Why is my variable unaltered after I modify it inside of a function? – Asynchronous code reference

Fabrício’s answer is spot on; but I wanted to complement his answer with something less technical, which focusses on an analogy to help explain the concept of asynchronicity.

An Analogy…

Yesterday, the work I was doing required some information from a colleague. I rang him up; here’s how the conversation went:

Me: Hi Bob, I need to know how we foo‘d the bar‘d last week. Jim wants a report on it, and you’re the only one who knows the details about it.

Bob: Sure thing, but it’ll take me around 30 minutes?

Me: That’s great Bob. Give me a ring back when you’ve got the information!

At this point, I hung up the phone. Since I needed information from Bob to complete my report, I left the report and went for a coffee instead, then I caught up on some email. 40 minutes later (Bob is slow), Bob called back and gave me the information I needed. At this point, I resumed my work with my report, as I had all the information I needed.

Imagine if the conversation had gone like this instead;

Me: Hi Bob, I need to know how we foo‘d the bar‘d last week. Jim want’s a report on it, and you’re the only one who knows the details about it.

Bob: Sure thing, but it’ll take me around 30 minutes?

Me: That’s great Bob. I’ll wait.

And I sat there and waited. And waited. And waited. For 40 minutes. Doing nothing but waiting. Eventually, Bob gave me the information, we hung up, and I completed my report. But I’d lost 40 minutes of productivity.

This is asynchronous vs. synchronous behavior

This is exactly what is happening in all the examples in our question. Loading an image, loading a file off disk, and requesting a page via AJAX are all slow operations (in the context of modern computing).

Rather than waiting for these slow operations to complete, JavaScript lets you register a callback function which will be executed when the slow operation has completed. In the meantime, however, JavaScript will continue to execute other code. The fact that JavaScript executes other code whilst waiting for the slow operation to complete makes the behaviorasynchronous. Had JavaScript waited around for the operation to complete before executing any other code, this would have been synchronous behavior.

var outerScopeVar;    
var img = document.createElement('img');

// Here we register the callback function.
img.onload = function() {
    // Code within this function will be executed once the image has loaded.
    outerScopeVar = this.width;
};

// But, while the image is loading, JavaScript continues executing, and
// processes the following lines of JavaScript.
img.src = 'lolcat.png';
alert(outerScopeVar);

In the code above, we’re asking JavaScript to load lolcat.png, which is a sloooow operation. The callback function will be executed once this slow operation has done, but in the meantime, JavaScript will keep processing the next lines of code; i.e. alert(outerScopeVar).

This is why we see the alert showing undefined; since the alert() is processed immediately, rather than after the image has been loaded.

In order to fix our code, all we have to do is move the alert(outerScopeVar) code into the callback function. As a consequence of this, we no longer need the outerScopeVar variable declared as a global variable.

var img = document.createElement('img');

img.onload = function() {
    var localScopeVar = this.width;
    alert(localScopeVar);
};

img.src = 'lolcat.png';

You’ll always see a callback is specified as a function, because that’s the only* way in JavaScript to define some code, but not execute it until later.

Therefore, in all of our examples, the function() { /* Do something */ } is the callback; to fix all the examples, all we have to do is move the code which needs the response of the operation into there!

^{* Technically you can use eval() as well, but eval() is evil for this purpose}

How do I keep my caller waiting?

You might currently have some code similar to this;

function getWidthOfImage(src) {
    var outerScopeVar;

    var img = document.createElement('img');
    img.onload = function() {
        outerScopeVar = this.width;
    };
    img.src = src;
    return outerScopeVar;
}

var width = getWidthOfImage('lolcat.png');
alert(width);

However, we now know that the return outerScopeVar happens immediately; before the onload callback function has updated the variable. This leads to getWidthOfImage() returning undefined, and undefined being alerted.

To fix this, we need to allow the function calling getWidthOfImage() to register a callback, then move the alert’ing of the width to be within that callback;

function getWidthOfImage(src, cb) {     
    var img = document.createElement('img');
    img.onload = function() {
        cb(this.width);
    };
    img.src = src;
}

getWidthOfImage('lolcat.png', function (width) {
    alert(width);
});

… as before, note that we’ve been able to remove the global variables (in this case width).

Here’s a more concise answer for people that are looking for a quick reference as well as some examples using promises and async/await.

Start with the naive approach (that doesn’t work) for a function that calls an asynchronous method (in this case setTimeout) and returns a message:

function getMessage() {
  var outerScopeVar;
  setTimeout(function() {
    outerScopeVar = 'Hello asynchronous world!';
  }, 0);
  return outerScopeVar;
}
console.log(getMessage());

undefined gets logged in this case because getMessage returns before the setTimeout callback is called and updates outerScopeVar.

The two main ways to solve it are using callbacks and promises:

Callbacks

The change here is that getMessage accepts a callback parameter that will be called to deliver the results back to the calling code once available.

function getMessage(callback) {
  setTimeout(function() {
    callback('Hello asynchronous world!');
  }, 0);
}
getMessage(function(message) {
  console.log(message);
});

Promises

Promises provide an alternative which is more flexible than callbacks because they can be naturally combined to coordinate multiple async operations. A Promises/A+ standard implementation is natively provided in node.js (0.12+) and many current browsers, but is also implemented in libraries like Bluebird and Q.

function getMessage() {
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      resolve('Hello asynchronous world!');
    }, 0);
  });
}

getMessage().then(function(message) {
  console.log(message);  
});

jQuery Deferreds

jQuery provides functionality that’s similar to promises with its Deferreds.

function getMessage() {
  var deferred = $.Deferred();
  setTimeout(function() {
    deferred.resolve('Hello asynchronous world!');
  }, 0);
  return deferred.promise();
}

getMessage().done(function(message) {
  console.log(message);  
});

async/await

If your JavaScript environment includes support for async and await (like Node.js 7.6+), then you can use promises synchronously within async functions:

function getMessage () {
    return new Promise(function(resolve, reject) {
        setTimeout(function() {
            resolve('Hello asynchronous world!');
        }, 0);
    });
}

async function main() {
    let message = await getMessage();
    console.log(message);
}

main();

To state the obvious, the cup represents outerScopeVar.

Asynchronous functions be like…

The other answers are excellent and I just want to provide a straight forward answer to this. Just limiting to jQuery asynchronous calls

All ajax calls (including the $.get or $.post or $.ajax) are asynchronous.

Considering your example

var outerScopeVar;  //line 1
$.post('loldog', function(response) {  //line 2
    outerScopeVar = response;
});
alert(outerScopeVar);  //line 3

The code execution starts from line 1, declares the variable and triggers and asynchronous call on line 2, (i.e., the post request) and it continues its execution from line 3, without waiting for the post request to complete its execution.

Lets say that the post request takes 10 seconds to complete, the value of outerScopeVar will only be set after those 10 seconds.

To try out,

var outerScopeVar; //line 1
$.post('loldog', function(response) {  //line 2, takes 10 seconds to complete
    outerScopeVar = response;
});
alert("Lets wait for some time here! Waiting is fun");  //line 3
alert(outerScopeVar);  //line 4

Now when you execute this, you would get an alert on line 3. Now wait for some time until you are sure the post request has returned some value. Then when you click OK, on the alert box, next alert would print the expected value, because you waited for it.

In real life scenario, the code becomes,

var outerScopeVar;
$.post('loldog', function(response) {
    outerScopeVar = response;
    alert(outerScopeVar);
});

All the code that depends on the asynchronous calls, is moved inside the asynchronous block, or by waiting on the asynchronous calls.

In all these scenarios outerScopeVar is modified or assigned a value asynchronously or happening in a later time(waiting or listening for some event to occur),for which the current execution will not wait.So all these cases current execution flow results in outerScopeVar = undefined

Let’s discuss each examples(I marked the portion which is called asynchronously or delayed for some events to occur):

1.

Here we register an eventlistner which will be executed upon that particular event.Here loading of image.Then the current execution continuous with next lines img.src = 'lolcat.png'; and alert(outerScopeVar); meanwhile the event may not occur. i.e, funtion img.onload wait for the referred image to load, asynchrously. This will happen all the folowing example- the event may differ.

2.

Here the timeout event plays the role, which will invoke the handler after the specified time. Here it is 0, but still it registers an asynchronous event it will be added to the last position of the Event Queue for execution, which makes the guaranteed delay.

3.

This time ajax callback.

4.

Node can be consider as a king of asynchronous coding.Here the marked function is registered as a callback handler which will be executed after reading the specified file.

5.

Obvious promise (something will be done in future) is asynchronous. see What are the differences between Deferred, Promise and Future in JavaScript?

https://www.quora.com/Whats-the-difference-between-a-promise-and-a-callback-in-Javascript

The short answer is : asynchronicity.

Why asynchronous is needed?

JavaScript is single-threaded, meaning that two bits of the script cannot run at the same time; they have to run one after another. In browsers, JavaScript shares a thread with a load of other stuff that differs from browser to browser. But typically JavaScript is in the same queue as painting, updating styles, and handling user actions (such as highlighting text and interacting with form controls). Activity in one of these things delays the others.

You’ve probably used events and callbacks to get around this. Here are events:

var img1 = document.querySelector('.img-1');

img1.addEventListener('load', function() {
  // image loaded
  console.log("Loaded");
});

img1.addEventListener('error', function() {
  // error caught
  console.log("Error printed");
});

<img class="img-1" src="#" alt="img">

This isn’t sneezy at all. We get the image, add a couple of listeners, then JavaScript can stop executing until one of those listeners is called.

Unfortunately, in the example above, it’s possible that the events happened before we started listening for them, so we need to work around that using the "complete" property of images:

var img1 = document.querySelector('.img-1');

function loaded() {
  // image loaded
  console.log("Loaded");
}

if (img1.complete) {
  loaded();
} else {
  img1.addEventListener('load', loaded);
}

img1.addEventListener('error', function() {
  // error caught
  console.log("Error printed");
});

<img class="img-1" src="#" alt="img">

This doesn’t catch images that errored before we got a chance to listen for them; unfortunately, the DOM doesn’t give us a way to do that. Also, this is loading one image. Things get even more complex if we want to know when a set of images have loaded.

Events aren’t always the best way

Events are great for things that can happen multiple times on the same object— keyup, touchstart etc. With those events, you don’t really care about what happened before you attached the listener.

The two main ways to do it correctly: are callbacks and promises.

Callbacks

Callbacks are functions that are passed inside the arguments of other functions, this procedure is valid in JavaScript because functions are objects and objects can be passed as arguments to functions. The basic structure of the callback function looks something like this:

function getMessage(callback) {
  callback();
}

function showMessage() {
  console.log("Hello world! I am a callback");
}
getMessage(showMessage);

Promise

Although there are ways to keep the callback hell at bay with vanilla JS, promises are growing in popularity and are currently being standardized in ES6 (see Promise).

A promise is a placeholder representing the eventual result (value) of an asynchronous operation

• the promise placeholder will be replaced by the result value (if successful) or reason for failure (if unsuccessful)

If you don’t need to know when something happened, but just whether it happened or not, then a promise is what you are looking for.

A promise is a bit like an event listener, except that:

• a promise can only succeed or fail once
• a promise can’t switch from fail to success, or vice versa
• once you have a result, the promise is immutable
• if a promise has succeeded or failed, and you later add a success/failure callback, the correct callback will be called
• it doesn’t matter that the event occurred before you added the callback

Note: Always return a result from a function inside a Promise, otherwise there’s nothing for the subsequent function to act on.

Promise Terminology

A promise can be:

• fulfilled: The action relating to the promise succeeded
  • the asynchronous operation has completed
  • the promise has a value
  • the promise will not change again
• rejected: The action relating to the promise failed
  • the asynchronous operation failed
  • the promise will never be fulfilled
  • the promise has a reason indicating why the operation failed
  • the promise will not change again
• pending: Hasn’t fulfilled or rejected yet
  • the asynchronous operation hasn’t been completed yet
  • can transition to fulfilled or rejected
• settled: Has been fulfilled or rejected and is thus immutable

How to Create a Promise

function getMessage() {
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      resolve('Hello world! I am a promise');
    }, 0);
  });
}

getMessage().then(function(message) {
  console.log(message);
});