92 lines
4.1 KiB
Markdown
92 lines
4.1 KiB
Markdown
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---
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title: Await Promises
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localeTitle: Ожидание обещаний
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---
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## Ожидание обещаний
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В `async` / `await` [операторы](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators) облегчают реализацию многого асинхронного Promises. Они также позволяют инженерам писать более четкий, более сжатый, проверяемый код.
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Чтобы понять эту тему, вы должны иметь четкое представление о том, как работают [Обещания](https://guide.freecodecamp.org/javascript/promises) .
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* * *
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## Основной синтаксис
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\`\` \`javascript функция медленноResolvedPromiseFunc (строка) { return new Promise (solve => { setTimeout (() => { разрешения (строка); }, 5000); }); }
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асинхронная функция doIt () { const myPromise = ждать медленноResolvedPromiseFunc ("foo"); console.log (myPromise); // "foo" }
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сделай это();
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```
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There are a few things to note:
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* The function that encompasses the `await` declaration must include the `async` operator. This will tell the JS interpreter that it must wait until the Promise is resolved or rejected.
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* The `await` operator must be inline, during the const declaration.
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* This works for `reject` as well as `resolve`.
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---
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## Nested Promises vs. `Async` / `Await`
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Implementing a single Promise is pretty straightforward. In contrast, Chained Promises or the creation of a dependency pattern may produce "spaghetti code".
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The following examples assume that the <a href='https://github.com/request/request-promise' target='_blank' rel='nofollow'>`request-promise`</a> library is available as `rp`.
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### Chained/Nested Promises
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```
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Javascript // Первое обещание const fooPromise = rp ("http://domain.com/foo");
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fooPromise.then (resultFoo => { // Должен дождаться "foo", чтобы разрешить console.log (resultFoo);
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```
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const barPromise = rp("http://domain.com/bar");
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const bazPromise = rp("http://domain.com/baz");
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return Promise.all([barPromise, bazPromise]);
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```
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}), затем (resultArr => { // Обрабатываем разрешения «bar» и «baz» здесь console.log (resultArr \[0\]); console.log (resultArr \[1\]); });
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```
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### `async` and `await` Promises
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```
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Javascript // Оберните все в асинхронной функции асинхронная функция doItAll () { // Получить данные из конечной точки «foo», но дождаться разрешения console.log (ожидание rp ("http://domain.com/foo"));
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```
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// Concurrently kick off the next two async calls,
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// don't wait for "bar" to kick off "baz"
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const barPromise = rp("http://domain.com/bar");
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const bazPromise = rp("http://domain.com/baz");
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// After both are concurrently kicked off, wait for both
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const barResponse = await barPromise;
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const bazResponse = await bazPromise;
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console.log(barResponse);
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console.log(bazResponse);
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```
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}
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// Наконец, вызовите функцию async doItAll (). then (() => console.log ('Done!'));
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```
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The advantages of using `async` and `await` should be clear. This code is more readable, modular, and testable.
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It's fair to note that even though there is an added sense of concurrency, the underlying computational process is the same as the previous example.
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---
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## Handling Errors / Rejection
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A basic try-catch block handles a rejected Promise.
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```
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Javascript async function errorExample () { пытаться { const rejectPromise = ждать Promise.reject («О-о!»); } catch (ошибка) { console.log (ошибка); // «О-о!» } }
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errorExample (); \`\` \`
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* * *
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#### Дополнительная информация:
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* `await` Операторы [MDN Docs](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await)
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* `async` Оператор функций [MDN Docs](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function)
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