了解NodeJS上的异步/等待 [英] Understanding async/await on NodeJS

问题描述

我认为我对它的理解可能会受到我对 .NET 的 async / await 的体验的影响，所以我想要一些代码示例：

I think my understanding of it might be affected by my experience with .NET's async/await, so I'd like some code example:

我正在尝试让快速控制器在返回响应之前等待5秒钟：

I'm trying to make a express controller wait 5 seconds before returning the response:

const getUsers = async (ms) => {
  var wait = ms => new Promise(resolve => setTimeout(resolve, ms));

  await wait(ms);
};


export const index = (req, res) => {
  async () => {
    await getUsers(5000);

    res.json([
      {
        id: 1,
        name: 'John Doe',
      },
      { id: 2,
        name: 'Jane Doe',
      },
    ]);
  };
};

此代码不起作用，浏览器不断加载和加载，从不显示任何内容。

This code doesn't work, the browser keeps loading and loading and never shows a thing.

我基于 getUser 函数38975138 / is-using-async-in-settimeout-valid＃answer-39027151>这个SO答案，以及控制器方法，基于我（错误）理解它的工作方式，所以我想澄清一下和更正：

The getUser function I built based on this SO answer, and the controller method, based on my (mistaken) understanding of how it works so I'd like some clarification and correction:

1。我应该何时使用等待？

1. when should I use await?

根据我的理解，你应该使用 await 在 async 函数调用之前。它是否正确？另外，为什么我可以在返回promise的非异步函数之前调用await？

To my understanding, you should use await before an async function call. Is this correct? Also, why can I call await before a non-async function that returns a promise?

2。我应该何时使用 async ？

2. When should I use async?

根据我的理解，您将函数标记为 async 一个，以便可以使用 await 关键字调用它。它是否正确？另外，[为什么]我必须在匿名异步函数中包装我的 await getUsers（5000）？

To my understanding, you mark a function as an async one so that it can be called with the await keyword. Is this correct? Also, [why] do I have to wrap my await getUsers(5000) call in an anonymous async function?

推荐答案

要清除一些疑问 -

To clear a few doubts -

1. 您可以使用 await 包含任何返回promise的函数。您正在等待的功能不一定是 async 。


2. 您应该使用 async 当你想在该函数中使用 await 关键字时的函数。如果您不打算在函数中使用等待关键字，那么您不需要创建该函数 async 。


3. async 默认情况下，函数返回一个promise。这就是你能够等待 async 函数的原因。

1. You can use await with any function which returns a promise. The function you're awaiting doesn't need to be async necessarily.
2. You should use async functions when you want to use the await keyword inside that function. If you're not gonna be using the await keyword inside a function then you don't need to make that function async.
3. async functions by default return a promise. That is the reason that you're able to await async functions.

来自 MDN -

当调用异步函数时，它返回一个Promise 。

When an async function is called, it returns a Promise.

就您的代码而言，它可以这样写 -

As far as your code is concerned, it could be written like this -

const getUsers = (ms) => { // No need to make this async
    return new Promise(resolve => setTimeout(resolve, ms));
};

// this function is async as we need to use await inside it
export const index = async (req, res) => {
    await getUsers(5000);

    res.json([
      {
        id: 1,
        name: 'John Doe',
      },
      { id: 2,
        name: 'Jane Doe',
      },
    ]);
};

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