careful of the semantic differences from a real array.
HTMLCollection.length Read only
Returns the number of items in the collection.
HTMLCollection.item()
Returns the specific element at the given zero-based index into the list. Returns null if the index is out of range.
An alternative to accessing collection[i] (which instead returns undefined when i is out-of-bounds). This is mostly useful for non-JavaScript DOM implementations.
HTMLCollection.namedItem()
Returns the specific node whose ID or, as a fallback, name matches the string specified by name. Matching by name is only done as a last resort, only in HTML, and only if the referenced element supports the name attribute. Returns null if no node exists by the given name.
An alternative to accessing collection[name] (which instead returns undefined when name does not exist). This is mostly useful for non-JavaScript DOM implementations.
HTMLCollection also exposes its members as properties by name and index. HTML IDs may contain : and . as valid characters, which would necessitate using bracket notation for property access. Currently, an HTMLCollection object does not recognize purely numeric IDs, which would cause conflict with the array-style access, though HTML does permit these.
For example, assuming there is one <form> element in the document and its id is myForm:
let elem1, elem2;
// document.forms is an HTMLCollection
elem1 = document.forms[0];
elem2 = document.forms.item(0);
alert(elem1 === elem2); // shows: "true"
elem1 = document.forms.myForm;
elem2 = document.forms.namedItem("myForm");
alert(elem1 === elem2); // shows: "true"
elem1 = document.forms["named.item.with.periods"];
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