javascript do something after executing a function

Question

I've seen a lot of post about it but I still don't understand callback. I got a function like this:

function getData (url) {
  request(url, function (error, response, data) {
      //...
      return data;
    }
  });
}

var data = getData(url);
// I got undefined because it's not finished
if (data) 
  ...

How do i wait the return of my function to continue. I've seen a lot of things about callback and promise but how do use it in this case? for example I've seen this post but I don't understand the answer.

I'm using node js

http://blog.slaks.net/2015-01-04/async-method-patterns/ – SLaks Sep 02 '15 at 21:23 — SLaks, Sep 02 '15 at 21:23
you're using node.js or trying to do this with ajax only ? – Nelson Teixeira Sep 02 '15 at 21:26 — Nelson Teixeira, Sep 02 '15 at 21:26

score 5 · Accepted Answer · answered Sep 02 '15 at 21:37

First of all it's natural you have this kind of question.

The problem is that you need to understand what async is in programming, when you use the 'request' function your code will execute in parallel so you don't have to wait the server respose.

You actually need to use your if(data) inside the request function. Something like this.

request(url,  function(error, response, data){
    //your if and your code after the response of this request need's to be placed here.
}

This is what you shold do if you want to process the request after the server response, but this way you always have the same code when your request returns isn't ?

So you can use a 'callback' for now think it's a variable that represents a function. It's a little strange but you get in time.
Ok, so you will create a variable called 'callback' that represent a function, but ! the magic happens here, if you are using a variable you can change before you write your code. Check my example below.

function getData(url, callback){
 request(url, function(error, response, data){
   callback(error, response, data);
 }

//so when you use this function you can use another function in response like this.
getData('someUrl', function(error, response, data)
{
   //this piece of code will only be executed AFTER the request function returns.
});

I googled a little bit and found that node has some synchronous call but if you are building a system in node using only sync function you should change your server code.

score 0 · Answer 2 · answered Sep 02 '15 at 21:32

0

getData should take the callback as an argument.

function getData(url, callback) {
    request(url, function (error, response, data) {
        callback(data);
    });
}

getData(url, function(data) {
    if (data) {
        ...
    }
});

answered Sep 02 '15 at 21:32

Barmar

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bcngr · Answer 3 · 2015-09-02T21:38:33.460

First, you need to understand that the main thread where you call getData is going to finish and another will be open on the request function, so you need to continue in that new thread. To do this you need a callback function, that is a piece of code you send as a parameter to the getData function and can be executed when the request function finishes. You can do something like this:

function getData (url, callback) {
  request(url, function (error, response, data) {
      callback(data);
    }
  });
}

getData(url, function(data){
   // do something with the data returned
});

javascript do something after executing a function

3 Answers3