(Javascript) Definitely can't return an array

Question

I've been watching for a while the questions here in Stack Overflow about returning arrays on JavaScript, but not a single answer works in my code. It always returns 'undefined'.

This is the important part of the code:

function VisualizadorIzquierdo () {
    // some stuff here
    this.dibujar = function () {
        var datos = obtenerDatos(0);
        alert(datos);                         //<--- This always returns 'undefined'
        // other stuff here
    }
    // more stuff here
}

function obtenerDatos(id) {                                                                             
    var xmlhttp = ConstructorXMLHttpRequest();
    if(!xmlhttp) alert('Error: No se pudo crear el objeto XMLHttpRequest');
    else {
        xmlhttp.onreadystatechange = function() {
            if(xmlhttp.readyState == 4)
            {
                alert(xmlhttp.responseText); //<--- This returns '[1,2,3,4]'    
                return xmlhttp.responseText; 
            }
        }
        xmlhttp.open('GET', rutaLector + '?archivo=' + archivos[id], false);
        xmlhttp.send(null);     
    }
}

I'v tried defining the variable datos as a new Array() and new Object(). Also I'm trying returning an Object (like {"arreglo" : [1,2,3,4]}), defining a new Array() and new Object() within obtenerDatos(id) function, changing between syncronic and asyncronic, etc ...

But every time it returns 'undefined' :c

So, How can I definitely return an Array from a function?

Thanks in advanced! :)

Assuming that the data returned from your _asynchronous_ call is valid, your alert will be executed prior to the code being returned, hence the undefined. — j08691, Oct 16 '13 at 19:30

score 0 · Answer 1 · edited May 23 '17 at 12:10

0

obtenerDatos() don't have a return. The only 'return' in its context is inside another function that is being dynamically declared to the onreadystatechange, but that one doesn't count. Its a delayed event, that will be ran at some other time when the AJAX request is concluded. obtenerDatos() itself isn't returning anything.

What you have to do is set up a callback function that will be called with the AJAX response to process it once the AJAX is done fetching the data.

This other question might explain it better.

edited May 23 '17 at 12:10

Community

1
1

answered Oct 16 '13 at 19:31

Havenard

27,022
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But if I set the *asyncronic* param to *false*, the declaration *var datos = obtenerDatos(0);* does move forward? – AronNeewart Oct 16 '13 at 19:37
It doesn't change the fact that `obtenerDatos()` is not returning anything. `onreadystatechange` is, but its going nowhere. – Havenard Oct 16 '13 at 19:38

score 0 · Answer 2 · edited Oct 16 '13 at 19:39

0

the reason why you are getting undefined is because you are calling obtenerDatos() which then calls

xmlhttp.send(null);

xmlhttp.send(null); <-- is an asynchronous call. so you obtenerDatos() function immediately returns after that. and since you don't have a return statement returning anything for obtenerDatos then the variable datos will get nothing or undefined as it is doing right now.

edited Oct 16 '13 at 19:39

thefourtheye

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answered Oct 16 '13 at 19:35

Sucaritas

51
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How can I make it to wait until the asynchronous call ends before returning *nothing*? (is that even possible?) – AronNeewart Oct 16 '13 at 19:43
straight from the docs. the open function accepts these parameters void open(DOMString method, DOMString url, optional boolean async, optional DOMString? user, optional DOMString? password); the the one that says async? thats where. here is a link to the docs https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest?redirectlocale=en-US&redirectslug=DOM%2FXMLHttpRequest – Sucaritas Oct 18 '13 at 00:17

score 0 · Answer 3 · answered Oct 16 '13 at 19:48

Try in jQuery.

function VisualizadorIzquierdo () {
    // some stuff here
    obtenerDatos(0).done(function (result) {
        //var datos = obtenerDatos(0);
        alert(result);                        
        // other stuff here
    });
    // more stuff here
}


function obtenerDatos(id) {

    return $.ajax({
        url: rutaLector + '?archivo=' + archivos[id],
        type: "GET",
        success: function (result) { },
        error: function () { },
        traditional: true
    });
}

(Javascript) Definitely can't return an array

3 Answers3