The define() method of the CustomElementRegistry interface adds a definition for a custom element to the custom element registry, mapping its name to the constructor which will be used to create it.
Optionaloptions: ElementDefinitionOptionsThe get() method of the previously-defined custom element.
The getName() method of the previously-defined custom element.
The upgrade() method of the elements in a Node subtree, even before they are connected to the main document.
The whenDefined() method of the resolves when the named element is defined.
The
CustomElementRegistryinterface provides methods for registering custom elements and querying registered elements.MDN Reference