JavaScript promise resolving with setTimeout

问题:

I don't understand why the first setTimeout function works but the second one doesn't. The first one is commented out when I run the second setTimeout. But instead of resolving after 3 seconds it resolves immediately.

I'm new to the whole 'promise' thing and the tutorial I'm working through uses promises with setTimeout a lot.

  let promise = new Promise( ( resolve, reject ) => {

     /* why does setTimeout work with this one... */
     setTimeout( () => resolve( 'Job\'s done!!!' ), 3000 );

     /* but not with this one */
     setTimeout( resolve('done'), 3000 );
  } );

  promise.then(
     result => alert( result )
  );

回答1:

/* why does setTimeout work with this one... */
 setTimeout( () => resolve( 'Job\'s done!!!' ), 3000 );

when the timeout occur you call a function () => ... wich when executed till resolve the promise

/* but not with this one */
 setTimeout( resolve('done'), 3000 );

here you actually resolve the promise (you execute the result function) and pass the result to the setTimeout function.

Writing

() => resolve( 'Job\'s done!!!' )

is the same as

function() {
    resolve( 'Job\'s done!!!' );
}