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;
}
Instead of
return
, you should usecallbacks
because inasynchronous
operations,return
does not wait for theI/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);
}
});
}