assign function return value to some variable using javascript

Question

I have a js function , after doing some business logic, the javascript function should return some result to another variable.Sample code below

var response="";
function doSomething() {  
    $.ajax({
        url:'action.php',
        type: "POST",
        data: dataString,
        success: function (txtBack) { 
            if(txtBack==1) {
                status=1;
            }
    });
    return status;     
}

Here i want to use like

response=doSomething();

I want to assign a return value "status" "var response".But the result is 'undefined'.

Stygian

Already got the correct wording in your title: return value. But you are not using return in your function ;) – Inappreciable

Note that usually you don't need to initialize new variables with values if those values are falsy (which an empty string is). You can just write: var response; The value of response will be set to the undefined value in this case (which is also a falsy value). Then later, when you want to check if the value has been set, you can just check if it is falsy or not... – Veroniqueverras

@Felix: i have a ajax call inside the function, it is returning "undefined". any idea? Thanks – Stygian

Search here on SO about this issue. This is normal. You cannot return a value form an Ajax call, because it is executed asynchronously, i.e. the return statement is executed before the Ajax call finished. You can solve this using callbacks... – Inappreciable

@Felix: Hi, i made a little change in the code, after success condition, i just include "async :false".... now the code is working fine as expected :) – Stygian

@Ra: But keep in mind that async: false is bad. Depending on how long the Ajax call takes, it can freeze the browser. And you loose imho the most important feature of an Ajax call. – Inappreciable

Answer 1

Or just...

var response = (function() {
    var a;
    // calculate a
    return a;
})();  

In this case, the response variable receives the return value of the function. The function executes immediately.

You can use this construct if you want to populate a variable with a value that needs to be calculated. Note that all calculation happens inside the anonymous function, so you don't pollute the global namespace.

Counterstamp

For the curious, this is called an immediately invoked function express (IIFE). And it is awesome. – Bea

Note that the function itself is inside parenthesis (there is an opening parenthesis before the word function) and that there is an extra () at the end. (It was not obvious to me at first sight) – Nissa

How do I pass parameters using this syntax? – Saltzman

@LeonardNiehaus (function(w) { /*…*/ })(window) – Veroniqueverras

Answer 2

AJAX requests are asynchronous. Your doSomething function is being exectued, the AJAX request is being made but it happens asynchronously; so the remainder of doSomething is executed and the value of status is undefined when it is returned.

Effectively, your code works as follows:

function doSomething(someargums) {
     return status;
}

var response = doSomething();

And then some time later, your AJAX request is completing; but it's already too late

You need to alter your code, and populate the "response" variable in the "success" callback of your AJAX request. You're going to have to delay using the response until the AJAX call has completed.

Where you previously may have had

var response = doSomething();

alert(response);

You should do:

function doSomething() {  
    $.ajax({
           url:'action.php',
           type: "POST",
           data: dataString,
           success: function (txtBack) { 
            alert(txtBack);
           })
    }); 
};

Candelaria

Answer 3

You could simply return a value from the function:

var response = 0;

function doSomething() {
    // some code
    return 10;
}
response = doSomething();

Parris

Its working... but i have ajax call inside, in thats time the result is 'undefined'. Any idea? Please look at the modified sample code. – Stygian

Answer 4

The only way to retrieve the correct value in your context is to run $.ajax() function synchronously (what actually contradicts to main AJAX idea). There is the special configuration attribute async you should set to false. In that case the main scope which actually contains $.ajax() function call is paused until the synchronous function is done, so, the return is called only after $.ajax().

function doSomething() {
    var status = 0;
    $.ajax({
        url: 'action.php',
        type: 'POST',
        data: dataString,
        async: false,
        success: function (txtBack) {
            if (txtBack == 1)
                status = 1;
        }
    });

    return status;
}

var response = doSomething();

Borneol

Answer 5

The result is undefined since $.ajax runs an asynchronous operation. Meaning that return status gets executed before the $.ajax operation finishes with the request.

You may use Promise to have a syntax which feels synchronous.

function doSomething() { 
    return new Promise((resolve, reject) => {
        $.ajax({
            url:'action.php',
            type: "POST",
            data: dataString,
            success: function (txtBack) { 
                if(txtBack==1) {
                    resolve(1);
                } else {
                    resolve(0);
                }
            },
            error: function (jqXHR, textStatus, errorThrown) {
                reject(textStatus);
            }
        });
    });
}

You can call the promise like this

doSomething.then(function (result) {
    console.log(result);
}).catch(function (error) {
    console.error(error);
});

or this

(async () => {
    try {
        let result = await doSomething();
        console.log(result);
    } catch (error) {
        console.error(error);
    }
})();

Geophyte