How to use data returned by an asynchronous function in Node.js?

Question

I want to define a function that gets me a list of certain IDs from a response of a GET request:

var getList = function (){

    var list = [];

    https.get(options).on('response', function (response) {
        var body = '';
        response.on('data', function (chunk) {
            body += chunk;
        });
        response.on('end', function () {
            var obj = JSON.parse(body);
            for (i=0 ; i<obj.length ; i++){
                list.push(obj[i].id);
            }
            //console.log(list);
            //return list;
        });
    });
};

Now, I'd like to use that list from this function in other functions or simply assign it to a variable. I understand that since the function is asynchronous (well, the https.get one), returning the list won't mean much as the other code will not wait for this function to finish. Do I have to put all the remaining code inside the response.end call? I know I'm missing something very obvious here...

Answer 1

You can accept a callback as a parameter and call it with the relevant data inside the response.end handler:

var getList = function (successCallback){

    var list = [];

    https.get(options).on('response', function (response) {
        var body = '';
        response.on('data', function (chunk) {
            body += chunk;
        });
        response.on('end', function () {
            var obj = JSON.parse(body);
            for (i=0 ; i<obj.length ; i++){
                list.push(obj[i].id);
            }

            // invoke the callback and pass the data
            successCallback(list);
        });
    });
};

Then you can call the getList function and pass in a callback:

getList(function(data) {
    // do some stuff with 'data'
});

Of course other options are to use some library implementing the Promises pattern to avoid callback hell.

Answer 2

make the list as global or accessible

var list = []

//declare a get method

function getUpdatedList(){
  return list;
}

if you call before ajax finish you will get null anfter you will get the actual data array, but in normal cases just uses callback functions like

function getList(callback){

    ///
    //
    response.on('end', function () {
        var obj = JSON.parse(body);
        for (i=0 ; i<obj.length ; i++){
            list.push(obj[i].id);
        }
        callback(list);
    });
}