HTML

Living Standard — Last Updated 16 January 2024

1. 1. 2.6 Common DOM interfaces
      1. 2.6.1 Reflecting content attributes in IDL attributes
      2. 2.6.2 Using reflect in specifications
      3. 2.6.3 Collections
         1. 2.6.3.1 The HTMLAllCollection interface
            1. 2.6.3.1.1 [[Call]] ( thisArgument, argumentsList )
         2. 2.6.3.2 The HTMLFormControlsCollection interface
         3. 2.6.3.3 The HTMLOptionsCollection interface
      4. 2.6.4 The DOMStringList interface

2.6 Common DOM interfaces

2.6.1 Reflecting content attributes in IDL attributes

The building blocks for reflecting are as follows:

• A reflected target is an element or ElementInternals object. It is typically clear from context and typically identical to the interface of the reflected IDL attribute. It is always identical to that interface when it is an ElementInternals object.

• A reflected IDL attribute is an attribute interface member.

• A reflected content attribute name is a string. When the reflected target is an element, it represents the local name of a content attribute whose namespace is null. When the reflected target is an ElementInternals object, it represents a key of the reflected target's target element's internal content attribute map.

A reflected IDL attribute can be defined to reflect a reflected content attribute name of a reflected target. In general this means that the IDL attribute getter returns the current value of the content attribute, and the setter changes the value of the content attribute to the given value.

If the reflected target is an element, then the reflected IDL attribute can additionally declare to support ElementInternals. This means that the ElementInternals interface also has a reflected IDL attribute, with the same identifier, and that reflected IDL attribute reflects the same reflected content attribute name.

The fooBar IDL attribute must reflect the foobar content attribute and support ElementInternals.

Reflected targets have these associated algorithms:

• get the element: takes no arguments; returns an element.
• get the content attribute: takes no arguments; returns null or a string.
• set the content attribute: takes a string value; returns nothing.
• delete the content attribute: takes no arguments; returns nothing.

For a reflected target that is an element element, these are defined as follows:

get the element

1. Return element.

get the content attribute

1. Let attribute be the result of running get an attribute by namespace and local name given null, the reflected content attribute name, and element.

2. If attribute is null, then return null.

3. Return attribute's value.

set the content attribute with a string value

1. Set an attribute value given element, the reflected content attribute name, and value.

delete the content attribute

1. Remove an attribute by namespace and local name given null, the reflected content attribute name, and element.

For a reflected target that is an ElementInternals object elementInternals, they are defined as follows:

get the element

1. Return elementInternals's target element.

get the content attribute

1. If elementInternals's target element's internal content attribute map[the reflected content attribute name] does not exist, then return null.

2. Return elementInternals's target element's internal content attribute map[the reflected content attribute name].

set the content attribute with a string value

1. Set elementInternals's target element's internal content attribute map[the reflected content attribute name] to value.

delete the content attribute

1. Remove elementInternals's target element's internal content attribute map[the reflected content attribute name].

This results in somewhat redundant data structures for ElementInternals objects as their target element's internal content attribute map cannot be directly manipulated and as such reflection is only happening in a single direction. This approach was nevertheless chosen to make it less error-prone to define IDL attributes that are shared between reflected targets and benefit from common API semantics.

IDL attributes of type DOMString or DOMString? that reflect enumerated content attributes can be limited to only known values. Per the processing models below, those will cause the getters for such IDL attributes to only return keywords for those enumerated attributes, or the empty string or null.

If a reflected IDL attribute has the type DOMString:

• The getter steps are:

  1. Let element be the result of running this's get the element.

  2. Let contentAttributeValue be the result of running this's get the content attribute.

  3. Let attributeDefinition be the attribute definition of element's content attribute whose namespace is null and local name is the reflected content attribute name.

  4. If attributeDefinition indicates it is an enumerated attribute and the reflected IDL attribute is defined to be limited to only known values:

     1. If contentAttributeValue does not correspond to any state of attributeDefinition (e.g., it is null and there is no missing value default), or that it is in a state of attributeDefinition with no associated keyword value, then return the empty string.

     2. Return the canonical keyword for the state of attributeDefinition that contentAttributeValue corresponds to.

  5. If contentAttributeValue is null, then return the empty string.

  6. Return contentAttributeValue.

• The setter steps are to run this's set the content attribute with the given value.

If a reflected IDL attribute has the type DOMString?:

• The getter steps are:

  1. Let element be the result of running this's get the element.

  2. Let contentAttributeValue be the result of running this's get the content attribute.

  3. Let attributeDefinition be the attribute definition of element's content attribute whose namespace is null and local name is the reflected content attribute name.

  4. Assert: attributeDefinition indicates it is an enumerated attribute.

  5. Assert: the reflected IDL attribute is limited to only known values.

  6. Assert: contentAttributeValue corresponds to a state of attributeDefinition.

  7. If contentAttributeValue corresponds to a state of attributeDefinition with no associated keyword value, then return null.

  8. Return the canonical keyword for the state of attributeDefinition that contentAttributeValue corresponds to.

• The setter steps are:

  1. If the given value is null, then run this's delete the content attribute.

  2. Otherwise, run this's set the content attribute with the given value.

If a reflected IDL attribute has the type USVString:

• The getter steps are:

  1. Let element be the result of running this's get the element.

  2. Let contentAttributeValue be the result of running this's get the content attribute.

  3. Let attributeDefinition be the attribute definition of element's content attribute whose namespace is null and local name is the reflected content attribute name.

  4. If attributeDefinition indicates it contains a URL:

     1. If contentAttributeValue is null, then return the empty string.

     2. Let urlString be the result of encoding-parsing-and-serializing a URL given contentAttributeValue, relative to element's node document.

     3. If urlString is not failure, then return urlString.

  5. Return contentAttributeValue, converted to a scalar value string.

• The setter steps are to run this's set the content attribute with the given value.

If a reflected IDL attribute has the type boolean:

• The getter steps are:

  1. Let contentAttributeValue be the result of running this's get the content attribute.

  2. If contentAttributeValue is null, then return false.

  3. Return true.

• The setter steps are:

  1. If the given value is false, then run this's delete the content attribute.

  2. If the given value is true, then run this's set the content attribute with the empty string.

This corresponds to the rules for boolean content attributes.

If a reflected IDL attribute has the type long, optionally limited to only non-negative numbers and optionally with a default value defaultValue:

• The getter steps are:

  1. Let contentAttributeValue be the result of running this's get the content attribute.

  2. If contentAttributeValue is not null:

     1. Let parsedValue be the result of integer parsing contentAttributeValue if the reflected IDL attribute is not limited to only non-negative numbers; otherwise the result of non-negative integer parsing contentAttributeValue.

     2. If parsedValue is not an error and is within the long range, then return parsedValue.

  3. If the reflected IDL attribute has a default value, then return defaultValue.

  4. If the reflected IDL attribute is limited to only non-negative numbers, then return −1.

  5. Return 0.

• The setter steps are:

  1. If the reflected IDL attribute is limited to only non-negative numbers and the given value is negative, then throw an "IndexSizeError" DOMException.

  2. Run this's set the content attribute with the given value converted to the shortest possible string representing the number as a valid integer.

If a reflected IDL attribute has the type unsigned long, optionally limited to only positive numbers, limited to only positive numbers with fallback, or clamped to the range [clampedMin, clampedMax], and optionally with a default value defaultValue:

• The getter steps are:

  1. Let contentAttributeValue be the result of running this's get the content attribute.

  2. Let minimum be 0.

  3. If the reflected IDL attribute is limited to only positive numbers or limited to only positive numbers with fallback, then set minimum to 1.

  4. If the reflected IDL attribute is clamped to the range, then set minimum to clampedMin.

  5. Let maximum be 2147483647 if the reflected IDL attribute is not clamped to the range; otherwise clampedMax.

  6. If contentAttributeValue is not null:

     1. Let parsedValue be the result of non-negative integer parsing contentAttributeValue.

     2. If parsedValue is not an error and is in the range minimum to maximum, inclusive, then return parsedValue.

     3. If parsedValue is not an error and the reflected IDL attribute is clamped to the range:

        1. If parsedValue is less than minimum, then return minimum.

        2. Return maximum.

  7. If the reflected IDL attribute has a default value, then return defaultValue.

  8. Return minimum.

• The setter steps are:

  1. If the reflected IDL attribute is limited to only positive numbers and the given value is 0, then throw an "IndexSizeError" DOMException.

  2. Let minimum be 0.

  3. If the reflected IDL attribute is limited to only positive numbers or limited to only positive numbers with fallback, then set minimum to 1.

  4. Let newValue be minimum.

  5. If the reflected IDL attribute has a default value, then set newValue to defaultValue.

  6. If the given value is in the range minimum to 2147483647, inclusive, then set newValue to it.

  7. Run this's set the content attribute with newValue converted to the shortest possible string representing the number as a valid non-negative integer.

  Clamped to the range has no effect on the setter steps.

If a reflected IDL attribute has the type double, optionally limited to only positive numbers and optionally with a default value defaultValue:

• The getter steps are:

  1. Let contentAttributeValue be the result of running this's get the content attribute.

  2. If contentAttributeValue is not null:

     1. Let parsedValue be the result of floating-point number parsing contentAttributeValue.

     2. If parsedValue is not an error and is greater than 0, then return parsedValue.

     3. If parsedValue is not an error and the reflected IDL attribute is not limited to only positive numbers, then return parsedValue.

  3. If the reflected IDL attribute has a default value, then return defaultValue.

  4. Return 0.

• The setter steps are:

  1. If the reflected IDL attribute is limited to only positive numbers and the given value is not greater than 0, then return.

  2. Run this's set the content attribute with the given value, converted to the best representation of the number as a floating-point number.

The values Infinity and Not-a-Number (NaN) values throw an exception on setting, as defined in Web IDL. [WEBIDL]

If a reflected IDL attribute has the type DOMTokenList, then its getter steps are to return a DOMTokenList object whose associated element is this and associated attribute's local name is the reflected content attribute name. Specification authors cannot use support ElementInternals for IDL attributes of this type.

If a reflected IDL attribute has the type T?, where T is either Element or an interface that inherits from Element, then:

• Let attr be the reflected content attribute name.

• Its reflected target has an explicitly set attr-element, which is a weak reference to an element or null. It is initially null.

• Its reflected target has an attr-associated element. To compute the attr-associated element for such a reflected target reflectedTarget:

  1. Let element be the result of running reflectedTarget's get the element.

  2. Let contentAttributeValue be the result of running reflectedTarget's get the content attribute.

  3. If reflectedTarget's explicitly set attr-element is not null:

     • If reflectedTarget's explicitly set attr-element is a descendant of any of element's shadow-including ancestors, then return reflectedTarget's explicitly set attr-element.

     • Return null.

  4. Otherwise, if contentAttributeValue is not null, return the first element candidate, in tree order, that meets the following criteria:

     • candidate's root is the same as element's root,
     • candidate's ID is contentAttributeValue, and
     • candidate implements T.

     If no such element exists, then return null.

  5. Return null.

• The getter steps are to return this's attr-associated element.

• The setter steps are:

  1. If the given value is null, then:

     1. Set this's explicitly set attr-element to null.

     2. Run this's delete the content attribute.

     3. Return.

  2. Run this's set the content attribute with the empty string.

  3. Set this's explicitly set attr-element to a weak reference to the given value.

• For element reflected targets only: the following attribute change steps, given element, localName, oldValue, value, and namespace, are used to synchronize between the content attribute and the IDL attribute:

  1. If localName is not attr or namespace is not null, then return.

  2. Set element's explicitly set attr-element to null.

Reflected IDL attributes of this type are strongly encouraged to have their identifier end in "Element" for consistency.

If a reflected IDL attribute has the type FrozenArray<T>?, where T is either Element or an interface that inherits from Element, then:

• Let attr be the reflected content attribute name.

• Its reflected target has an explicitly set attr-elements, which is either a list of weak references to elements or null. It is initially null.

• Its reflected target has a cached attr-associated elements, which is a FrozenArray<T>?. It is initially null.

• Its reflected target has an attr-associated elements. To compute the attr-associated elements for such a reflected target reflectedTarget:

  1. Let elements be an empty list.

  2. Let element be the result of running reflectedTarget's get the element.

  3. If reflectedTarget's explicitly set attr-elements is not null:

     1. For each attrElement in reflectedTarget's explicitly set attr-elements:

        1. If attrElement is not a descendant of any of element's shadow-including ancestors, then continue.

        2. Append attrElement to elements.

  4. Otherwise:

     1. Let contentAttributeValue be the result of running reflectedTarget's get the content attribute.

     2. If contentAttributeValue is null, then return null.

     3. Let tokens be contentAttributeValue, split on ASCII whitespace.

     4. For each id of tokens:

        1. Let candidate be the first element, in tree order, that meets the following criteria:

           • candidate's root is the same as element's root,
           • candidate's ID is id, and
           • candidate implements T.

           If no such element exists, then continue.

        2. Append candidate to elements.

  5. Return elements.

• The getter steps are:

  1. Let elements be this's attr-associated elements.

  2. If the contents of elements is equal to the contents of this's cached attr-associated elements, then return this's cached attr-associated elements.

  3. Let elementsAsFrozenArray be elements, converted to a FrozenArray<T>?.

  4. Set this's cached attr-associated elements to elementsAsFrozenArray.

  5. Return elementsAsFrozenArray.

  This extra caching layer is necessary to preserve the invariant that element.reflectedElements === element.reflectedElements.

• The setter steps are:

  1. If the given value is null:

     1. Set this's explicitly set attr-elements to null.

     2. Run this's delete the content attribute.

     3. Return.

  2. Run this's set the content attribute with the empty string.

  3. Let elements be an empty list.

  4. For each element in the given value:

     1. Append a weak reference to element to elements.

  5. Set this's explicitly set attr-elements to elements.

• For element reflected targets only: the following attribute change steps, given element, localName, oldValue, value, and namespace, are used to synchronize between the content attribute and the IDL attribute:

  1. If localName is not attr or namespace is not null, then return.

  2. Set element's explicitly set attr-elements to null.

Reflected IDL attributes of this type are strongly encouraged to have their identifier end in "Elements" for consistency.

2.6.2 Using reflect in specifications

Reflection is primarily about improving web developer ergonomics by giving them typed access to content attributes through reflected IDL attributes. The ultimate source of truth, which the web platform builds upon, is the content attributes themselves. That is, specification authors must not use the reflected IDL attribute getter or setter steps, but instead must use the content attribute presence and value. (Or an abstraction on top, such as the state of an enumerated attribute.)

Two important exceptions to this are reflected IDL attributes whose type is one of the following:

• T?, where T is either Element or an interface that inherits from Element

• FrozenArray<T>?, where T is either Element or an interface that inherits from Element

For those, specification authors must use the reflected target's attr-associated element and attr-associated elements, respectively. The content attribute presence and value must not be used as they cannot be fully synchronized with the reflected IDL attribute.

A reflected target's explicitly set attr-element, explicitly set attr-elements, and cached attr-associated elements are to be treated as internal implementation details and not to be built upon.

2.6.3 Collections

The HTMLFormControlsCollection and HTMLOptionsCollection interfaces are collections derived from the HTMLCollection interface. The HTMLAllCollection interface is a collection, but is not so derived.

2.6.3.1 The HTMLAllCollection interface

The HTMLAllCollection interface is used for the legacy document.all attribute. It operates similarly to HTMLCollection; the main differences are that it allows a staggering variety of different (ab)uses of its methods to all end up returning something, and that it can be called as a function as an alternative to property access.

All HTMLAllCollection objects are rooted at a Document and have a filter that matches all elements, so the elements represented by the collection of an HTMLAllCollection object consist of all the descendant elements of the root Document.

Objects that implement the HTMLAllCollection interface are legacy platform objects with an additional [[Call]] internal method described in the section below. They also have an [[IsHTMLDDA]] internal slot.

[Exposed=Window,
 LegacyUnenumerableNamedProperties]
interface HTMLAllCollection {
  readonly attribute unsigned long length;
  getter Element (unsigned long index);
  getter (HTMLCollection or Element)? namedItem(DOMString name);
  (HTMLCollection or Element)? item(optional DOMString nameOrIndex);

  // Note: HTMLAllCollection objects have a custom [[Call]] internal method and an [[IsHTMLDDA]] internal slot.
};

The object's supported property indices are as defined for HTMLCollection objects.

The supported property names consist of the non-empty values of all the id attributes of all the elements represented by the collection, and the non-empty values of all the name attributes of all the "all"-named elements represented by the collection, in tree order, ignoring later duplicates, with the id of an element preceding its name if it contributes both, they differ from each other, and neither is the duplicate of an earlier entry.

The length getter steps are to return the number of nodes represented by the collection.

The indexed property getter must return the result of getting the "all"-indexed element from this given the passed index.

The namedItem(name) method steps are to return the result of getting the "all"-named element(s) from this given name.

The item(nameOrIndex) method steps are:

1. If nameOrIndex was not provided, return null.

2. Return the result of getting the "all"-indexed or named element(s) from this, given nameOrIndex.

The following elements are "all"-named elements: a, button, embed, form, frame, frameset, iframe, img, input, map, meta, object, select, and textarea

To get the "all"-indexed element from an HTMLAllCollection collection given an index index, return the index^th element in collection, or null if there is no such index^th element.

To get the "all"-named element(s) from an HTMLAllCollection collection given a name name, perform the following steps:

1. If name is the empty string, return null.

2. Let subCollection be an HTMLCollection object rooted at the same Document as collection, whose filter matches only elements that are either:

   • "all"-named elements with a name attribute equal to name, or,

   • elements with an ID equal to name.

3. If there is exactly one element in subCollection, then return that element.

4. Otherwise, if subCollection is empty, return null.

5. Otherwise, return subCollection.

To get the "all"-indexed or named element(s) from an HTMLAllCollection collection given nameOrIndex:

1. If nameOrIndex, converted to a JavaScript String value, is an array index property name, return the result of getting the "all"-indexed element from collection given the number represented by nameOrIndex.

2. Return the result of getting the "all"-named element(s) from collection given nameOrIndex.

2.6.3.1.1 [[Call]] ( thisArgument, argumentsList )

1. If argumentsList's size is zero, or if argumentsList[0] is undefined, return null.

2. Let nameOrIndex be the result of converting argumentsList[0] to a DOMString.

3. Let result be the result of getting the "all"-indexed or named element(s) from this HTMLAllCollection given nameOrIndex.

4. Return the result of converting result to an ECMAScript value.

The thisArgument is ignored, and thus code such as Function.prototype.call.call(document.all, null, "x") will still search for elements. (document.all.call does not exist, since document.all does not inherit from Function.prototype.)

2.6.3.2 The HTMLFormControlsCollection interface

The HTMLFormControlsCollection interface is used for collections of listed elements in form elements.

[Exposed=Window]
interface HTMLFormControlsCollection : HTMLCollection {
  // inherits length and item()
  getter (RadioNodeList or Element)? namedItem(DOMString name); // shadows inherited namedItem()
};

[Exposed=Window]
interface RadioNodeList : NodeList {
  attribute DOMString value;
};

collection.length

Returns the number of elements in collection.

element = collection.item(index)

element = collection[index]

Returns the item at index index in collection. The items are sorted in tree order.

element = collection.namedItem(name)

✔MDN

HTMLFormControlsCollection/namedItem

Support in all current engines.

Firefox33+Safari4+Chrome1+

---

Opera12.1+Edge79+

---

Edge (Legacy)12+Internet ExplorerNo

---

Firefox Android?Safari iOS1+Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+

radioNodeList = collection.namedItem(name)

element = collection[name]

radioNodeList = collection[name]

Returns the item with ID or name name from collection.

If there are multiple matching items, then a RadioNodeList object containing all those elements is returned.

radioNodeList.value

Returns the value of the first checked radio button represented by radioNodeList.

radioNodeList.value = value

Checks the first first radio button represented by radioNodeList that has value value.

The object's supported property indices are as defined for HTMLCollection objects.

The supported property names consist of the non-empty values of all the id and name attributes of all the elements represented by the collection, in tree order, ignoring later duplicates, with the id of an element preceding its name if it contributes both, they differ from each other, and neither is the duplicate of an earlier entry.

The namedItem(name) method must act according to the following algorithm:

1. If name is the empty string, return null and stop the algorithm.
2. If, at the time the method is called, there is exactly one node in the collection that has either an id attribute or a name attribute equal to name, then return that node and stop the algorithm.
3. Otherwise, if there are no nodes in the collection that have either an id attribute or a name attribute equal to name, then return null and stop the algorithm.
4. Otherwise, create a new RadioNodeList object representing a live view of the HTMLFormControlsCollection object, further filtered so that the only nodes in the RadioNodeList object are those that have either an id attribute or a name attribute equal to name. The nodes in the RadioNodeList object must be sorted in tree order.
5. Return that RadioNodeList object.

Members of the RadioNodeList interface inherited from the NodeList interface must behave as they would on a NodeList object.

The value IDL attribute on the RadioNodeList object, on getting, must return the value returned by running the following steps:

1. Let element be the first element in tree order represented by the RadioNodeList object that is an input element whose type attribute is in the Radio Button state and whose checkedness is true. Otherwise, let it be null.

2. If element is null, return the empty string.

3. If element is an element with no value attribute, return the string "on".

4. Otherwise, return the value of element's value attribute.

On setting, the value IDL attribute must run the following steps:

1. If the new value is the string "on": let element be the first element in tree order represented by the RadioNodeList object that is an input element whose type attribute is in the Radio Button state and whose value content attribute is either absent, or present and equal to the new value, if any. If no such element exists, then instead let element be null.

   Otherwise: let element be the first element in tree order represented by the RadioNodeList object that is an input element whose type attribute is in the Radio Button state and whose value content attribute is present and equal to the new value, if any. If no such element exists, then instead let element be null.

2. If element is not null, then set its checkedness to true.

2.6.3.3 The HTMLOptionsCollection interface

The HTMLOptionsCollection interface is used for collections of option elements. It is always rooted on a select element and has attributes and methods that manipulate that element's descendants.

[Exposed=Window]
interface HTMLOptionsCollection : HTMLCollection {
  // inherits item(), namedItem()
  [CEReactions] attribute unsigned long length; // shadows inherited length
  [CEReactions] setter undefined (unsigned long index, HTMLOptionElement? option);
  [CEReactions] undefined add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
  [CEReactions] undefined remove(long index);
  attribute long selectedIndex;
};

collection.length

Returns the number of elements in collection.

collection.length = value

When set to a smaller number than the existing length, truncates the number of option elements in the container corresponding to collection.

When set to a greater number than the existing length, if that number is less than or equal to 100000, adds new blank option elements to the container corresponding to collection.

element = collection.item(index)

element = collection[index]

Returns the item at index index in collection. The items are sorted in tree order.

collection[index] = element

When index is a greater number than the number of items in collection, adds new blank option elements in the corresponding container.

When set to null, removes the item at index index from collection.

When set to an option element, adds or replaces it at index index in collection.

element = collection.namedItem(name)

element = collection[name]

Returns the item with ID or name name from collection.

If there are multiple matching items, then the first is returned.

collection.add(element[, before])

Inserts element before the node given by before.

The before argument can be a number, in which case element is inserted before the item with that number, or an element from collection, in which case element is inserted before that element.

If before is omitted, null, or a number out of range, then element will be added at the end of the list.

Throws a "HierarchyRequestError" DOMException if element is an ancestor of the element into which it is to be inserted.

collection.remove(index)

Removes the item with index index from collection.

collection.selectedIndex

Returns the index of the first selected item, if any, or −1 if there is no selected item.

collection.selectedIndex = index

Changes the selection to the option element at index index in collection.

The object's supported property indices are as defined for HTMLCollection objects.

The length getter steps are to return the number of nodes represented by the collection.

The length setter steps are:

1. Let current be the number of nodes represented by the collection.

2. If the given value is greater than current, then:

   1. If the given value is greater than 100,000, then return.

   2. Let n be value − current.

   3. Append n new option elements with no attributes and no child nodes to the select element on which this is rooted. Mutation events must be fired as if a DocumentFragment containing the new option elements had been inserted.

3. If the given value is less than current, then:

   1. Let n be current − value.

   2. Remove the last n nodes in the collection from their parent nodes.

Setting length never removes or adds any optgroup elements, and never adds new children to existing optgroup elements (though it can remove children from them).

The supported property names consist of the non-empty values of all the id and name attributes of all the elements represented by the collection, in tree order, ignoring later duplicates, with the id of an element preceding its name if it contributes both, they differ from each other, and neither is the duplicate of an earlier entry.

When the user agent is to set the value of a new indexed property or set the value of an existing indexed property for a given property index index to a new value value, it must run the following algorithm:

1. If value is null, invoke the steps for the remove method with index as the argument, and return.

2. Let length be the number of nodes represented by the collection.

3. Let n be index minus length.

4. If n is greater than zero, then append a DocumentFragment consisting of n-1 new option elements with no attributes and no child nodes to the select element on which the HTMLOptionsCollection is rooted.

5. If n is greater than or equal to zero, append value to the select element. Otherwise, replace the indexth element in the collection by value.

The add(element, before) method must act according to the following algorithm:

1. If element is an ancestor of the select element on which the HTMLOptionsCollection is rooted, then throw a "HierarchyRequestError" DOMException.

2. If before is an element, but that element isn't a descendant of the select element on which the HTMLOptionsCollection is rooted, then throw a "NotFoundError" DOMException.

3. If element and before are the same element, then return.

4. If before is a node, then let reference be that node. Otherwise, if before is an integer, and there is a beforeth node in the collection, let reference be that node. Otherwise, let reference be null.

5. If reference is not null, let parent be the parent node of reference. Otherwise, let parent be the select element on which the HTMLOptionsCollection is rooted.

6. Pre-insert element into parent node before reference.

The remove(index) method must act according to the following algorithm:

1. If the number of nodes represented by the collection is zero, return.

2. If index is not a number greater than or equal to 0 and less than the number of nodes represented by the collection, return.

3. Let element be the indexth element in the collection.

4. Remove element from its parent node.

The selectedIndex IDL attribute must act like the identically named attribute on the select element on which the HTMLOptionsCollection is rooted

2.6.4 The DOMStringList interface

The DOMStringList interface is a non-fashionable retro way of representing a list of strings.

[Exposed=(Window,Worker)]
interface DOMStringList {
  readonly attribute unsigned long length;
  getter DOMString? item(unsigned long index);
  boolean contains(DOMString string);
};

New APIs must use sequence<DOMString> or equivalent rather than DOMStringList.

strings.length

Returns the number of strings in strings.

strings[index]

strings.item(index)

Returns the string with index index from strings.

strings.contains(string)

Returns true if strings contains string, and false otherwise.

Each DOMStringList object has an associated list.

The DOMStringList interface supports indexed properties. The supported property indices are the indices of this's associated list.

The length getter steps are to return this's associated list's size.

The item(index) method steps are to return the indexth item in this's associated list, or null if index plus one is greater than this's associated list's size.

The contains(string) method steps are to return true if this's associated list contains string, and false otherwise.