Promise.resolve 与解决

Question

I have this code:

var promise1 = new Promise(function(resolve, reject) {
  setTimeout(() => {
         console.warn('Elo');
         resolve('First response');
      },
      1000);
})

promise1
.then((resp) => {
    console.warn('First then!');
    
});

And It resolves promise after one second going to then and console.warning 'First then!'.

But when I change line:

resolve('First response');

for

Promise.resolve('First response');

It won't work. Some idea why ?

Also tried

return Promise.resolve('First response');

But it also doesn't work. I don't know why.

Can you help me understand it?

Answer 1

The new Promise constructor passes a specific function into your callback, which becomes your resolve parameter. That promise (the one you're constructing there with new Promise ) can only be resolved by calling that specific resolve function.

Promise.resolve simply creates a new "pre-resolved" promise. It does not resolve any existing promise (nor would it have any way of knowing which promise it's supposed to resolve).

Answer 2

Another way to think about it is:

Resolve === resolveCallback === first argument provided to your Promise callback Promise.resolve === a function to returns a new Promise

These two are functional equivalents:

`const foo = Promise.resolve('First response').then( /* ... */ );`

`const bar = new Promise(function(resolveCallback, rejectCallback) {
    resolveCallback('First response');
  }).then( /* ... */ );`