JavaScript Promises: chaining promises with non-promise objects. Why it works? - Stack Overflow

admin管理员组

文章数量:1325229

I'm learning about JavaScript Fetch API and I'm a bit confusing about Promises.

Consider this dummy example that prints "ok" in the console:

fetch(".")
.then(function(response) { // first then() call
    return response;
}).then(function(response) { // second then() call
    console.log("ok");
});

The page about the Response Object of the Fetch API says:

The fetch() call returns a promise, which resolves with the Response object associated with the resource fetch operation.

Well, since fetch() returns a Promise object I can understand that the first then() call works fine because a Promise object has this method. But the Response object returned in the chained call is not a Promise object. However the second call to then() method works!

Altering the dummy example prints undefined in the first console.log():

fetch(".")
.then(function(response) { // first then() call
    console.log(response.then)
    return response;
}).then(function(response) { // second then() call
    console.log("ok");
});

My questions is: Why this works? How the second call to then() works since the returned object does not have this method? Is it a kind of syntax sugar?

Thanks!

I'm learning about JavaScript Fetch API and I'm a bit confusing about Promises.

Consider this dummy example that prints "ok" in the console:

fetch(".")
.then(function(response) { // first then() call
    return response;
}).then(function(response) { // second then() call
    console.log("ok");
});

The page about the Response Object of the Fetch API says:

The fetch() call returns a promise, which resolves with the Response object associated with the resource fetch operation.

Well, since fetch() returns a Promise object I can understand that the first then() call works fine because a Promise object has this method. But the Response object returned in the chained call is not a Promise object. However the second call to then() method works!

Altering the dummy example prints undefined in the first console.log():

fetch(".")
.then(function(response) { // first then() call
    console.log(response.then)
    return response;
}).then(function(response) { // second then() call
    console.log("ok");
});

My questions is: Why this works? How the second call to then() works since the returned object does not have this method? Is it a kind of syntax sugar?

Thanks!

• javascript
• promise
• es6-promise

Share Improve this question Follow edited May 18, 2017 at 13:39 Bruno Peres asked May 17, 2017 at 2:40 Bruno PeresBruno Peres 16.4k66 gold badges5656 silver badges9292 bronze badges 2

• 3 "How the second call to then() works since the returned object does not have this method?" - The .then() function calls themselves occur immediately and each return a promise object, they're not dependent on the previous callback in the chain returning something with a .then() method. – nnnnnn Commented May 17, 2017 at 2:45
• To support @nnnnnn, here: stackoverflow./a/27716518/4279440 is a good explanation of this behavior. – Travis Clarke Commented May 17, 2017 at 2:49

Add a ment  | 

1 Answer 1

Sorted by: Reset to default 11

But the Response object returned in the chained call is not a Promise object. However the second call to then() method works!

Yes, because the second .then() invocation is on the return value of the first then call, not on the response. The promise then method always returns a promise - which makes it chainable. It does not return the return value of the asynchronous callback - for that it would need to peek into the future.

Look closely:

const promise1 = fetch(".");
const promise2 = promise1.then(function(response) { // first then() call
    return response;
});
const promise3 = promise2.then(function(response) { // second then() call
    console.log("ok");
});

It's not

fetch(".").then(function(response) { // outer then() call
    return response.then(function() { // inner then() call
        console.log("ok");
    });
});

which indeed doesn't work without response being a promise.

本文标签： JavaScript Promises chaining promises with nonpromise objects Why it worksStack Overflow