Javascript函数在nodejs中返回未定义的值

Question

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

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;
}

Answer 1

Instead of return , you should use callbacks because in asynchronous operations, return does not wait for the I/O operation to complete.

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.

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

Using 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 :

 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); } }); }