返回一个空洞的承诺

Question

I have a function that returns a jQuery promise. It looks like this:

addBooks(books: Array<Books>) {
    return $.ajax({
        url: '/Books/AddBooks/',
        type: 'POST',
        data: ko.toJSON(books),
        contentType: 'application/json'
    });
}

I do this so I can reuse this function and chain promise callbacks like:

addBooks.done(() => { alert("Books added!"); })

My question is, what if I want to break out of addBooks early and prevent a trip to the server. For example:

addBooks(books: Array<Books>) {

    // An empty array was passed in for some reason.
    // Theres nothing to add so dont try to POST
    if (books <= 0) return null;

    return $.ajax({
        url: '/Books/AddBooks/',
        type: 'POST',
        data: ko.toJSON(books),
        contentType: 'application/json'
    });
}

My example will not compile because my chained done callback example is expecting addBooks to return a promise object, not null. How can I return an empty promise (or whatever the correct object is I should return in the situation)?

Answer 1

How can I return an empty promise (or whatever the correct object is I should return in the situation)?

Yes, an "empty promise" is appropriate here, if you mean a promise that is already fulfilled with nothing ( undefined , null ).

The jQuery syntax to create such is using $.when with a single (or no) argument:

if (books <= 0) return $.when(null);

Answer 2

You can return a resolved promise: Instead of

if (books <= 0) return null;

use

if (books <= 0) return $.Deferred().resolve();

Beware , though, that jQuery's Promise API does something surprising: Sometimes it calls your done / then /etc. callback synchronously with the done / then /etc. call, sometimes it doesn't. Most promises libraries ensure that the call is always asynchronous, even if you're calling done / then /etc. on a resolved promise. jQuery doesn't, so you get subtle differences.

For instance, this code:

addBooks(() => console.log(1));
console.log(2);

...will log

2
1

...if you did the ajax call, but

1
2

...if you returned a resolved promise.

Answer 3

From https://api.jquery.com/jquery.when/

If you don't pass it any arguments at all, jQuery.when() will return a resolved promise.

eg

if (books <= 0) return $.when();

and, if you need a value that is not undefined passed on:

If a single argument is passed to jQuery.when() and it is not a Deferred or a Promise , it will be treated as a resolved Deferred and any doneCallbacks attached will be executed immediately. The doneCallbacks are passed the original argument.

eg an empty array is passed on:

if (books <= 0) return $.when([]);

eg an empty object is passed on:

if (books <= 0) return $.when({});