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Node.js中从回调函数返回一个值时遇到小问题，我将尽可能简单地解释我的情况。考虑我有一个片段，它接受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());
});

我试图将其包装在函数中并返回如下值：

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 将决定 urlToCall 的值的声明，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中的所有语句.request 将保持不变，但 urlToCall 的值除外。所以我肯定需要将这些公共代码放在函数中。我试过同样的但是在 doCall 总是会给我 undefined 。我试过这样：

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

但是如果我在里面打印值 doCall（）它打印完美，但它总会返回 undefined 。根据我的研究，我发现我们无法从回调函数返回值！ （是真的吗？）如果是的话，任何人都可以建议我如何处理这种情况，因为我想在每个 if-else 块中防止重复代码。

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

其未定义因为， console.log（响应）在 doCall（urlToCall）; 之前运行。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，您的函数。传递回调：

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，发布了好资源。阅读节点中的回调以及它们的工作原理。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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