A class or another interface is the only type that an interface is allowed to extend

We are currently using typescript version 2.9.2

I encountered an issue while trying to extend the interface DropDownOption. I received the error "error TS2312: An interface may only extend a class or another interface."

Is there an alternate approach to defining this that does not involve using types so that it remains a pure interface and can still be extended?

export interface BaseDropdownOption {
  id: string;
}
export interface ValueDropdownOption extends BaseDropdownOption {
  value: string;
  template?: never;
}
export interface TemplateDropdownOption extends BaseDropdownOption {
  template: TemplateRef<any>;
  value?: never;
}
export type DropdownOption = ValueDropdownOption | TemplateDropdownOption;

For instance, the following snippet results in the aforementioned error:

export interface DropdownOptionWithPayload extends DropdownOption {
  payload: any
  name: string
}

Answer №1

In order to enhance a union, consider utilizing an intersection method to incorporate additional properties:

export type EnhancedDropdownOption = DropdownOption & {
  payload: any
  name: string
}

This can also be expressed as:

export type EnhancedDropdownOption = 
     | (ValueDropdownOption & { payload: any; name: string; }) 
     | (TemplateDropdownOption & { payload: any; name: string; })

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