This section describes features that apply most directly to Web browsers. Having said that, except where specified otherwise, the requirements defined in this section do apply to all user agents, whether they are Web browsers or not.
A browsing context is an environment in which Document
objects are presented to the user.
A tab or window in a Web browser typically contains a browsing
context, as does an iframe or frames in a
frameset.
A browsing context has a corresponding WindowProxy object.
A browsing context has an opener browsing context, which is null or a browsing context. It is initially null.
A browsing context has a disowned boolean. It is initially false.
A browsing context has an is closing boolean. It is initially false.
A browsing context has a session history, which lists the
Document objects that the browsing context has presented, is presenting,
or will present. A browsing context's active document is
its WindowProxy object's [[Window]]
internal slot value's associated
Document. A Document's browsing context is the
browsing context whose session history contains the
Document, if any such browsing context exists and has not been discarded, and null otherwise.
In general, there is a 1-to-1 mapping from the Window object to the
Document object, as long as the Document object has a non-null browsing context. There is one exception. A
Window can be reused for the presentation of a second Document in the
same browsing context, such that the mapping is then 1-to-2. This occurs when a
browsing context is navigated from the initial
about:blank Document to another, with replacement enabled.
A Document does not necessarily have a non-null browsing context. In particular, data mining tools are likely
to never instantiate browsing contexts. A Document created using an API such as createDocument() never has a non-null browsing context. And the Document originally
created for an iframe element, which has since been removed from the document, has no associated browsing context, since that
browsing context was discarded.
To set the active document of a browsing context
browsingContext to a Document object document, run these
steps:
Let window be document's relevant global object.
Per this standard document can be created before window, which does not make much sense. See issue #2688.
Set browsingContext's WindowProxy object's [[Window]] internal slot value to
window.
Set window's associated
Document to document.
Set window's relevant settings object's execution ready flag.
A browsing context has an associated creator origin (null or returns an origin), creator URL (null or returns a URL), and creator base URL (null or returns a URL). These are all initially null.
To determine the origin, given browsing context browsingContext, URL url, sandboxing flag set sandboxFlags, and two origins invocationOrigin and activeDocumentNavigationOrigin:
If sandboxFlags has its sandboxed origin browsing context flag set, then return a new opaque origin.
If url is null, then return a new opaque origin.
If activeDocumentOrigin is not null, and url's scheme is "javascript", then return
activeDocumentNavigationOrigin.
If invocationOrigin is not null, and url is
about:blank, then return invocationOrigin.
If url is about:srcdoc, then return the origin of
browsingContext's browsing context container's node
document.
Return url's origin.
To create a new browsing context, given
null or a Document object creator and browsing context group
group:
Let browsingContext be a new browsing context.
If creator is non-null, then set browsingContext's creator origin to return creator's origin, browsingContext's creator URL to return creator's URL, and browsingContext's creator base URL to return creator's base URL.
Let sandboxFlags be the result of determining active sandboxing flags given browsingContext.
Let origin be the result of determining the
origin given browsingContext, about:blank,
sandboxFlags, browsingContext's creator origin, and null.
Let featurePolicy be the result of creating a feature policy given browsingContext and origin. [FEATUREPOLICY]
Let agent be the result of obtaining a similar-origin window agent given origin and group.
Let realm execution context be the result of creating a new JavaScript realm with the following customizations:
For the agent, use agent. This pointer is not yet defined in the JavaScript specification; see tc39/ecma262#1357.
For the global object, create a new Window object.
For the global this binding, use browsingContext's
WindowProxy object.
Set up a window environment settings object with realm execution context, and let settingsObject be the result.
Let document be a new Document, marked as an HTML document in quirks mode, whose content type is "text/html",
origin is origin, active sandboxing flag set is
sandboxFlags, feature policy is
feature policy, and which is both ready for post-load tasks and
completely loaded immediately.
Ensure that document has a single child html node, which itself
has two empty child nodes: a head element, and a body element.
Set the active document of browsingContext to document.
If browsingContext's creator URL is non-null, then set document's referrer to the serialization of it.
If creator is non-null, then set document's referrer policy to creator's referrer policy.
Add document to browsingContext's session history.
Return browsingContext.
To create a new top-level browsing context:
Let group be the result of creating a new browsing context group.
Return group's browsing context set[0].
This creates a top-level browsing context.
To create a new auxiliary browsing context, given a browsing context opener:
Let group be opener's top-level browsing context's group
Assert: group is non-null, as navigating invokes this directly.
Let browsingContext be the result of creating a new browsing context with opener's active document and group.
Append browsingContext to group.
Set browsingContext's opener browsing context to opener.
Assert: browsingContext's creator origin is non-null.
If browsingContext's creator origin is
same origin with browsingContext's active document's
origin, then copy the sessionStorage
storage area of opener into browsingContext's set of session storage areas.
These areas must be considered separate, not affecting each other in any way.
Return browsingContext.
This creates a top-level browsing context that is also an auxiliary browsing context.
To create a new nested browsing context, given an element element:
Let browsingContext be the result of creating a new browsing context with element's node document and element's node document's browsing context's top-level browsing context's group.
Set element's nested browsing context to browsingContext.
If element has a name attribute, then set
browsingContext's name to the value of
this attribute.
Certain elements (for example, iframe elements) can instantiate further browsing contexts. These elements are called browsing
context containers.
Each browsing context container has a nested browsing context, which is either a browsing context or null.
If a browsing context is the nested browsing context of a browsing context container, then the browsing context is said to be nested through the browsing context container's node document.
A browsing context child is said to be a child browsing context of another browsing context parent, if all of the following conditions hold:
child is a nested browsing context of a browsing context container element
element is connected
element's shadow-including root's browsing context is parent
A browsing context child is then a document-tree child browsing context of parent if it is a child browsing context and its browsing context container is not just connected, but also in a document tree.
A browsing context child may have a parent browsing context. This is the unique browsing context that has child as a child browsing context, if any such browsing context exists. Otherwise, the browsing context has no parent browsing context.
A browsing context A is said to be an ancestor of a browsing context B if there exists a browsing context A' that is a child browsing context of A and that is itself an ancestor of B, or if the browsing context A is the parent browsing context of B.
A browsing context that has no parent browsing context is the top-level browsing context for itself and all of the browsing contexts for which it is an ancestor browsing context.
A top-level browsing context has an associated group (null or a browsing context group). It is initially null.
It is possible to create new browsing contexts that are related to a top-level browsing context without being nested through an element. Such browsing contexts are called auxiliary browsing contexts. Auxiliary browsing contexts are always top-level browsing contexts.
The transitive closure of parent browsing contexts for a browsing context that is a nested browsing context gives the list of ancestor browsing contexts.
The list of the descendant browsing contexts of a Document d
is the (ordered) list returned by the following algorithm:
Let list be an empty list.
For each child browsing context of d that is nested through an element that is in
the Document d, in the tree order of the elements
nesting those browsing contexts, run these substeps:
Append that child browsing context to the list list.
Append the list of the descendant browsing contexts of the active document of that child browsing context to the list list.
Return the constructed list.
A Document is said to be fully
active when its browsing context is non-null and
it is the active document of that browsing context, and either its browsing context is a top-level browsing context,
or it has a parent browsing context and the Document through which it is nested is itself fully active.
Because they are associated with an element, child
browsing contexts are always tied to a specific Document in their parent
browsing context. User agents must not allow the user to interact with child browsing contexts of elements that are in Documents
that are not themselves fully active.
The following example illustrates the differences between active and fully active Document objects. Here a.html is loaded into a browser window, b-1.html starts
out loaded into an iframe as shown, and b-2.html and c.html are omitted (they can simply be an empty document).
<!-- a.html -->
<!DOCTYPE html>
<html lang="en">
<title>Browsing context A</title>
<iframe src="b-1.html"></iframe>
<button onclick="frames[0].location.href = 'b-2.html'">Click me</button>
<!-- b-1.html -->
<!DOCTYPE html>
<html lang="en">
<title>Browsing context B</title>
<iframe src="c.html"></iframe>At this point, the documents given by a.html, b-1.html, and c.html are all the active documents of their respective browsing
contexts. They are also all fully active.
After clicking on the button, and thus loading a new Document from
b-2.html into browsing context B, we have the following results:
The a.html Document remains both the active
document of browsing context A, and fully active.
The b-1.html Document is now not the
active document of browsing context B. As such it is also not fully
active.
The new b-2.html Document is now the active
document of browsing context B, and is also fully active.
The c.html Document is still the active
document of browsing context C. However, since it is nested through the b-1.html Document,
which is itself not fully active, this means the c.html
Document is now not fully active (even though it is active).
For more explorations of the complexities involved here, especially as it impacts the session history, see A Model of Navigation History. [NAVMODEL]
A browsing context that is a nested browsing context can be put into
a delaying load events mode. This is used when it is
navigated, to delay the load event of its
browsing context container before the new Document is created.
The document family of a browsing context consists of the union of all
the Document objects in that browsing context's session
history and the document families of all those
Document objects. The document family of a Document object
consists of the union of all the document families of the
browsing contexts that are nested through the Document object.
The content document of a browsing context container container is the result of the following algorithm:
If container's nested browsing context is null, then return null.
Let context be container's nested browsing context.
Let document be context's active document.
If document's origin and the origin specified by the current settings object are not same origin-domain, then return null.
Return document.
topReturns the WindowProxy for the top-level browsing context.
opener [ = value ]Returns the WindowProxy for the opener browsing context.
Returns null if there isn't one or if it has been set to null.
Can be set to null.
parentReturns the WindowProxy for the parent browsing context.
frameElementReturns the Element for the browsing context container.
Returns null if there isn't one, and in cross-origin situations.
The top attribute's getter must run these steps:
If this Window object's browsing context is
null, then return null.
Return this Window object's browsing
context's top-level browsing context's WindowProxy
object.
The opener attribute's getter must run these
steps:
Let current be this Window object's browsing context.
If current is null, then return null.
If current's disowned is true, then return null.
If current's opener browsing context is null, then return null.
Return current's opener browsing context's
WindowProxy object.
The opener attribute's setter must run these steps:
If the given value is null and this Window object's browsing context is non-null, then set this Window object's browsing context's disowned to true.
If the given value is non-null, then return ? OrdinaryDefineOwnProperty(this
Window object, "opener", { [[Value]]: the given value,
[[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }).
If a browsing context's disowned is true, its window.opener attribute is null. That prevents scripts in the
browsing context from changing any properties of its opener browsing
context's Window object (i.e., the Window object from which the
browsing context was created).
Otherwise, if a browsing context's disowned is false, then scripts
in that browsing context can use window.opener to
change properties of its opener browsing context's Window object. For
example, a script running in the browsing context can change the value of window.opener.location, causing the opener browsing context to
navigate to a completely different document.
The parent attribute's getter must run these
steps:
Let current be this Window object's browsing context.
If current is null, then return null.
If current is a child browsing context of another browsing
context parent, then return parent's WindowProxy
object.
Assert: current is a top-level browsing context.
Return current's WindowProxy object.
The frameElement attribute's getter must run
these steps:
Let current be this Window object's browsing context.
If current is null, then return null.
If current is not a child browsing context, then return null.
Let container be current's browsing context container.
If container's node document's origin is not same origin-domain with the current settings object's origin, then return null.
Return container.
An example of when these IDL attributes can return null is as follows:
<!DOCTYPE html>
<iframe></iframe>
<script>
"use strict";
const element = document.querySelector("iframe");
const iframeWindow = element.contentWindow;
element.remove();
console.assert(iframeWindow.top === null);
console.assert(iframeWindow.parent === null);
console.assert(iframeWindow.frameElement === null);
</script>Here the browsing context corresponding to iframeWindow
was discarded when element was removed from the document.
A browsing context A is familiar with a second browsing context B if one of the following conditions is true:
A browsing context A is allowed to navigate a second browsing context B if the following algorithm returns true:
If A is not the same browsing context as B, and A is not one of the ancestor browsing contexts of B, and B is not a top-level browsing context, and A's active document's active sandboxing flag set has its sandboxed navigation browsing context flag set, then return false.
Otherwise, if B is a top-level browsing context, and is one of the ancestor browsing contexts of A, then:
If A's WindowProxy's [[Window]] value has transient
activation and A's active document's active sandboxing
flag set has its sandboxed top-level navigation with user activation browsing
context flag set, then return false.
Otherwise, if A's WindowProxy's [[Window]] value does not have
transient activation and A's active document's
active sandboxing flag set has its sandboxed top-level navigation without
user activation browsing context flag set, then return false.
Otherwise, if B is a top-level browsing context, and is
neither A nor one of the ancestor
browsing contexts of A, and A's
Document's active sandboxing flag set has its sandboxed
navigation browsing context flag set, and A is not the one
permitted sandboxed navigator of B, then return false.
Return true.
An element has a browsing context scope origin if its Document's
browsing context is a top-level browsing
context or if all of its Document's ancestor browsing contexts all have active
documents whose origin are the same origin as the element's
node document's origin. If an element has a browsing context scope
origin, then its value is the origin of the element's node
document.
A user agent holds a browsing context group set (a set of browsing context groups).
A browsing context group holds a browsing context set (a set of top-level browsing contexts).
A browsing context group has an associated agent cluster map (a weak map of agent cluster keys to agent clusters). User agents are responsible for collecting agent clusters when it is deemed that nothing can access them anymore.
To create a new browsing context group, run these steps:
Let group be a new browsing context group.
Append group to the user agent's browsing context group set.
Let browsingContext be the result of creating a new browsing context with null and group.
Append browsingContext to group.
Return group.
To append a top-level browsing context browsingContext to a browsing context group group, run these steps:
Append browsingContext to group's browsing context set.
Set browsingContext's group to group.
To remove a top-level browsing context browsingContext, run these steps:
Assert: browsingContext's group is non-null, because a browsing context only gets discarded once.
Let group be browsingContext's group.
Set browsingContext's group to null.
Remove browsingContext from group's browsing context set.
If group's browsing context set is empty, then remove group from the user agent's browsing context group set.
Append and remove are primitive operations that help define the lifetime of a browsing context group. They are called from creating a new browsing context group, creating a new auxiliary browsing context, and discarding a browsing context.
The HTML Standard used to define "" and "". These have been removed as they were not adequate.
Browsing contexts can have a browsing context name. Unless stated otherwise, it is the empty string.
A valid browsing context name is any string with at least one character that does not start with a U+005F LOW LINE character. (Names starting with an underscore are reserved for special keywords.)
A valid browsing context name or keyword is any string that is either a valid
browsing context name or that is an ASCII case-insensitive match for one of:
_blank, _self, _parent, or
_top.
These values have different meanings based on whether the page is sandboxed or not, as
summarized in the following (non-normative) table. In this table, "current" means the
browsing context that the link or script is in, "parent" means the parent
browsing context of the one the link or script is in, "top" means the top-level
browsing context of the one the link or script is in, "new" means a new top-level
browsing context or auxiliary browsing context is to be created, subject to
various user preferences and user agent policies, "none" means that nothing will happen, and
"maybe new" means the same as "new" if the "allow-popups" keyword is also specified on the
sandbox attribute (or if the user overrode the
sandboxing), and the same as "none" otherwise.
| Keyword | Ordinary effect | Effect in an iframe with...
| |
|---|---|---|---|
sandbox=""
| sandbox="allow-top-navigation"
| ||
| none specified, for links and form submissions | current | current | current |
| empty string | current | current | current |
_blank
| new | maybe new | maybe new |
_self
| current | current | current |
_parent if there isn't a parent
| current | current | current |
_parent if parent is also top
| parent/top | none | parent/top |
_parent if there is one and it's not top
| parent | none | none |
_top if top is current
| current | current | current |
_top if top is not current
| top | none | top |
| name that doesn't exist | new | maybe new | maybe new |
| name that exists and is a descendant | specified descendant | specified descendant | specified descendant |
| name that exists and is current | current | current | current |
| name that exists and is an ancestor that is top | specified ancestor | none | specified ancestor/top |
| name that exists and is an ancestor that is not top | specified ancestor | none | none |
| other name that exists with common top | specified | none | none |
| name that exists with different top, if familiar and one permitted sandboxed navigator | specified | specified | specified |
| name that exists with different top, if familiar but not one permitted sandboxed navigator | specified | none | none |
| name that exists with different top, not familiar | new | maybe new | maybe new |
Most of the restrictions on sandboxed browsing contexts are applied by other algorithms, e.g. the navigation algorithm, not the rules for choosing a browsing context given below.
The rules for choosing a browsing context, given a browsing context name name, a browsing context current, and a boolean noopener are as follows:
Let chosen be null.
Let new be false.
Let sandboxingFlagSet be current's active document's active sandboxing flag set.
If name is the empty string or an ASCII case-insensitive match for
"_self", then set chosen to current.
Otherwise, if name is an ASCII case-insensitive match for "_parent", set chosen to current's parent browsing
context, if any, and current otherwise.
Otherwise, if name is an ASCII case-insensitive match for "_top", set chosen to current's top-level browsing
context, if any, and current otherwise.
Otherwise, if name is not an ASCII case-insensitive match for "_blank", there exists a browsing context whose name is the same as name, current is familiar
with that browsing context, and the user agent determines that the two browsing contexts
are related enough that it is ok if they reach each other, set chosen to that
browsing context. If there are multiple matching browsing contexts, the user agent should set
chosen to one in some arbitrary consistent manner, such as the most recently opened,
most recently focused, or more closely related.
This will be made more precise in issue #313.
Otherwise, a new browsing context is being requested, and what happens depends on the user agent's configuration and abilities — it is determined by the rules given for the first applicable option from the following list:
WindowProxy's [[Window]] value does
not have transient activation and the user agent has been configured to not show
popups (i.e. the user agent has a "popup blocker" enabled)The user agent may inform the user that a popup has been blocked.
The user agent may offer the user one of:
Set chosen to the result of creating a new top-level browsing context and set new to true.
Set chosen to an existing top-level browsing context.
If this case occurs, it means that an author has explicitly sandboxed the document that is trying to open a link.
If the user declines or the user agent doesn't offer the above, the variables remain unchanged.
Set new to true.
If noopener is true, then set chosen to the result of creating a new top-level browsing context.
Otherwise:
Set chosen to the result of creating a new auxiliary browsing context with current.
If sandboxingFlagSet's sandboxed navigation browsing context flag is set, then current must be set as chosen's one permitted sandboxed navigator.
If sandboxingFlagSet's sandbox propagates to auxiliary browsing contexts flag is set, then all the flags that are set in sandboxingFlagSet must be set in chosen's popup sandboxing flag set.
If name is not an ASCII case-insensitive match for "_blank", then set chosen's name to name.
If the newly created browsing context is immediately navigated, then the navigation will be done with replacement enabled.
Set chosen to current.
Do nothing.
User agents are encouraged to provide a way for users to configure the user agent to always reuse current.
Return chosen and new.
Window,
WindowProxy, and Location objectsAlthough typically objects cannot be accessed across origins, the web platform would not be true to itself if it did not have some legacy exceptions to that rule that the web depends upon.
When perform a security check is invoked, with a platformObject, identifier, and type, run these steps:
If platformObject is a Window or Location object,
then:
Repeat for each e that is an element of ! CrossOriginProperties(platformObject):
If SameValue(e.[[Property]], identifier) is true, then:
If type is "method" and e has neither
[[NeedsGet]] nor [[NeedsSet]], then return.
Otherwise, if type is "getter" and
e.[[NeedsGet]] is true, then return.
Otherwise, if type is "setter" and
e.[[NeedsSet]] is true, then return.
If ! IsPlatformObjectSameOrigin(platformObject) is false, then
throw a "SecurityError" DOMException.
Window and Location objects both have a
[[CrossOriginPropertyDescriptorMap]] internal slot, whose value is initially an empty
map.
The [[CrossOriginPropertyDescriptorMap]] internal slot contains a map
with entries whose keys are (currentGlobal, objectGlobal,
propertyKey)-tuples and values are property descriptors, as a memoization of what is
visible to scripts when currentGlobal inspects a Window or
Location object from objectGlobal. It is filled lazily by
CrossOriginGetOwnPropertyHelper, which consults it on future lookups.
User agents should allow a value held in the map to be garbage collected along with its corresponding key when nothing holds a reference to any part of the value. That is, as long as garbage collection is not observable.
For example, with const href =
Object.getOwnPropertyDescriptor(crossOriginLocation, "href").set the value and its
corresponding key in the map cannot be garbage collected as that would be observable.
User agents may have an optimization whereby they remove key-value pairs from the map when
document.domain is set. This is not observable as document.domain cannot revisit an earlier value.
For example, setting document.domain
to "example.com" on www.example.com means user agents can remove all
key-value pairs from the map where part of the key is www.example.com, as that can never be part
of the origin again and therefore the corresponding value could never be retrieved
from the map.
If O is a Location object, then return «
{ [[Property]]: "href", [[NeedsGet]]: false, [[NeedsSet]]: true },
{ [[Property]]: "replace" } ».
Let crossOriginWindowProperties be «
{ [[Property]]: "window", [[NeedsGet]]: true, [[NeedsSet]]: false },
{ [[Property]]: "self", [[NeedsGet]]: true, [[NeedsSet]]: false },
{ [[Property]]: "location", [[NeedsGet]]: true, [[NeedsSet]]: true },
{ [[Property]]: "close" },
{ [[Property]]: "closed", [[NeedsGet]]: true, [[NeedsSet]]: false },
{ [[Property]]: "focus" },
{ [[Property]]: "blur" },
{ [[Property]]: "frames", [[NeedsGet]]: true, [[NeedsSet]]: false },
{ [[Property]]: "length", [[NeedsGet]]: true, [[NeedsSet]]: false },
{ [[Property]]: "top", [[NeedsGet]]: true, [[NeedsSet]]: false },
{ [[Property]]: "opener", [[NeedsGet]]: true, [[NeedsSet]]: false },
{ [[Property]]: "parent", [[NeedsGet]]: true, [[NeedsSet]]: false },
{ [[Property]]: "postMessage" } ».
Repeat for each e that is an element of O's document-tree child browsing context name property set:
Add { [[Property]]: e, [[HideFromKeys]]: true } as the last element of crossOriginWindowProperties.
Return crossOriginWindowProperties.
Indexed properties do not need to be safelisted as they are handled directly by
the WindowProxy object.
If P is "then", @@toStringTag,
@@hasInstance, or @@isConcatSpreadable, then return
PropertyDescriptor{
[[Value]]: undefined,
[[Writable]]: false,
[[Enumerable]]: false,
[[Configurable]]: true }.
Throw a "SecurityError" DOMException.
Return true if the current settings object's origin is same origin-domain with O's relevant settings object's origin, and false otherwise.
If this abstract operation returns undefined and there is no custom behavior, the
caller needs to throw a "SecurityError" DOMException. In
practice this is handled by the caller calling CrossOriginPropertyFallback.
Let crossOriginKey be a tuple consisting of the current settings object, O's relevant settings object, and P.
Repeat for each e that is an element of ! CrossOriginProperties(O):
If SameValue(e.[[Property]], P) is true, then:
If the value of the [[CrossOriginPropertyDescriptorMap]] internal slot of O contains an entry whose key is crossOriginKey, then return that entry's value.
Let originalDesc be OrdinaryGetOwnProperty(O, P).
Let crossOriginDesc be undefined.
If e.[[NeedsGet]] and e.[[NeedsSet]] are absent, then:
Let value be originalDesc.[[Value]].
If ! IsCallable(value) is true, then set value to an anonymous built-in function, created in the current Realm Record, that performs the same steps as the IDL operation P on object O.
Set crossOriginDesc to PropertyDescriptor{ [[Value]]: value, [[Enumerable]]: false, [[Writable]]: false, [[Configurable]]: true }.
Otherwise:
Let crossOriginGet be undefined.
If e.[[NeedsGet]] is true, then set crossOriginGet to an anonymous built-in function, created in the current Realm Record, that performs the same steps as the getter of the IDL attribute P on object O.
Let crossOriginSet be undefined.
If e.[[NeedsSet]] is true, then set crossOriginSet to an anonymous built-in function, created in the current Realm Record, that performs the same steps as the setter of the IDL attribute P on object O.
Set crossOriginDesc to PropertyDescriptor{ [[Get]]: crossOriginGet, [[Set]]: crossOriginSet, [[Enumerable]]: false, [[Configurable]]: true }.
Create an entry in the value of the [[CrossOriginPropertyDescriptorMap]] internal slot of O with key crossOriginKey and value crossOriginDesc.
Return crossOriginDesc.
Return undefined.
The reason that the property descriptors produced here are configurable is to preserve the invariants of the essential internal methods required by the JavaScript specification. In particular, since the value of the property can change as a consequence of navigation, it is required that the property be configurable. (However, see tc39/ecma262 issue #672 and references to it elsewhere in this specification for cases where we are not able to preserve these invariants, for compatibility with existing Web content.) [JAVASCRIPT]
The reason the property descriptors are non-enumerable, despite this mismatching the same-origin behavior, is for compatibility with existing Web content. See issue #3183 for details.
Let desc be ? O.[[GetOwnProperty]](P).
Assert: desc is not undefined.
If ! IsDataDescriptor(desc) is true, then return desc.[[Value]].
Assert: IsAccessorDescriptor(desc) is true.
Let getter be desc.[[Get]].
If getter is undefined, then throw a "SecurityError"
DOMException.
Return ? Call(getter, Receiver).
Let desc be ? O.[[GetOwnProperty]](P).
Assert: desc is not undefined.
If desc.[[Set]] is present and its value is not undefined, then:
Perform ? Call(setter, Receiver, «V»).
Return true.
Throw a "SecurityError" DOMException.
Let keys be a new empty List.
Repeat for each e that is an element of ! CrossOriginProperties(O):
If e.[[HideFromKeys]] is not true, append e.[[Property]] to keys.
If keys does not contain "then", then append "then" to keys.
Return the concatenation of keys and « @@toStringTag, @@hasInstance, @@isConcatSpreadable ».