不能用异步执行http请求并等待 [英] can't do http request with async and await

问题描述

试图在Node.js中使用 async 和 await 进行http请求，但出现错误.有任何想法吗?谢谢

Trying to use async and await to do http request in nodejs, but got error. Any ideas? Thx

got response: 
undefined
/home/tom/learn/node/node_modules/node-rest-client/lib/node-rest-client.js:539
            callback(parsedData,res);
            ^

TypeError: callback is not a function
    at /home/tom/learn/node/node_modules/node-rest-client/lib/node-rest-client.js:539:13
    at Object.parse (/home/tom/learn/node/node_modules/node-rest-client/lib/nrc-parser-manager.js:151:3)
    at ConnectManager.handleResponse (/home/tom/learn/node/node_modules/node-rest-client/lib/node-rest-client.js:538:32)
    at ConnectManager.handleEnd (/home/tom/learn/node/node_modules/node-rest-client/lib/node-rest-client.js:531:18)
    at IncomingMessage.<anonymous> (/home/tom/learn/node/node_modules/node-rest-client/lib/node-rest-client.js:678:34)
    at emitNone (events.js:110:20)
    at IncomingMessage.emit (events.js:207:7)
    at endReadableNT (_stream_readable.js:1059:12)
    at _combinedTickCallback (internal/process/next_tick.js:138:11)
    at process._tickCallback (internal/process/next_tick.js:180:9)

这是脚本的源代码

var Client = require('node-rest-client').Client;
var client = new Client();
async function test1() {
    response = await client.get("http://localhost/tmp.txt");
    console.log("got response: ");
    console.log(response.headers);
};
test1();

nodejs的版本为v8.4.0，在Ubuntu 14.04上.

nodejs is of version v8.4.0, on ubuntu 14.04.

推荐答案

async/await 不仅可以神奇地与需要回调的函数一起使用.如果client.get()希望将回调作为参数，则如果要使用它，则必须传递一个回调. async/await 与返回承诺的异步操作一起使用，并根据这些承诺进行操作.它们不可思议地让您跳过将回调传递到为回调设计的函数的过程.我建议您阅读更多有关如何实际使用 async 和 await 的文章.

async/await don't just magically work with functions that expect callbacks. If client.get() is expecting a callback as an argument, you HAVE to pass a callback if you're going to use it. async/await work with asynchronous operations that return promises and they operate on those promises. They do not magically let you skip passing callbacks to functions designed for a callback. I'd suggest a lot more reading about how to actually use async and await.

通常， async/await 的路径是首先设计所有 async 操作以使用promise和 .then()处理程序.然后，在所有工作完成之后，您可以将一个函数声明为要在其中使用await的异步函数，然后在这些异步声明的函数内部，您可以调用使用await返回promise的函数，而不是使用 .then()处理程序.这里没有魔术快捷键.从承诺设计开始.

In general, the path to async/await is to first design all your async operations to use promises and .then() handlers. Then, after that is all working, you can declare a function as async that you want to use await in and then inside those async-declared functions you can call functions that return promises with await instead of using .then() handlers with them. There are no magic shortcuts here. Start with a promise design.

这是一个简单的promise示例:

Here's a simple promise example:

// asynchronous function that returns a promise that resolves to
// the eventual async value
function delay(t, val) {
   return new Promise(resolve => {
       setTimeout(() => {
           resolve(val);
       }, t);
   });
}


function run() {
    return delay(100, "hello").then(data => {
        console.log(data);
        return delay(200, "goodbye").then(data => {
           console.log(data);
        });
    }).then(() => {
        console.log("all done");
    });
}

run();

而且，这与使用 async/await 相同:

// function returning a promise declared to be async
function delay(t, val) {
   return new Promise(resolve => {
       setTimeout(() => {
           resolve(val);
       }, t);
   });
}

async function run() {
    console.log(await delay(100, "hello"));
    console.log(await delay(200, "goodbye"));
    console.log("all done");
}

run();

这两个示例均产生相同的输出和相同的输出时序，因此希望您可以看到从promise到async/await的映射.

Both of these examples produce the same output and same output timing so hopefully you can see the mapping from promises to async/await.

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