Make customElements.define faster

third_party/WebKit/Source/core/dom/custom/README.md

 # Custom Elements
 
 Custom elements let authors create their own elements, with their own
 methods, behavior, and attribute handling. Custom elements shipped in
 M33. We colloquially refer to that version as "v0."
 
-Contact Dominic Cooney
-([dominicc@chromium.org](mailto:dominicc@chromium.org)) with
+Contact [dom-dev@chromium.org](mailto:dom-dev@chromium.org) with
 questions.
 
 ### Code Location
 
 The custom elements implementation is split between core/dom and
 bindings/core/v8.
 
 ## Design
 
 ### Some Important Classes
 
 ###### CustomElementDefinition
 
 The definition of one ‘class’ of element. This type is
 abstract to permit different kinds of definitions, although at the
 moment there is only one: ScriptCustomElementDefinition.
 
-ScriptCustomElementDefinition is linked to its constructor by an ID
-number. The ID number is stored in a map, keyed by constructor, in a
-hidden value of the CustomElementRegistry wrapper. The ID is an index
-into a list of definitions stored in V8PerContextData.
+###### ScriptCustomElementDefinition
+
+A constructor and its callbacks. To go from a custom element
+definition to a constructor, just access the member.
+
+Mapping a constructor back to a definition is more complicated because
+the same constructor could be defined as a custom element in different
+contexts. Each context has its own v8::Private that's a key to the
+property on the constructor; the value is an index into the table of
+definitions in the custom element registry.
 
 ###### CustomElementDescriptor
 
 A tuple of local name, and custom element name. For autonomous custom
 elements, these strings are the same; for customized built-in elements
 these strings will be different. In that case, the local name is the
-element's tag name and the custom element name is related to the value
-of the “is” attribute.
+element's local name and the custom element name is related to the
+value of the “is” attribute.
 
 ###### CustomElementRegistry
 
 Implements the `window.customElements` property. This maintains the
-set of registered names. The wrapper of this object is used by
-ScriptCustomElementDefinition to cache state in V8.
+set of registered names and a list of registered definitions.
 
-###### V8HTMLElement Constructor
+###### V8HTMLElementConstructor
 
 The `HTMLElement` interface constructor. When a custom element is
 created from JavaScript all we have to go on is the constructor (in
-`new.target`); this uses ScriptCustomElementDefinition's state to find
-the definition to use.
-
-### Memory Management
-
-Once defined, custom element constructors and prototypes have to be
-kept around indefinitely because they could be created in future by
-the parser. On the other hand, we must not leak when a window can no
-longer run script.
-
-We use a V8HiddenValue on the CustomElementRegistry wrapper which
-points to a map that keeps constructors and prototypes alive. See
-ScriptCustomElementDefinition.
+`new.target`); this uses the context to map the constructor back to
+its definition as described in ScriptCustomElementDefintion above.
 
 ## Style Guide
 
 In comments and prose, write custom elements, not Custom Elements, to
 match the HTML standard.
 
 Prefix type names with CustomElement (singular).
 
 ## Testing
 
@@ skipping 75 matching lines @@
    may remove it even sooner, if we have evidence that sites are using
    feature detection correctly.
 
 ## References
 
 These have links to the parts of the DOM and HTML specs which define
 custom elements:
 
 * [WHATWG DOM Wiki: Custom Elements](https://github.com/whatwg/dom/wiki#custom-elements)
 * [WHATWG HTML Wiki: Custom Elements](https://github.com/whatwg/html/wiki#custom-elements)