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 CustomElementRegistry interface returns the constructor for a previously-defined custom element.
The getName() method of the CustomElementRegistry interface returns the name for a previously-defined custom element.
The upgrade() method of the CustomElementRegistry interface upgrades all shadow-containing custom elements in a Node subtree, even before they are connected to the main document.
The whenDefined() method of the CustomElementRegistry interface returns a Promise that resolves when the named element is defined.
The
CustomElementRegistryinterface provides methods for registering custom elements and querying registered elements. To get an instance of it, use the window.customElements property.MDN Reference