The add() method of the HTMLOptionsCollection interface adds an HTMLOptionElement or HTMLOptGroupElement to this HTMLOptionsCollection.
add(item)
add(item, before)
item
before Optional
An element of the collection, or a numeric 0-based index representing the element that the item should be inserted before. If omitted, null, or the index does not exist, the new element is appended to the end of the collection.
None (undefined).
HierarchyRequestError DOMException
Thrown if the item passed to the method is an ancestor of the element into which it is to be inserted.
By default, the add() appends the <option> or <optgroup> passed as the parameter to the end of the collection. You can define where the added <option> or <optgroup> should be placed by specifying the before parameter. The before is the <option> element or a numeric 0-based index of the <option> element the added element should precede.
If the before parameter is null or out of range (or omitted), the <option> or <optgroup> will be appended as the last element in the collection, outside of any <optgroup>. If the <option> referenced by the before parameter is in an <optgroup>, an added HTMLOptionElement will be in the same group.
The <optgroup> element can only contain <option> elements as child nodes. The add() method will successfully add an HTMLOptGroupElement to the end of the HTMLOptionsCollection or between <optgroup> elements only. In other words, attempting to add an HTMLOptGroupElement before an <option> within an <optgroup> may silently fail if the <option> referenced by the before parameter is not the first <option> within its <optgroup>.
const optionList = document.querySelector("select").options;
const firstOption = document.createElement("option");
firstOption.text = "new item";
optionList.add(firstOption, 0); // added as the first item
optionList.add(optionList[0]); // moves the first item to the end
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