typescript 返回一个空头承诺
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Return an empty promise
提问by Jonathan Eckman
I have a function that returns a jQuery promise. It looks like this:
我有一个返回 jQuery 承诺的函数。它看起来像这样：
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:
我这样做是为了可以重用这个函数和链式 promise 回调，例如：
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:
我的问题是，如果我想提前退出 addBooks 并防止访问服务器怎么办。例如：
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 addBooksto 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)?
我的示例将无法编译，因为我的链式完成回调示例期望addBooks返回一个 promise 对象，而不是 null。我怎样才能返回一个空的承诺（或者在这种情况下我应该返回的任何正确的对象）？
回答by Bergi
How can I return an empty promise (or whatever the correct object is I should return in the situation)?
我怎样才能返回一个空的承诺（或者在这种情况下我应该返回的任何正确的对象）？
Yes, an "empty promise" is appropriate here, if you mean a promise that is already fulfilled with nothing (undefined, null).
是的，如果您的意思是已经没有任何东西实现的承诺 ( undefined, null) ，那么这里使用“空承诺”是合适的。
The jQuery syntax to create such is using $.whenwith a single (or no) argument:
创建此类的 jQuery 语法使用$.when单个（或无）参数：
if (books <= 0) return $.when(null);
回答by T.J. Crowder
You can return a resolvedpromise: Instead of
您可以返回一个已解决的承诺：而不是
if (books <= 0) return null;
use
利用
if (books <= 0) return $.Deferred().resolve();
Beware, though, that jQuery's PromiseAPI 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.
但是请注意，jQuery 的PromiseAPI 会做一些令人惊讶的事情：有时它会调用您的done/ then/etc. 与同步回调done/then的/ etc。打电话，有时没有。大多数承诺库确保调用始终是异步的，即使您正在调用done/ then/etc。在已解决的承诺上。jQuery 没有，所以你会得到细微的差异。
For instance, this code:
例如，这段代码：
addBooks(() => console.log(1));
console.log(2);
...will log
...将记录
2 1
...if you did the ajax call, but
...如果你做了 ajax 调用，但是
1 2
...if you returned a resolved promise.
...如果你返回了一个已解决的承诺。
回答by Gone Coding
From https://api.jquery.com/jquery.when/
来自https://api.jquery.com/jquery.when/
If you don't pass it any arguments at all,
jQuery.when()will return a resolved promise.
如果你根本不传递任何参数，
jQuery.when()将返回一个已解决的承诺。
e.g.
例如
if (books <= 0) return $.when();
and, if you need a value that is not undefined passed on:
并且，如果您需要传递一个非未定义的值：
If a single argumentis 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.
如果将单个参数传递给 jQuery.when() 并且它不是 Deferred 或 Promise，它将被视为已解决的 Deferred 并且任何附加的 doneCallbacks 将立即执行。doneCallbacks 传递了原始参数。
e.g. an empty array is passed on:
例如传递一个空数组：
if (books <= 0) return $.when([]);
e.g. an empty object is passed on:
例如传递一个空对象：
if (books <= 0) return $.when({});
