Javascript Javascript函数在nodejs中返回未定义的值

Javascript function returning undefined value in nodejs

提问by iam

I am writing code for getting data. First I call **getsomedata**function to get data and inside getsomedatafunction I am calling another function getRandomdatato get data and returning it back to the previous function but it is returning undefined. But in getRandomdatadata could be seen in console.log. Do I need to use callbacks?

我正在编写用于获取数据的代码。首先，我调用**getsomedata**函数来获取数据，在getsomedata函数中，我调用另一个函数getRandomdata来获取数据并将其返回到前一个函数，但它正在返回undefined. 但在getRandomdata数据中可以看到console.log。我需要使用callbacks吗？

router.get('/get-data', function (req, res, next) {
    var result = getsomedata(some_parameter);
    console.log(result);   // receiving undefined
    res.send(result);
});

function getsomedata(some_parameter_recieved) {
    var getsomedata = getRandomdata(random_params);
    console.log(getsomedata);    // receiving undefined
    return getsomedata;
}

function getRandomdata(random_params_recieved) {
    // after some calculation 
    console.log(random_data);           // receiving proper data
    return random_data;
}

回答by Rayon

Instead of return, you should use callbacksbecause in asynchronousoperations, returndoes not wait for the I/Ooperation to complete.

而不是return，您应该使用，callbacks因为在asynchronous操作中，return不等待I/O操作完成。

Callback- In JavaScript, higher order functions could be passed as parameters in functions. Since JavaSCript is a single threaded, only one operations happens at a time, each operation thats going to happen is queued in single thread. This way, passed functions(as parameter) could be executed when rest of the parent functions operation(async) is completed and script can continue executing while waiting for results.

Callback- 在 JavaScript 中，高阶函数可以作为函数中的参数传递。由于 JavaSCRipt 是单线程的，一次只发生一个操作，每个将要发生的操作都在单线程中排队。这样，传递的函数（作为参数）可以在父函数 operation( async) 的其余部分完成时执行，脚本可以在等待结果的同时继续执行。

Usually this callbackfunction is passed in as the last argument in the function.

通常这个callback函数作为函数的最后一个参数传入。

Using Callbacks:

使用Callbacks：

router.get('/get-data', function(req, res, next) {
  getsomedata(some_parameter, function(result) {
    console.log(result);
    res.send(result);
  });
});

function getsomedata(some_parameter_recieved, callback) {
  getRandomdata(random_params, function(random_data) {
    callback(random_data);
  });
}

function getRandomdata(random_params_recieved, callback) {
  // after some calculation
  callback(random_data);
}

Using Promise:

使用Promise：

router.get('/get-data', function(req, res, next) {
  getsomedata(some_parameter, function(result) {
    console.log(result);
    res.send(result);
  });
});

function getsomedata(some_parameter_received, callback) {
  getRandomdata(random_params).then(function(random_data) {
    callback(random_data);
  }).catch(function(e) {
    //handle error here
  });
}

function getRandomdata(random_params_received, callback) {
  return new Promise(function(resolve, reject) {
    // after some calculation
    if (RandomDataGeneratedSuccessfully) {
      resolve(random_data);
    } else {
      reject(reason);
    }
  });
}