Discussion on: Javascript fetch, retry upon failure.

[enoch1017]

Hello, I am a beginner of javascript . After watching your blog about promise ,especially this part-->
.....
.catch(function(error) {
fetch_retry(url, options, n - 1)
.then(resolve) // <--- simply resolve
.catch(reject); // <--- simply reject
})

I don't understand why resolve & reject can be included in then & catch functions directly... Is anything omitted?
Could you tell me why or where I can reference about this issue?
Sorry for my poor English.
I truely appreciate for your time.

[YCM Jason]

Thanks for your comments! I am not sure if I understand your question correctly.

But it sounds like you are confused with the idea of "functions as first-class citizens", which is a fancy way of saying you can pass functions as a values around.

Are you familiar with setTimeout? Have a read about it here.

Consider

setTimeout(function() {
  print('hello')
}, 3000);

We are passing a function function () { print('hello'); } to setTimeout as the first argument. So if we call that function f, then we can equivilantly pass f to setTimeout.

const f = function () {
  print('hello');
}
setTimeout(f, 3000);

So in another words, my code:

.catch(function(error) {
  fetch_retry(url, options, n - 1)
    .then(resolve)
    .catch(reject);
})

Is basically equivalent to:

.catch(function(error) {
  fetch_retry(url, options, n - 1)
    .then(() => resolve())
    .catch(() => reject());
})

It is also important to note that the following is incorrect (or not the same):

.catch(function(error) {
  fetch_retry(url, options, n - 1)
    .then(resolve())
    .catch(reject());
})

Because Javascript will first evaluate resolve() and use the value it returns to pass on to then.

I hope I did help a little.