finally.js

class PromiseCopy{  
  
  finally( callback ){
    // resolveOrReject : forward current #state.
    // value : forward current #result.
    
    /** curried function to pass Promise.resolve or 
        Promise.reject forward based on current Promise #state. **/
    const commonCallback = ( resolveOrReject ) => ( value ) => {
      const response = callback?.() ;
      return Promise.resolve( response ).then( _ => resolveOrReject( value ) );
    } 
    return this.then( commonCallback( Promise.resolve ) , commonCallback( Promise.reject ) );
  }
}