promise.js

const error = null ; 
//const error = 'error' ; // change to a truthy value to reject promise.
const promise = new Promise( ( resolve, reject ) => {
    //asynchronous operation 
    setTimeout( () => {
       return error ? reject( error ) : resolve( 'final data' ) ;
    }, 3000 );
} );

// before 3 seconds:
Promise {<pending>} 
[[PromiseState]]: "pending"
[[PromiseResult]]: undefined

//resolved after 3 seconds :
Promise {<fulfilled>: 'final data'}
[[PromiseState]]: "fulfilled"
[[PromiseResult]]: "final data"
         
//rejected after 3 seconds :
Promise {<rejected>: 'error'}
[[PromiseState]]: "rejected"
[[PromiseResult]]: "error"