then.js

class PromiseCopy{
    
    then( successcallback , failurecallback ){
        return new PromiseCopy( ( resolve, reject ) => { 
            /** 
              1. handlers is an Array which will store success handlers and error handlers.
              2. Each entry will have :
                - success method called with resolved value.
                - failure method called with rejected value.
            */
            this.#handlers.push( {
                success( value ){
                    if( !successcallback )return resolve( value );

                    try{
                        resolve( successcallback( value ) );
                    }
                    catch( e ){
                        reject( e )
                    }
                },
                fail( error ){
                    if( !failurecallback )return reject( error );

                    try{
                        resolve( failurecallback( error ) );
                    }
                    catch( e ){
                        reject( e )
                    }
                }
             } );

           // execute handlers if promise is already settled.
           this.#dispatchCallbacks(); 
        });
    }
}