从 Node.js 中的回调函数返回一个值 [英] Returning a value from callback function in Node.js

问题描述

我在从 Node.js 中的回调函数返回值时遇到了小麻烦，我将尝试尽可能简单地解释我的情况.考虑我有一个片段，它获取 URL 并点击该 URL 并给出输出:

I am facing small trouble in returning a value from callback function in Node.js, I will try to explain my situation as easy as possible. Consider I have a snippet, which takes URL and hits that url and gives the output:

urllib.request(urlToCall, { wd: 'nodejs' }, function (err, data, response) {                              
    var statusCode = response.statusCode;
    finalData = getResponseJson(statusCode, data.toString());
});

我试图将它包装在一个函数中并返回一个这样的值:

I tried to wrap it inside a function and return a value like this:

function doCall(urlToCall) {
urllib.request(urlToCall, { wd: 'nodejs' }, function (err, data, response) {                              
    var statusCode = response.statusCode;
    finalData = getResponseJson(statusCode, data.toString());
    return finalData;
});
}

因为在我的 Node.js 代码中，我有很多 if-else 语句，其中 urlToCall 的值将被确定，如下所示:

Because in my Node.js code, I have a lot of if-else statement where value of urlToCall will be decided, like this:

if(//somecondition) {
   urlToCall = //Url1;
} else if(//someother condition) {
   urlToCall = //Url2;
} else {
   urlToCall = //Url3;
}

事情是urllib.request 中的所有语句都将保持不变，除了urlToCall 的值.所以我肯定需要把这些通用代码放在一个函数中.我尝试了同样的方法，但在 doCall 中总是会返回我 undefined.我是这样试的:

The thing is all of the statements inside a urllib.request will remain same, except value of urlToCall. So definitely I need to put those common code inside a function. I tried the same but in doCall will always return me undefined. I tried like this:

response = doCall(urlToCall);
console.log(response) //Prints undefined

但是如果我在 doCall() 中打印值，它会完美打印，但它总是会返回 undefined.根据我的研究，我开始知道我们不能从回调函数返回值！(是真的吗)?如果是，任何人都可以建议我如何处理这种情况，因为我想防止每个 if-else 块中的重复代码.

But if I print value inside doCall() it prints perfectly, but it will always return undefined. As per my research I came to know that we cannot return values from callback functions! (is it true)? If yes, can anyone advice me how to handle this situation, as I want to prevent duplicate code in every if-else blocks.

推荐答案

Its undefined 因为，console.log(response) 在 doCall(urlToCall) 之前运行; 完成.您还必须传入一个回调函数，该函数在您的 请求 完成时运行.

Its undefined because, console.log(response) runs before doCall(urlToCall); is finished. You have to pass in a callback function aswell, that runs when your request is done.

首先，你的功能.传递一个回调:

First, your function. Pass it a callback:

function doCall(urlToCall, callback) {
    urllib.request(urlToCall, { wd: 'nodejs' }, function (err, data, response) {                              
        var statusCode = response.statusCode;
        finalData = getResponseJson(statusCode, data.toString());
        return callback(finalData);
    });
}

现在:

var urlToCall = "http://myUrlToCall";
doCall(urlToCall, function(response){
    // Here you have access to your variable
    console.log(response);
})

@Rodrigo，在评论中发布了很好的资源.阅读节点中的 回调 及其工作原理.请记住，它是异步代码.

@Rodrigo, posted a good resource in the comments. Read about callbacks in node and how they work. Remember, it is asynchronous code.

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