I'm developing a site in javascript and jquery. Sometimes when I refresh I just get different random errors in firebug. What's the deal?
edit: I'm getting errors like a variable isn't defined, when clearly it is and working, and when i refresh again, the error is gone..
using Firefox V3.5.5 Firebug V.1.5.3 and I'm primarily working with jQuery 1.4.2
OK. While it's more or less impossible to give a reasonable solution to such a general question, I'll just add my 2 cents' worth:
One possible source of "undefined variable" errors comes from including several scripts, which may or may not always load and execute in the same order. If you define a variable in one script (let's call that script declare.js) and use it in another (let's say use.js), and use.js is executed before declare.js, then you will get such an error. If the scripts execute the other way around, everything will appear fine.
If you're interested in this very topic, have a look at e.g. Steve Souders' book Even faster web sites, published by O'Reilly. More specifically, look at the chapter about non-blocking script loading.
Most common cause is that you're trying to execute Javascript before the DOM is loaded and thus before all HTML elements are available in the DOM tree, which in turn may cause that simple calls like document.getElementById(id) and jQuery's $(selector) may return undefined elements. That it sometimes works is pure coincidence and a matter of timing.
You need to ensure that any Javascript/jQuery code which is supposed to be executed during page load and relies on the availability of the elements in the DOM tree, also really get executed after the DOM is loaded. In plain vanilla JS you can do so:
window.onload = function() {
document.getElementById(someId);
}
and in jQuery:
$(document).ready(function() {
$(someSelector);
});
Related
I'm using a framework which features auto-connecting to server on page load. I can disable it by passing options arguments, but the line that confuses me is this:
You can prevent this initial socket from connecting automatically by disabling io.sails.autoConnect before the first cycle of the event loop elapses.
My questions are:
When does the first cycle of the event loop elapses?
Is this behaviour the same across ALL modern (IE9+) browsers?
I have a bunch of scripts (in <body>) loading between the lib and my entry file. Does this affect when the first cycle elapses? EDIT: Yes, it does.
How can I ensure my code runs before the first cycle elapses?
Is this kind of implementation of auto-connect considered good practice?
The documentation for the source file is a little more explicit; it says "This can be disabled or configured by setting io.socket.options within the first cycle of the event loop."
Basically what's happening is that there exists within the library a setTimeout(fn, 0) call, which is idiomatic for starting a parallel process. However, in the JS standards it's explicitly stated that JS is single-threaded: in other words, even though setTimeout and setInterval are asynchronous they are not actually parallel in the sense that any of their code will be executing simultaneously with any other code. So they wait until the current function is over before they execute. This queueing mechanism is known as the JavaScript event loop.
I believe that what you are asked to do by the script author is to modify the source to include the relevant change, perhaps at the bottom of the file for your convenience.
It is also likely that a similar effect will be achieved by putting a <script> tag underneath the one that loads the given JS. This has not been explicitly standardized by HTML 4, but may be implicitly standardized in the new HTML 5 spec (it's a complicated interaction between different parts of the specs).
In terms of HTML5, it looks like the current specs say that there is a afterscriptexecute event and a load event which occur immediately after any remote script is loaded (or, if it's an inline script, the load event is scheduled as a task -- I am not sure when those occur). So you might be able to guarantee it without modifying the script by instead doing:
<script>
function do_not_autoload() { /* ... */ }
</script>
<script onload="do_not_autoload()" src="./path/to/sails.io.js"></script>
but I'm not sure what the compatibility table for script#onload is going to look like.
I made you a jsfiddle which can be used to grab a 'fingerprint' for different browsers to get an idea of what evaluation orders are out there in the wild. The * is the document.body.onload event. On my system it produces:
Firefox 32.0.3 : cafdbe*
Chrome 37.0.2062 : cafd*be
IE 11.0.9600 : cafd*be
In other words,
I have been asked to fix an issue with a website and I am encountering an issue with a JavaScript error.
On the home page, [removed website link], I am receiving the error Uncaught TypeError: undefined is not a function. However, the function being called on that line (line 30) should exist, as the jQuery plugin is being loaded.
The call-stack just shows a series of anonymous functions pointing towards the jQuery file. I am having trouble determining why this is producing an undefined function error.
I have tried using the Chrome debugger to step through the code where the error occurs but it just seems to highlight the jQuery source file for every step.
My question is this:
How do I go about tracking down the source of the issue when the trail is just a series of anonymous functions in the jQuery source file?
Is there something I am missing here or that I am not considering?
Thank you.
Edit:
As is it not clear, the method being called, jQuery.ContentSlider is in fact being included within the page within the file testimonials.js.
This is not just a "What's wrong with my code" question, but also an inquiry into how I handle situations such as this in the context of JavaScript & jQuery specifically.
A call stack of anonymous functions is confusing to me, and I have already attempted to take the obvious steps, such as verifying the plugin is included and that this inclusion takes place before attempting to utilize that plugin.
Sorry for the confusion.
Edit - Solution Found
It appears that although jQuery and the plugin were included prior to use, another copy of the same jQuery file was being injected by a Joomla! module. Since this was the exact same Google hosted jQuery file, it did NOT appear twice in the Resources tab in the Chrome Developer Tools. It appears that Chrome will parse jQuery twice, but doesn't show it as being included twice. So, the version with the plugin attached was being overwritten.
Thank you to those who answered. Thanks to A. Wolff for bringing that piece of information about the Resource tab to my attention.
You're loading the slider after you instantiate it.
Reverse the order of these two blocks:
<script type="text/javascript">
$(function() {
jQuery('#two').ContentSlider({
width : '440px',
height : '240px',
speed : 400,
easing : 'easeOutQuad',
textResize : true
});
});
</script>
<script src="/templates/sp/javascript/jquery.sudoSlider.min.js"></script>
Edit: To the heart of your well-formed question about debugging, generally, Undefined is not a function, especially when dealing with frameworks, is a symptom of trying to access a method before it exists, which is why your attempted function call returns undefined rather than a function.
It's almost always the result of loading a framework after trying to call it, or in an asynchronous context, of not waiting for the framework to load or do something important.
EDIT 2: The above answer is not correct, as A.Wolff points out: it's not that you must reverse the order of the two blocks, but that:
1) The second framework is probably not the one you want, or
2) You have called jQuery('#two').ContentSlider when you meant to call .sudoSlider, (or whatever is appropriate for that framework).
I'd like to write a test case (using Selenium, but not the point of this question) to validate that my web application has no script errors\warnings or unhanded exceptions at certain points in time (like after initializing a major library).
This information can easily be seen in the debug consoles of most browsers. Is it possible to execute a javascript statement to get this information programatically?
It's okay if it's different for each browser, I can deal with that.
not so far read about your issue (as far as I understood your problem) here
The idea be the following:
I found, however, that I was often getting JavaScript errors when the page first loaded (because I was working on the JS and was introducing errors), so I was looking for a quick way to add an assert to my test to check whether any JS errors occurred. After some Googling I came to the conclusion that there is nothing built into Selenium to support this, but there are a number of hacks that can be used to accomplish it. I'm going to describe one of them here. Let me state again, for the record, that this is pretty hacky. I'd love to hear from others who may have better solutions.
I simply add a script to my page that will catch any JS errors by intercepting the window.onerror event:
<script type="text/javascript">
window.onerror=function(msg){
$("body").attr("JSError",msg);
}
</script>
This will cause an attribute called JSError with a value corresponding to the JavaScript error message to be added to the body tag of my document if a JavaScript error occurs. Note that I'm using jQuery to do this, so this specific example won't work if jQuery fails to load. Then, in my Selenium test, I just use the command assertElementNotPresent with a target of //body[#JSError]. Now, if any JavaScript errors occur on the page my test will fail and I'll know I have to address them first. If, for some strange reason, I want to check for a particular JavaScript error, I could use the assertElementPresent command with a target of //body[#JSError='the error message'].
Hope this fresh idea helps you :)
try {
//code
} catch(exception) {
//send ajax request: exception.message, exception.stack, etc.
}
More info - MDN Documentation
How to know which Javascript (.js) executed a GET or Ajax call using Firebug or Google Chrome Plugin?
For example, a request for a image or html file executed by a Javascript, how to know which Javascript on the page executed that
Update:
I have to (shamefully) admit that the original below was wrong. Due to the nature of the js execution flow this works at the first execution time:
console.log($("script").last().attr("class")); //or whatever
That, however, is not good enough, as illustrated by a call on a timeout. We need to keep the reference to the script element, this can be achieved by wrapping script contents into a closure and creating a variable to store the jQuery reference:
(function(){
var $scriptElement = $("script").last();
console.log($scriptElement.attr("class")); //or whatever
})();
Now, I have to disclaim that with the markup as above it is unlikely to be practical unless you come up with a better way to store the script element reference... this pretty much became a rather bad example of what could be done but really shouldn't :(
Unless you have to get the reference in-code, you would be much better off looking at the console output, it actually tells you where the output originated from, with the line# and everything:
Original:
Not sure how applicable this would be to external js (script tag with a src), but for inline scripts you could do something like this w/jQuery:
$(this).closest("script");//gets you reference to the script element
I'm assuming it would just a matter of getting its src attribute! Let us know if it works.
console.log($(this).closest("script").attr("src"));
In chrome you can break on any xhr request. This will only set breakpoints for AJAX calls though.
I have an ASP.NET MVC project that uses some simple AJAX functionality through jQuery's $.get method like so:
$.get(myUrl, null, function(result) {
$('#myselector').html(result);
});
The amount of content is relatively low here -- usually a single div with a short blurb of text. Sometimes, however, I am also injecting some javascript into the page. At some point when I dynamically include script into content that was itself dynamically added to the page, the script still runs, but it ceases to be available to the debugger. In VS2008, any breakpoints are ignored, and when I use the "debugger" statement, I get a messagebox saying that "no source code is available at this location." This fails both for the VS2008 debugger and the Firebug debugger in Firefox. I have tried both including the script inline in my dynamic content and also referencing a separate js file from this dynamic content -- both ways seemed to result in script that's unavailable to the debugger.
So, my question is twofold:
Is there any way to help the debugger recognize the existence of this script?
If not, what's the best way to include scripts that are used infrequently and in dynamically generated content in a way that is accessible to the debuggers?
I can not comment yet, but I can maybe help answer. As qwerty said, firefox console can be the way to go. I'd recommend going full bar and getting firebug. It hasn't ever missed code in my 3 years using it.
You could also change the way the injected javascript is added and see if that effects the debugger you're using. (I take it you're using Microsoft's IDE?).
In any case, I find the best way to inject javascript for IE is to put it as an appendChild in the head. In the case that isn't viable, the eval function (I hate using it as much as you do) can be used. Here is my AJAX IE fixer code I use. I use it for safari too since it has similar behavior. If you need that too just change the browser condition check (document.all for IE, Safari is navigator.userAgent.toLowerCase() == 'safari';).
function execajaxscripts(obj){
if(document.all){
var scripts = obj.getElementsByTagName('script');
for(var i=0; i<scripts.length; i++){
eval(scripts[i].innerHTML);
}
}
}
I've never used jquery, I preferred prototype then dojo but... I take it that it would look something like this:
$.get(myUrl, null, function(result) {
$('#myselector').html(result);
execajaxscripts(result);
});
The one problem is, eval debug errors may not be caught since it creates another instance of the interpreter. But it is worth trying.. and otherwise. Use a different debugger :D
This might be a long shot, but I don't have access to IE right now to test.
Try naming the anonymous function, e.g.:
$.get(myUrl, null, function anon_temp1(result) {
$('#myselector').html(result);
});
I'm surprised firebug is not catching the 'debugger' statement. I've never had any problems no matter how complicated the JS including method was
If this is javascript embedded within dynmically generated HTML, I can see where that might be a problem since the debugger would not see it in the initial load. I am surprised that you could put it into a seperate .js file and the debugger still failed to see the function.
It seems you could define a function in a seperate static file, nominally "get_and_show" (or whatever, possibly nested in a namespace of sorts) with a parameter of myUrl, and then call the function from the HTML. Why won't that trip the breakpoint (did you try something like this -- the question is unclear as to whether the reference to the .js in the dynamic HTML was just a func call, or the actual script/load reference as well)? Be sure to first load the external script file from a "hard coded" reference in the HTML file? (view source on roboprogs.com/index.html -- loads .js files, then runs a text insertion func)
We use firebug for debug javascript, profile requests, throw logs, etc.
You can download from http://getfirebug.com/
If firebug don't show your javascript source, post some url to test your example case.
I hope I've been of any help!
If you add // # sourceURL=foo.js to the end of the script that you're injecting then it should show up in the list of scripts in firebug and webkit inspector.
jQuery could be patched to do this automatically, but the ticket was rejected.
Here's a related question: Is possible to debug dynamic loading JavaScript by some debugger like WebKit, FireBug or IE8 Developer Tool?