101 lines
3.5 KiB
Markdown
101 lines
3.5 KiB
Markdown
---
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title: Await Promises
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localeTitle: Await Promise
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---
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## Await Promise
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`async` / `await` [关键字](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators)可以更轻松地实现许多异步Promise。它们能帮助工程师编写更清晰，更简洁，可测试的代码。
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要理解这个主题，您需要对[Promise](https://guide.freecodecamp.org/javascript/promises)的工作机制有充分的了解。
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* * *
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## 基本语法
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```javascript
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function slowlyResolvedPromiseFunc (string) {
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return new Promise（resolve => {
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setTimeout（() => { resolve（string}, 5000;
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};
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}
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async function doIt () {
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const myPromise = await slowResolvedPromiseFunc ('foo');
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console.log(myPromise); // 'foo'
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}
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doIt();
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```
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有几点需要注意:
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* 包含`await`关键字的函数在定义时必须有`async`关键字修饰. 它会阻塞javascript进程，直到Promise执行了resolve或者reject。
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* `await`关键字必须和声明的变量在同一行。
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* 对`reject`和`resolve`效果相同。
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---
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## 嵌套 Promises vs. `Async` / `Await`
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实现一个Promise很简单。 然而，链式的Promise或有依赖的模式会导致“意大利面条”式的代码。
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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的 //第一承诺 const fooPromise = rp（“http://domain.com/foo”）;
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fooPromise.then（resultFoo => { //必须等待“foo”才能解决 的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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}）。then（resultArr => { //在此处理“bar”和“baz”分辨率 的console.log（resultArr \[0\]）; 的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的 //将所有内容包装在异步函数中 异步函数doItAll（）{ //从“foo”端点获取数据，但等待解决 console.log（等待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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//最后，调用异步函数 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的 异步函数errorExample（）{ 尝试{ const rejectedPromise = await Promise.reject（“哦 - 哦！”）; } catch（error）{ 的console.log（误差）; //“呃 - 哦！” } }
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errorExample（）; \`\`\`
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* * *
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#### 更多信息：
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* `await`运营商[MDN文档](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await)
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* `async`功能操作员[MDN文档](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/async_function)
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