assign function return value to some variable using javascript

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) { 
           开发者_开发问答 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'.

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.

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);
           })
    }); 
};

You could simply return a value from the function:

var response = 0;

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

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);
    }
})();

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();