JavaScript Asynchronous Return Value/Assignment with Jquery

JavaScript asynchronous return value / assignment with jQuery

So, this question has been asked a million times over, and I'm sure that everyone (myself included) tried this once.

It is just the nature of an asynchronous call, you can't use their results as a return value. Thats why they have you passing in a function that gets the result of the call, they can't return it either! Also notice that the elqTracker.pageTrack() function call returns IMMEDIATELY, therefore your returnValue is simply undefined.

Most people (see dfsq's answer) solve this problem by introducing a callback function as a paramater. This method is tried, and true – however jQuery has $.Deferred. This allows you to make your own asynchronous logic return a promise which you can then attach any number of callbacks to:

function trackPage(){
    var elqTracker = new jQuery.elq( 459 ),
        dfd = $.Deferred();

    elqTracker.pageTrack({
        success: function() {
            elqTracker.getGUID(function( guid ) {
                dfd.resolve( guid );
            });
        }
    });

    return dfd.promise();
}

// example use:
trackPage().done(function( guid ) {
    alert( "Got GUID:" + guid );
});

Notice now that your trackPage() returns an object that you can attach callbacks to? You don't have to attach them immediately either.

var pageHit = trackPage().done(function( guid ) {
    alert( "Page Hit GUID:" +guid );
});

$("button").click(function() {
    pageHit.done( function( guid ) {
        alert( "Clicked on Page GUID:" + guid );
    });
});

Also, the jQuery AJAX module always returns promises as well, so the interface for all your AJAX stuff should be very similar if you make your own logic return promises.

As a side note: I'd like to point out that your var returnValue was in the wrong "scope" anyway. It needed to be declared in the outer scope of the trackPage function. Even with this fix, the concept still doesn't work.

Scope for using a JS variable set by an asynchronous call

Change the entry point of your script so that getSubstanceLimits gets called first, before anything else. In the callback, call your previous entry point, which runs the rest of your script. You could do something like:

function getSubstanceLimits(callback) {
    $.ajax({
        url: '/get-data',
        success: function (response) {
            callback(response.download_list_groups_limit || 10);
        }
    });
}

and instead of your prior entry point, eg:

runApp();

getSubstanceLimits(runApp);

function runApp(result) {
  // code here that uses async `result`
}

Your whole app doesn't have to be inside the callback, but wherever you need to depend on result needs to be inside the callback.

You could also use the jQuery then-able instead of using a callback.

How can I get the return value of jquery post function?

The best thing to do is use a callback:

function readMemory(ptr, size, callback)
{
    $.post("readMemory.php", { ptr : ptr, size : size}, function(data)
    {
        callback(data, ptr, size);
    });     
}

(Note that we have ptr and size being passed to callback as well as data; this is usually good practice.)

So code you were expecting to use readMemory like this:

var data = readMemory(ptr, 100);
doSomethingWith(data);
doSomethingElseWith(data);

...will instead look like this:

readMemory(ptr, 100, function(data) {
    doSomethingWith(data);
    doSomethingElseWith(data);
});

This is because your readMemory function only starts the POST operation; it returns before the operation completes (the operation is asynchronous).

Ajax jquery async return value

This is not possible.

Javascript runs on the UI thread; if your code waits for the server to reply, the browser must remain frozen.

Instead, you need to return the value using a callback:

function get_char_val(merk, callback)
{  
    var returnValue = null;
    $.ajax({   
                type:       "POST",
                url:            "char_info2.php",   
                data:   { name: merk },   
                dataType: "html",  
                success:    function(data) {
                    callback(data);
                } 
        }); 
}

get_char_val('x', function(px) { ... });
get_char_val('y', function(py) { ... });

Note that the two callbacks will run in an unpredictable order.

You should modify your design so that you can get all twenty values in a single AJAX request.

For example, you can take a comma-separated list of values, and return a JSON object like { x: "...", y: "..." }.

How do I return the response from an asynchronous call?

→ For a more general explanation of asynchronous behaviour with different examples, see Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference
→ If you already understand the problem, skip to the possible solutions below.

The problem

The A in Ajax stands for asynchronous. That means sending the request (or rather receiving the response) is taken out of the normal execution flow. In your example, $.ajax returns immediately and the next statement, return result;, is executed before the function you passed as success callback was even called.

Here is an analogy which hopefully makes the difference between synchronous and asynchronous flow clearer:

Synchronous

Imagine you make a phone call to a friend and ask him to look something up for you. Although it might take a while, you wait on the phone and stare into space, until your friend gives you the answer that you needed.

The same is happening when you make a function call containing "normal" code:

function findItem() {
    var item;
    while(item_not_found) {
        // search
    }
    return item;
}

var item = findItem();

// Do something with item
doSomethingElse();

Even though findItem might take a long time to execute, any code coming after var item = findItem(); has to wait until the function returns the result.

Asynchronous

You call your friend again for the same reason. But this time you tell him that you are in a hurry and he should call you back on your mobile phone. You hang up, leave the house, and do whatever you planned to do. Once your friend calls you back, you are dealing with the information he gave to you.

That's exactly what's happening when you do an Ajax request.

findItem(function(item) {
    // Do something with the item
});
doSomethingElse();

Instead of waiting for the response, the execution continues immediately and the statement after the Ajax call is executed. To get the response eventually, you provide a function to be called once the response was received, a callback (notice something? call back ?). Any statement coming after that call is executed before the callback is called.

Solution(s)

Embrace the asynchronous nature of JavaScript! While certain asynchronous operations provide synchronous counterparts (so does "Ajax"), it's generally discouraged to use them, especially in a browser context.

Why is it bad do you ask?

JavaScript runs in the UI thread of the browser and any long-running process will lock the UI, making it unresponsive. Additionally, there is an upper limit on the execution time for JavaScript and the browser will ask the user whether to continue the execution or not.

All of this results in a really bad user experience. The user won't be able to tell whether everything is working fine or not. Furthermore, the effect will be worse for users with a slow connection.

In the following we will look at three different solutions that are all building on top of each other:

Promises with async/await (ES2017+, available in older browsers if you use a transpiler or regenerator)
Callbacks (popular in node)
Promises with then() (ES2015+, available in older browsers if you use one of the many promise libraries)

All three are available in current browsers, and node 7+.

ES2017+: Promises with `async/await`

The ECMAScript version released in 2017 introduced syntax-level support for asynchronous functions. With the help of async and await, you can write asynchronous in a "synchronous style". The code is still asynchronous, but it's easier to read/understand.

async/await builds on top of promises: an async function always returns a promise. await "unwraps" a promise and either result in the value the promise was resolved with or throws an error if the promise was rejected.

Important: You can only use await inside an async function or in a JavaScript module. Top-level await is not supported outside of modules, so you might have to make an async IIFE (Immediately Invoked Function Expression) to start an async context if not using a module.

You can read more about async and await on MDN.

Here is an example that elaborates the delay function findItem() above:

// Using 'superagent' which will return a promise.
var superagent = require('superagent')

// This is isn't declared as `async` because it already returns a promise
function delay() {
  // `delay` returns a promise
  return new Promise(function(resolve, reject) {
    // Only `delay` is able to resolve or reject the promise
    setTimeout(function() {
      resolve(42); // After 3 seconds, resolve the promise with value 42
    }, 3000);
  });
}

async function getAllBooks() {
  try {
    // GET a list of book IDs of the current user
    var bookIDs = await superagent.get('/user/books');
    // wait for 3 seconds (just for the sake of this example)
    await delay();
    // GET information about each book
    return superagent.get('/books/ids='+JSON.stringify(bookIDs));
  } catch(error) {
    // If any of the awaited promises was rejected, this catch block
    // would catch the rejection reason
    return null;
  }
}

// Start an IIFE to use `await` at the top level
(async function(){
  let books = await getAllBooks();
  console.log(books);
})();

Current browser and node versions support async/await. You can also support older environments by transforming your code to ES5 with the help of regenerator (or tools that use regenerator, such as Babel).

Let functions accept callbacks

A callback is when function 1 is passed to function 2. Function 2 can call function 1 whenever it is ready. In the context of an asynchronous process, the callback will be called whenever the asynchronous process is done. Usually, the result is passed to the callback.

In the example of the question, you can make foo accept a callback and use it as success callback. So this

var result = foo();
// Code that depends on 'result'

becomes

foo(function(result) {
    // Code that depends on 'result'
});

Here we defined the function "inline" but you can pass any function reference:

function myCallback(result) {
    // Code that depends on 'result'
}

foo(myCallback);

foo itself is defined as follows:

function foo(callback) {
    $.ajax({
        // ...
        success: callback
    });
}

callback will refer to the function we pass to foo when we call it and we pass it on to success. I.e. once the Ajax request is successful, $.ajax will call callback and pass the response to the callback (which can be referred to with result, since this is how we defined the callback).

You can also process the response before passing it to the callback:

function foo(callback) {
    $.ajax({
        // ...
        success: function(response) {
            // For example, filter the response
            callback(filtered_response);
        }
    });
}

It's easier to write code using callbacks than it may seem. After all, JavaScript in the browser is heavily event-driven (DOM events). Receiving the Ajax response is nothing else but an event.
Difficulties could arise when you have to work with third-party code, but most problems can be solved by just thinking through the application flow.

ES2015+: Promises with then()

The Promise API is a new feature of ECMAScript 6 (ES2015), but it has good browser support already. There are also many libraries which implement the standard Promises API and provide additional methods to ease the use and composition of asynchronous functions (e.g., bluebird).

Promises are containers for future values. When the promise receives the value (it is resolved) or when it is canceled (rejected), it notifies all of its "listeners" who want to access this value.

The advantage over plain callbacks is that they allow you to decouple your code and they are easier to compose.

Here is an example of using a promise:

function delay() {
  // `delay` returns a promise
  return new Promise(function(resolve, reject) {
    // Only `delay` is able to resolve or reject the promise
    setTimeout(function() {
      resolve(42); // After 3 seconds, resolve the promise with value 42
    }, 3000);
  });
}

delay()
  .then(function(v) { // `delay` returns a promise
    console.log(v); // Log the value once it is resolved
  })
  .catch(function(v) {
    // Or do something else if it is rejected
    // (it would not happen in this example, since `reject` is not called).
  });

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JavaScript Asynchronous Return Value/Assignment with Jquery