Given the following examples, why is outerScopeVar undefined in all cases?
var outerScopeVar;
var img = document.createElement('img');
img.onload = function() {
outerScopeVar = this.width;
};
img.src = 'lolcat.png';
alert(outerScopeVar);
var outerScopeVar;
setTimeout(function() {
outerScopeVar = 'Hello Asynchronous World!';
}, 0);
alert(outerScopeVar);
// Example using some jQuery
var outerScopeVar;
$.post('loldog', function(response) {
outerScopeVar = response;
});
alert(outerScopeVar);
// Node.js example
var outerScopeVar;
fs.readFile('./catdog.html', function(err, data) {
outerScopeVar = data;
});
console.log(outerScopeVar);
// with promises
var outerScopeVar;
myPromise.then(function (response) {
outerScopeVar = response;
});
console.log(outerScopeVar);
// with observables
var outerScopeVar;
myObservable.subscribe(function (value) {
outerScopeVar = value;
});
console.log(outerScopeVar);
// geolocation API
var outerScopeVar;
navigator.geolocation.getCurrentPosition(function (pos) {
outerScopeVar = pos;
});
console.log(outerScopeVar);
Why does it output undefined in all of these examples? I don't want workarounds, I want to know why this is happening.
Note: This is a canonical question for JavaScript asynchronicity. Feel free to improve this question and add more simplified examples which the community can identify with.
One word answer: asynchronicity.
Forewords
This topic has been iterated at least a couple of thousands of times here in Stack Overflow. Hence, first off I'd like to point out some extremely useful resources:
#Felix Kling's answer to "How do I return the response from an asynchronous call?". See his excellent answer explaining synchronous and asynchronous flows, as well as the "Restructure code" section.
#Benjamin Gruenbaum has also put a lot of effort into explaining asynchronicity in the same thread.
#Matt Esch's answer to "Get data from fs.readFile" also explains asynchronicity extremely well in a simple manner.
The answer to the question at hand
Let's trace the common behavior first. In all examples, the outerScopeVar is modified inside of a function. That function is clearly not executed immediately; it is being assigned or passed as an argument. That is what we call a callback.
Now the question is, when is that callback called?
It depends on the case. Let's try to trace some common behavior again:
img.onload may be called sometime in the future when (and if) the image has successfully loaded.
setTimeout may be called sometime in the future after the delay has expired and the timeout hasn't been canceled by clearTimeout. Note: even when using 0 as delay, all browsers have a minimum timeout delay cap (specified to be 4ms in the HTML5 spec).
jQuery $.post's callback may be called sometime in the future when (and if) the Ajax request has been completed successfully.
Node.js's fs.readFile may be called sometime in the future when the file has been read successfully or thrown an error.
In all cases, we have a callback that may run sometime in the future. This "sometime in the future" is what we refer to as asynchronous flow.
Asynchronous execution is pushed out of the synchronous flow. That is, the asynchronous code will never execute while the synchronous code stack is executing. This is the meaning of JavaScript being single-threaded.
More specifically, when the JS engine is idle -- not executing a stack of (a)synchronous code -- it will poll for events that may have triggered asynchronous callbacks (e.g. expired timeout, received network response) and execute them one after another. This is regarded as Event Loop.
That is, the asynchronous code highlighted in the hand-drawn red shapes may execute only after all the remaining synchronous code in their respective code blocks have executed:
In short, the callback functions are created synchronously but executed asynchronously. You can't rely on the execution of an asynchronous function until you know it has been executed, and how to do that?
It is simple, really. The logic that depends on the asynchronous function execution should be started/called from inside this asynchronous function. For example, moving the alerts and console.logs inside the callback function would output the expected result because the result is available at that point.
Implementing your own callback logic
Often you need to do more things with the result from an asynchronous function or do different things with the result depending on where the asynchronous function has been called. Let's tackle a bit more complex example:
var outerScopeVar;
helloCatAsync();
alert(outerScopeVar);
function helloCatAsync() {
setTimeout(function() {
outerScopeVar = 'Nya';
}, Math.random() * 2000);
}
Note: I'm using setTimeout with a random delay as a generic asynchronous function; the same example applies to Ajax, readFile, onload, and any other asynchronous flow.
This example clearly suffers from the same issue as the other examples; it is not waiting until the asynchronous function executes.
Let's tackle it by implementing a callback system of our own. First off, we get rid of that ugly outerScopeVar which is completely useless in this case. Then we add a parameter that accepts a function argument, our callback. When the asynchronous operation finishes, we call this callback, passing the result. The implementation (please read the comments in order):
// 1. Call helloCatAsync passing a callback function,
// which will be called receiving the result from the async operation
helloCatAsync(function(result) {
// 5. Received the result from the async function,
// now do whatever you want with it:
alert(result);
});
// 2. The "callback" parameter is a reference to the function which
// was passed as an argument from the helloCatAsync call
function helloCatAsync(callback) {
// 3. Start async operation:
setTimeout(function() {
// 4. Finished async operation,
// call the callback, passing the result as an argument
callback('Nya');
}, Math.random() * 2000);
}
Code snippet of the above example:
// 1. Call helloCatAsync passing a callback function,
// which will be called receiving the result from the async operation
console.log("1. function called...")
helloCatAsync(function(result) {
// 5. Received the result from the async function,
// now do whatever you want with it:
console.log("5. result is: ", result);
});
// 2. The "callback" parameter is a reference to the function which
// was passed as an argument from the helloCatAsync call
function helloCatAsync(callback) {
console.log("2. callback here is the function passed as argument above...")
// 3. Start async operation:
setTimeout(function() {
console.log("3. start async operation...")
console.log("4. finished async operation, calling the callback, passing the result...")
// 4. Finished async operation,
// call the callback passing the result as argument
callback('Nya');
}, Math.random() * 2000);
}
Most often in real use cases, the DOM API and most libraries already provide the callback functionality (the helloCatAsync implementation in this demonstrative example). You only need to pass the callback function and understand that it will execute out of the synchronous flow and restructure your code to accommodate for that.
You will also notice that due to the asynchronous nature, it is impossible to return a value from an asynchronous flow back to the synchronous flow where the callback was defined, as the asynchronous callbacks are executed long after the synchronous code has already finished executing.
Instead of returning a value from an asynchronous callback, you will have to make use of the callback pattern, or... Promises.
Promises
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 - MDN).
Promises (a.k.a. Futures) provide a more linear, and thus pleasant, reading of the asynchronous code, but explaining their entire functionality is out of the scope of this question. Instead, I'll leave these excellent resources for the interested:
JavaScript Promises - HTML5 Rocks
You're Missing the Point of Promises - domenic.me
More reading material about JavaScript asynchronicity
The Art of Node - Callbacks explains asynchronous code and callbacks very well with vanilla JS examples and Node.js code as well.
Note: I've marked this answer as Community Wiki. Hence anyone with at least 100 reputations can edit and improve it! Please feel free to improve this answer or submit a completely new answer if you'd like as well.
I want to turn this question into a canonical topic to answer asynchronicity issues that are unrelated to Ajax (there is How to return the response from an AJAX call? for that), hence this topic needs your help to be as good and helpful as possible!
FabrÃcio'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);
});
I'm creating online chat, but I'm wondering while using jQuery .load() in my script, my browser seems to get slow. When i checked the inspect element "Net" section, it loads bunches of GET-data... etc.
I would like to know if there's a better script solution with this code to prevent chat being heavy in the background while the data keeps looping in the background to check who's keep coming online/offline.
setInterval('loadThis()', 5000);
function loadThis () {
$("#loads").load('includes/users.php', function(){
$(".chat-side-panel li").each(function(i){
i = i+1;
$(this).addClass("stats"+i);
var status = $(".stats"+i).find("span.bullet").data("status"),
flag = $(".stats"+i).find("span.mail").data("flag");
if(status == 1) {
$(".stats"+i).find("span.bullet").addClass("online");
}
if(flag == 1) {
$(".stats"+i).find("span.mail").addClass("active");
}
});
});
}
the Chat-Side-Panel will be the main panel, and LI will be the listings of users including their status (online/offline) and flag (message received). As for the standard, what can you suggest for the setInterval time loading (if 5sec. is enough) or should i increase it.
Thanks for your input for this.
PS. We're doing this with both PHP/MySQL also.
One issue I see is that you keep re-querying the DOM for the same elements. Get them once, re-use them thereafter:
var load_target = $('#loads');
function loadThis () {
load_target.load('includes/users.php', function () {
load_target.find('.chat-side-panel li').each(function (i) {
var stats_li = $(this),
bullet = stats_li.find('span.bullet'),
mail = stats_li.find('span.mail');
bullet.toggleClass('online', (bullet.data('status') == 1))
mail.toggleClass('active', (mail.data('flag') == 1));
});
});
}
I don't know all of your involved logic or what the rest of your system looks like, so this particular code may not work exactly. It should simply serve as a re-factor done in a vacuum to show what that function could look like if you stopped hitting the DOM so hard.
Also, use of setInterval is not generally recommended. If the load of the remote file takes a while, you could end up calling loadThis() again before a previous one was completed. This would compound your DOM issues if calls to loadThis() began stacking up. Recursive use of setTimeout is preferred in a situation like this. Here is the above code modified to run recursively, and some usage examples below that:
var load_target = $('#loads'),
loadThis = function (start_cycle) {
$.ajax({
url: 'includes/users.php',
dataType: 'html',
type: 'GET',
success: function (response) {
load_target
.html(response)
.find('.chat-side-panel li').each(function (i) {
var stats_li = $(this),
bullet = stats_li.find('span.bullet'),
mail = stats_li.find('span.mail');
bullet.toggleClass('online', (bullet.data('status') == 1))
mail.toggleClass('active', (mail.data('flag') == 1));
});
},
complete: function () {
if (typeof start_cycle !== 'boolean' || start_cycle) {
load_target.data('cycle_timer', setTimeout(loadThis, 5000));
}
}
});
};
//to run once without cycling, call:
loadThis(false);
//to run and start cycling every 5 seconds
loadThis(true);
// OR, since start_cycle is assumed true
loadThis();
//to stop cycling, you would clear the stored timer
clearTimeout(load_target.data('cycle_timer'));
Last years (around 2012) I developed a chat system for a social network, and saw that
Using setInterval issue is when the request is being sent regularly, without waiting or carry about the result of the first requests in the queue. Sometimes the script can not respond and Mozilla or IE asks the user whether he should block or wait for the non-responding script.
I finally decided to use setTimeout instead. Here is what I did (I use $.getJSON so please study the example and how can use load instead)
function loadThis () {
$.getJSON('url').done(function(results){
//--use the results here
//then send another request
setTimeOut(function(){
loadThis();
},5000);
}).fail(function(err){
//console.log(print(err))
setTimeOut(function(){
loadThis();
},1000);
});
}
loadThis();
PS.: I would like to mention that the time depends on our many items are to be retrieved in your users.php file. Maybe you should use the paging tip. Your users.php can then treat url params users.php?page=1&count=100 for the first request, users.php?page=2&count=100 for the second until the results rows number is 0.
EDITS: In addition, I suggest you consider not interacting with the DOM every time. It is important too.
I want to know that suppose you did an ajax request to a page that runs PHP code. The page outputs some data (using flush() method or otherwise) but because of a 30second timeout or some other error, the php request ends.
Now when the request ends, I want to find out about it on client side and restart the request.
i.e suppose I have something like this
xmlhttp.onreadystatechange=function(){
if(xmlhttp.readyState==3 && xmlhttp.status==200){
document.getElementById("A").innerHTML=xmlhttp.responseText;
}
else if(xmlhttp.readyState==4 && xmlhttp.status==200){
document.getElementById("A").innerHTML=xmlhttp.responseText;
//plus some additional code to end the process
}
else if(xmlhttp.status== SOMETHING /*WHAT DO I ADD HERE TO KNOW THAT THE SERVER HAS TIMED OUT OR SOMETHING SO I CAN RESTART THE XMLHTTP CYCLE */){
//code to resend xmlhttp request.
}
}
One strategy I used was to set a timer in JS, and then clear it if the call was successful.
var myTimer = setTimeout(stuffToDoOnFailure, 6000); // 6 secs
ajaxCall(function callBack() {
clearTimeout(myTimer);
});
EDIT:
Of course, if the ajax call succeeds after 6 secs, you might end up with both stuffToDoOnFailure and callBack being executed, so you want to handle that case somehow. That depends on your app, though.
I'm trying to use this AJAX request to call a file with some PHP which is working ok and some JavaScript which is not. any ideas?
function showpart2(){
if(window.XMLHttpRequest){
xmlhttp = new XMLHttpRequest();
xmlhttp.open("GET","atuamae.org/parte2-encomendar.php",false);
xmlhttp.send(null);
}
document.getElementById('part2').innerHTML = xmlhttp.responseText;
eval(xmlhttp.responseText.getElementById('part2').innerHTML)
setTimeout('showpart2()',15000);
}
showpart2();
One big problem with the sample code is that making XMLHttpRequest.send synchronous means all JS execution must pause while waiting for the request to be received. There's no reason not to use an asynchronous call.
Asynchronous calls can improve responsiveness, but what they don't give you is coordination, which means a task won't run until the data it needs is ready. The standard way of coordinating asynchronous code is to pass to the asynchronous function a function that, when executed, performs the rest of the computation that relies on the data. This function has the technical name "continuation", which is simply a function that represents the rest of the computation from a given point forward. That is, turn:
f1();
f2();
async();
f3();
f4();
into:
f1();
f2();
async(function() {
f3();
f4();
});
Because you're passing around a continuation, this is known as "continuation passing style". XMLHttpRequest is a special case in that rather than passing a function to the asynchronous function, you set it as a listener for the readystatechange event on the XHR object. That is, you assign the continuation to xmlhttp.onreadystatechange.
There are a few more improvements to make. First, add error detection. The status property of the XHR instance holds the HTTP status, which you can use to check for errors.
As a number of others have mentioned, eval can be problematic and should be avoided when there's another option. For one thing, you have to make sure the string comes from a trusted source. The particular problem with eval here is that the script is evaluated in the same context as the call to eval. If the eval happens inside a function, anything defined by the script isn't visible outside the function. If your script doesn't need to define anything (and will never need to define anything; always consider the future of your code), you can use eval. Otherwise, dynamically create a script element with the script as content and add it to the document; you can define a function that does this (see globaleval in the sample below).
xmlhttp is a global variable, which is bad. Instead, declare it as a local variable.
Rather than setTimeout, which is for one-shot calls, use setInterval, which calls the passed function periodically. Note that both setTimeout and setInterval may take longer than the given delay to run, though that shouldn't be an issue here.
(function () {
// keep variable from polluting global namespace
var showpart2Interval = 0,
scriptElt = {parentNode: {removeChild: function() {}}};
function globaleval(script) {
scriptElt.parentNode.removeChild(scriptElt);
scriptElt = document.createElement('script');
scriptElt.type = 'text/javascript'
scriptElt.appendChild(document.createTextNode(script));
document.body.appendChild(scriptElt);
}
function showpart2(){
var xmlhttp = new XMLHttpRequest();
xmlhttp.open("GET","atuamae.org/parte2-encomendar.php",false);
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4) {
if (200 <= xmlhttp.status && xmlhttp.status < 300) {
globaleval(xmlhttp.responseText);
} else {
// HTTP error
...
}
}
}
xmlhttp.send(null);
}
function startShowpart2() {
if (window.XMLHttpRequest && !showpart2Interval) {
showpart2();
showpart2Interval = setInterval(showpart2, 15000);
}
}
function stopShowpart2() {
clearInterval(showpart2Interval);
showpart2Interval = 0;
}
window.startShowpart2 = startShowpart2;
window.stopShowpart2 = stopShowpart2;
})();
startShowpart2();
If you don't care about implementing all of this yourself, have jQuery do the heavy lifting. It's good to know how to do things yourself, but (for production code) using standard libraries with standard interfaces speeds up development in a number of ways.
See also
Javascript: Set the order of functions
You need to actually evaluate the JS-tags (manually, either through eval or through DOM insertion). A library like jQuery would do this for you, but if you need to use a custom solution you'll need to add that as well.
You write:
xmlhttp.responseText.getElementById('part2')
The responseText is a string, and won't have any getElementById method.
You either want to use xmlhttp.responseXML.getElementById("part2") or use string methods to extract the second part.
Like Adam Rackis I recommend you use $.ajax is reallyyyy easy try it.....but if it is not an option for you here is a function as used to use when making ajax calls and worked in IE and Firefox, Chrome I recommend to you if you can not use Jquery.
The problem in your function not works in IE as it has no xmlhttprequest object.
Hope this helps you.
function newAjaxObject()
{
var oHttp=false;
var asParsers=[
"Msxml2.XMLHTTP.5.0",
"Msxml2.XMLHTTP.4.0",
"Msxml2.XMLHTTP.3.0",
"Msxml2.XMLHTTP",
"Microsoft.XMLHTTP"
];
if ( !oHttp && typeof XMLHttpRequest != 'undefined')
{
oHttp=new XMLHttpRequest();
}
if( !oHttp){
for (var iCont=0; !oHttp && iCont < asParsers.length; iCont++)
{
try
{
oHttp=new ActiveXObject(asParsers[iCont]);
}
catch(e)
{
oHttp=false;
}
}
}
return oHttp;
}
I have a problem with Ajax. I'm totally noob with Ajax, and I apologize for such a stupid question.
I have a list of elements (loaded by a db) that I want to manage, i.e. Remove, Modify their name..; I want to use ajax to change db and the list.
But I want that the page is modified only AFTER the db has been modified. I can modify the page before the db is modified but it's not what I want.
That's my code:
function setXMLHttpRequest() {
var xhr = null;
if (window.XMLHttpRequest) {
xhr = new XMLHttpRequest();
}
else if (window.ActiveXObject) {
xhr = new ActiveXObject("Microsoft.XMLHTTP");
}
return xhr;
}
function modifyCat(n,newN){
xhrObj = setXMLHttpRequest();
var url = "modifyCat.php?action=modify&cat="+n+"&newCat="+newN;
xhrObj.open("GET", url, true);
links = document.getElementById("cat").getElementsByTagName("a");
updatePage(links);
xhrObj.send(null);
}
function updatePage(links) {
if (xhrObj.readyState == 4) {
var risp = xhrObj.responseText;
//code that works... if not put inside this if!
}
xhrObj.send(null);
}
ModifyCat.php is
//...
else if($action='modify'){
$n = cleanSimpleString($_GET['cat']);
$nN = cleanSimpleString($_GET['newCat']);
$qry = "UPDATE Categorie Set Nome='$nN' WHERE Nome='$n'";
$check = mysql_query($qry) or $db=0;
}
As I understand if (xhrObj.readyState == 4) should do want I'm asking. Instead with that If nothing happens (in the page, the php is correctly loaded). Without that If the page is correctly reloaded but while the db is working..
edit.
I would like to do that without framework, I think it's a simple thing that can be solved simply.
Thank you.
If you're "totally noob with Ajax" I'd recommend using an Ajax library, such as jQuery. Have a look at their Ajax page and you'll see that it's much more straightforward than working directly with XHR objects.
Also, you've got a typo - you've use hrObj in modifyCat.
You can either use the XMLHttpRequest to do a synchronous or asynchronous request. A synchronous request is easier to program, but will block your page until the result becomes available. An asynchronous request will execute a callback function when the result has become available. There are a number of events for which the callback will be executed, readyState = 4 means the result is available, see also:
http://en.wikipedia.org/wiki/XMLHttpRequest#The_onreadystatechange_event_listener
(and of course the rest of that article)
Having said that, take the suggestions elsewhere to heart, it is much easier (and more cross-browser compatible) to use jquery (or similar javascript/ajax library) to do this stuff.
It would save you a lot of headache if you just used jquery - it can be as easy as doing this:
$.get("modifyCat.php", { action: "modify", cat: n, newCat: newN },
function(data){
alert("Data Loaded: " + data);
});
http://api.jquery.com/jQuery.get/