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:

我试图让快速控制器在返回响应之前等待 5 秒:

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',
      },
    ]);
  };
};

这段代码不起作用，浏览器一直在加载和加载，从不显示任何东西.

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

我基于 getUser 函数>this SO answer，以及控制器方法，基于我对其工作原理的(错误的)理解，所以我想要一些澄清和更正:

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.我什么时候应该使用await?

据我所知，您应该在 async 函数调用之前使用 await.这个对吗?另外，为什么我可以在返回承诺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.我什么时候应该使用 async?

据我所知，您将函数标记为 async 函数，以便可以使用 await 关键字调用它.这个对吗?另外，[为什么] 我必须将我的 await getUsers(5000) 调用包装在匿名异步函数中?

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

澄清几个疑惑 -

1. 您可以将 await 与任何返回承诺的函数一起使用.您正在等待的函数不一定是 async.
2. 当您想在该函数中使用 await 关键字时，您应该使用 async 函数.如果您不打算在函数内使用 await 关键字，那么您不需要将该函数设为 async.
3. async 函数默认返回一个 promise.这就是您能够await 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 -

当异步函数被调用时，它会返回一个 Promise.

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

就你的代码而言，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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