How much content being replaced with an AJAX call is too much? - javascript

I run into a common problem when trying to do AJAX development. Where possible, I like to try and just update data in an existing layout, and not the layout itself. For example, take the div below:
<div id="content-5">Here is some content</div>
I would get the updated value for content-5 from the server and just replace the contents of content-5 with the value. This makes a lot of sense for simple data replacements where the value is always going to be displayed in its pure form.
Sometimes the content is more complicated, and I have to actually get more than just raw data... maybe there is some logic to determine how a value is displayed and perhaps the style needs to be different depending on the data inside. In that case, I generally produce the HTML on the server side and inject the HTML into the element instead of just raw data.
Example: A status field from the controller comes back as "complete", but from the design doc, "complete" is supposed to show the user the text "Available" and it needs to be styled in a way different from other statuses.
Doing this in Javascript would require some in-depth view knowledge that the template layer probably already handles. The end result would be the same (code snippet below), but the difference is that there could possibly be some code duplication and a far more complicated Javascript layer.
<div id="content-5"><span class="success">Available</span></div>
Without fail, the requirement comes up that the system will need to handle "new" contents as well. The easiest solution to implement is to just get all of the content's at the same time so that I do not need to handle the extra complexity of injecting a new element instead of just replacing existing content.
So, I create a new template, wrap the contents in another element with an ID, and bulk replace all of the content divs at the same time any time there is a change.
<div id="allContent">
<div id="content-1">Some content A</div>
<div id="content-2">Some content B</div>
<div id="content-3">Some content C</div>
<div id="content-4">Some content D</div>
<div id="content-5">Some content E</div>
</div>
At some point, I have to wonder: Where is the line? At some point it feels like I'll eventually just be replacing the whole page with an AJAX request. Would this really be a problem?
I realize this may be pretty subjective, but what are some good strategies for determining to which level you should be replacing content with AJAX? Replacing just the data seems to be my preferred method when possible as it makes the AJAX controllers very simple. Replacing larger chunks of HTML from a template seems to be the easiest for handling more complicating layout and design issues and also feels like it could be more easily maintained. Are there other options I have not considered?
I expect there will be some discussion about manipulating the DOM programatically, but I personally really dislike this. The code ends up looking pretty horrible and really starts to integrate too much layout and design into the JS layer for my liking. Since I generally work with template libraries of some sort (whether raw PHP, PHP templates like Smarty or JSP in Java) it seems to make more sense to leave as much visual design there as possible.
EDIT
Based on the first few answers, it seems like this is being read as trying to keep the user on the same page but navigating around around the site or otherwise changing the page in a radical way with each update. The question is more about how to determine where the layout work for AJAX calls should happen and whether or not it is an acceptable practice to change large chunks of code with an AJAX request, knowing that replacement code may look nearly identical to what had been there before.

I think the most important requirement is the refresh requirement. If after several AJAX updates I hit refresh, the page I was just looking at should be the page that arrives. If the page reverts to a previous state for any reason then the URL is wrong. If for any reason your AJAX data is going to make the URL in the browser invalid then you should not be using AJAX to fetch that data.
There are exceptions, of course for data the is even newer than the last AJAX request. But that's obviously not what I'm talking about. A live chat screen could receive an update between the last AJAX request and the refresh. No big deal. I'm talking about the logical content and the URL describing it should always be in sync.

Complete personal opinion ex nihil, my rule of thumb is to change no more than 1 "panel" unit or 33% of the page whichever is less.
The basis for this is that the user should be able to clearly recognise the previous page state is related to the new state - how would you feel if you were suddenly teleported into the building to your right? Be gentle with your poor user.
There are also serious technical questions about the benefits of moving and inserting basically a page worth of data, which I think is a bit of an AJAX anti-pattern. What benefit does AJAX provide if you're going to do that?
Your specific question seems dependant on the supposition that the response coming back from your AJAX request isn't "just" data. This feels wrong to me from a separation of concerns point of view: I would expect a page to have all the layout information it requires already, the AJAX response itself to provide nothing more than dumb data/markup, and the JS event handler which created the request to sew the two together, MVC style. In that respect I think, yes, you're doing too much.
(by panel, I mean one logical design element - a menu, a ribbon, an item metadata panel, etc..)
edit: now that I think about it, I think SO's user profile page breaks my rule of thumb with those tab clicks

Depending on whether you want people to be able to to link to / bookmark etc the current page, you might want to navigate the user's browser.
This isn't a concern for some apps like GMail etc, and they won't ever refresh the page.
For myself, I tend to think it's a good practice to navigate the browser when navigating to a logically different place. eg. a person's profile vs. a list of their messages.
Sorry if this is vague, it's rather subjective :-)

A good guideline for something like this is to ask yourself, "Is this dynamic application 'content', or is it content-content?" Your use case sounds like application content that will change with each user. This is probably the best place for Ajax, but with everything, it's always nice not to just have one hammer. You don't want to do too much on one page. For instance, if one part breaks, the entire thing might, thereby frustrating the user.
Anywhere you're looking at actual page content or anything where the information is static, I strongly suggest avoiding the use of JavaScript, as it runs the risk of being invisible to search engines. Make sure anything linking to information like this is crawlable. The first step towards this is dynamic generation on the server side rather than browser side.

If you're using Smarty templates to produce a page, just fragment a template into various meaningful sections - news.tpl, email.tpl, weather.tpl - and have a master.tpl producing the structure of the page and calling child templates.
Then, if you're for example using an AJAX call triggered by a timeout to refresh the news, you can just call the server, cram the necessary data into news.tpl, and return the results into the news div you set up with master.tpl. This way your news layout is always following the pattern of news.tpl. (If you used JavaScript to manipulate formatting bits or set up event handling on document load, you'll need to attach that post-processing to fire after the AJAX call.)
You haven't really gotten specific about the types of things you're trying to replace here, and my initial reaction is that, if a single event is triggering multiple sections of the page to update at once, that's a sign that maybe you should be coallating those sections into a single display.
How much formatting gets done on the server end versus how much gets done on the client end with JavaScript? I'd say server-side formatting if possible, that way you have code that reflects discussions you've made about display layout and logic. Client-side formatting can be used for more interface-based issues - sorting rows in a table and alternating row colors with :odd and :even selectors, showing and hiding divs to create a "tabbed display" without hitting the server since the data won't change just from selecting a new tab, that sort of thing.
Finally, AJAX is one-way. If your web page is a view on a database, this isn't as much of a problem, but using AJAX manipulation to take the place of normal navigation is a terrible idea.

If you were habitually replacing the entire contents of a page using AJAX calls, I would agree that you have a problem. However, it appears to me that you are attempting to carefully think through the implications of your design and attempting, where possible, to avoid what annakata has called this "AJAX anti-pattern."
My rule is a bit simpler: as long as a substantial amount of context (e.g. menu on the left, header, various controls, page title, etc.) remains on a page, I am Ok with replacing almost anything with an AJAX call. That being said, I've never struggled with a page that has as much AJAX-generated code as you are.
I do have one question though: isn't it possible to encode state so that you can just replace some of the Divs in your example rather than all of them? If not, have you thought about doing so?

Related

Conditionally showing elements: should I createElement or hide the elements?

I have a section on my website, which I would like to have users see depending on whether the user is logged in or not.
I was wondering if it would be more efficient to create/remove the elements conditionally (or even dynamically write) with jQuery, or to simply hide/show the elements.
Which would be the best practice method that you would use? jQuery tutorials that I've done always makes me add and remove the elements.
Further(I hope I do not anger the meta gods with this), what is the general guidelines on when I should be creating/removing elements and when I should be showing/hiding them?
Thank you in advance!
At one point, long ago, the recommendation was that you never put elements on the document that you weren't using, and you removed the element once it was done.
You have to realize, however, that this recommendation was at a time when browsers didn't always handle CSS and layout control was horrible. This was an era of text-based browsers such as lynx.
The world is much different now and even most blind users [citation needed] use full-fledged browsers with screen readers and these screen readers are smart enough to understand hidden elements.
Today, it is much easier to simply put everything that isn't sensitive in the document and style sheet rules or JavaScript to hide, unhide and re-arrange as needed.
That said, sensitive information should be restricted. If a end user isn't allowed to see something, then don't give it to them, even hidden.
Remember, the client can never be trusted.
EDIT PER COMMENT
Lets imagine that your system has a sensitive field that we'll call coolness. Coolness is something that regular users can't ever see, but the web admins get to set on folks they like and don't like.
If you simply say:
<div id="coolness_rating" style="display:none">
<label>Coolness
<input name="coolness" value="4">
</label>
</div>
then even though it is hidden on the screen, a user can still look at the HTML markup and see that value. Its hidden, but easily findable.
Continuing on the same though, the back end code also needs a guard around that coolness value, so that someone can't tweak what is being sent and add that in.
The back end code needs to look something like this:
if (loggedInuser.AccessLevel === AccessLevel.Admin) {
// Only let Admins change the level
referencedAccount.Coolness = formData.coolness;
}

Can I load a javascript file with a partialView in asp.net-mvc?

I have an asp.net-mvc website where there is a top section with a bunch of filter information and the middle section is a reports. I now have a few different report formats and I want to toggle between a few reports. I have it working by making them all partial views and loading them via ajax (to avoid loading the common info over and over again) but one issue i realized is that some of the different reports have different javascript that goes along with them. For now, I am loading up all of the javascript files in the main parent page but I realized that I am wasting a lot of resources by download and wiring up all of the jquery events even if i never actually view a report
Is there anyway I can pass some javascript along with downloading a partial view in asp.net-mvc so I only load this and wire up the events "on demand" as required (instead of always)
Of course you can. Just be aware that the effects of the code will stick around even if you later remove the code itself: any functions you defined will remain defined, any listeners you attached will remain attached (as long as their target elements persist)... so it would be a good idea to make a setup() and teardown() methods for the loading code, that you'd invoke from your controlling code, rather than just drop a bunch of code to execute as it loads.
However, I would say it would need to be a rather unique set of circumstances for me to employ this method; for most part, it would be much easier and efficient to just load all the code you need at once, to benefit from client caching if nothing else. Toggle the behaviour of your code, don't toggle the code.

Make DerbyJS only re-render certain DOM nodes

The way DerbyJS (http://derbyjs.com) seems to work at the moment is that it replaces everything in the body tag of the document whenever you click a link.
Is there anyway to say use the template, but replace the content inside #main-content with that instead of the whole body?
Navigation on the left is fixed and doesn't need the benefits of realtime interaction.
Why this is an issue is needing to run some Javascript on the page load to set the size of some containers based on the size of a users browser window, and once I click a link, this setup gets wiped and recreated, and of course, the Javascript doesn't run again, because the document itself hasn't refreshed, just the body.
This would also allow me to write nicer jQuery bindings for the most part, $('element').click(, rather than $('html').on('click','element', ...
Any thoughts, or is this a step too far for this framework at this point in time?
P.S. As I'm only just getting started with Derby, and realtime frameworks in general, maybe what I'm trying to do isn't best practice anyway? I chose Derby because I like the UX part of initial render on the server, then the rest in the client, but sharing routers, which reduces the duplication of code. Open to any better ways of achieving this.
There is no way to rerender part of body on page reload. Just whole body.
You can use app.enter hook to run js code after every page render.
No need to use jQuery bindings, use Derby bindings
I fully agree with Vladimir's answer, just trying to add something to it.
It should be possible to re-render part of the UI through transitional routes (http://derbyjs.com/#routes). In your case it seems like app.enter is the way to go though.

Why focus an input on page load instead of inline?

Almost all web pages that I see designed to set the focus to an input box add the code into a body onload event. This causes the code to execute once the entire html document has loaded. In theory, this seems like good practice.
However, in my experience, what this usually causes is double work on the user, as they have already entered data into two or three fields and are typing into another when their cursor is jumped back without their knowledge. I've seen a staggering number of users type the last 2/3 of their password into the beginning of a username field. As such, I've always placed the JS focus code immediately after the input to insure there is no delay.
My question is: Is there any technical reason not to place this focus code inline? Is there an advantage to calling it at the end of the page, or within an onload event? I'm curious why it has become common practice considering the obvious practical drawbacks.
A couple thoughts:
I would use a framework like jQuery and have this type of code run on $(document).ready(.... window.onload doesn't run until everything on the page is fully loaded, which explains the delay you have experienced. $(document).ready(... runs when jQuery determines the DOM has been loaded. You could probably write the same sort of logic without jQuery, but it varies by browser.
I prefer to keep my Javascript separate from my HTML because it allows for a cleaner separation of concerns. Then your behavior is then kept separate from your document structure which is separate from your presentation in your CSS. This also allows you to more easily re-use logic and maintain that code — possibly across projects.
Google and Yahoo both suggest placing scripts at the bottom of the html page for performance reasons.
The Yahoo article: http://developer.yahoo.com/performance/rules.html#js_bottom
You should definitely place the script in the appropriate place if it means the correct user experience -- in fact I would load that part of the script (Used for tabbing inputs) before the inputs to ensure it always works no matter how slow the connection.
The "document.ready" function allows you to ensure the elements you want to reference are in the dom and fires right when your whole document dom is loaded (This does not mean images are fully loaded).
If you want you could have the inputs start out as disabled and then reenable them on document ready. This would handle the rare case the script is not ready yet when the inputs are displayed.
Well if you call it before whole page has loaded you really don't know if the element already has been loaded when you make your call. And if you make your call in pre-hand you should check if the element really exists even if you know it always should.
Then to make the call inline, which might seem ideal. But on the other hand it's really bad if a page takes that long to load that you can make several inputs during the loading phase.
Also you could check if input has been made etc.
Also it is possible to check if any input on page contains focus if($("input::focus, textarea::focus").length)... and otherwise set focus on desired input.
Use the autofocus HTML attribute to specify which element should initially receive focus. This decouples JavaScript and gracefully degrades in older browsers.

Will infinite scroll cause browser crashes?

I implemented infinite scroll like so:
new_page_value = 1;
$(window).scroll(function() {
if($(window).scrollTop() >= $(document).height() - $(window).height() - 200) {
new_page_value = parseInt(new_page_value) + 1;
get_page(new_page_value);
}
});
When the user almost reaches the bottom of the page (200px left) the function get_page() is called. This contains an ajax call that gets all the contents of the new page and appends it to the <body> of the document.
Now I just realized if my site gets big and instead of having 10 small pages I have a gazillion giant pages then the user's browser might crash if they are persistent enough to keep infinite scrolling for long time.
Would this be a possible solution to this problem:
I will keep appending the new pages to the document <body> until the 10th page, after that I will be replacing the <body> content entirely instead of appending. So using html() rather than append().
I just don't know if this will actually work to prevent crashes. Will .html() clear the "memory" of prior html that was brought in via ajax?
I really think this is a common issue for many sites with AJAX list content. So let's take an example at some of the most popular ( think of scale = experience ) websites and their solutions :
Google Images
If you check out images.google.com and you search for whatever, for e.g. "guiness", you will see a page full of results (actually the images are ajax loaded, not the html-code, so the page is with fixed height) and when you scroll at the bottom there is a button "Show more results". This might be solution one of your problem, but is it really necessary to place a button at the bottom after, for e.g. the 10-th page? I really think it is generally a good solution for page usability and memory leaks, but it is really not a necessary option as we can see in :
Facebook
Facebook Newsfeed is another story. There is a button "Show more posts", but I really don't know when exactly it is displayed rather than loading the next page of posts. It happened to me once to load 10-15 pages of posts, only by scrolling. And you know Facebook posts include videos, photos, AJAX comments and a lot of more Javascript fancy stuff, which take a lot of memory. I think they've managed to do this after a lot of research, how much of the users scroll to the bottom.
Youtube
Youtube has "Load more videos" at every page, so the solution is basically similar to Google, except that Google renders the whole html of the page and on scrolling just loads the images.
Twitter
Twitter supports infinite scrolling. Yep, they do it may be because tweet is 140 characters and they don't need to worry about memory so much. After all who is willing to read more than 1000 pages of tweets at one page load. So they don't have a button for "load more" and they don't need one.
So there are two solutions :
Use infinite scrolling ( you should consider how much content you load and how rich it is )
Use button : "Load More"
Most of all, you should not delete already loaded content of a list.
Nowadays everything is Javascript and Javascript has garbage collection, so it is very hard to unload the DOM ( if it has Javascript, not plain text ) and manage to remove the Garbage from Javascript. Which means that you won't free the whole allocated memory of the unloaded content from the browser.
Also think about of your requests, why would you need to load again something, that you have already loaded at first place. It costs another server request, meaning another database request and so on.
I have worked with this before and here are some of my thoughts:
a) If you are appending data to the memory page(s) at a time then it is not an issue, some browsers might not respond well but most of the lastest browsers will render without any problem so long as there is enough memory on the target machine, you could probably see how the ram usage increases as you append pages. Use chrome for this as each page is a separate process and it has an inbuilt task manager
b) regarding usage of html(), it indeed removes the markup but it does so at a heavy cost as it tries to take care of special conditions and has an overhead and accesses all the controls nested within the container that you are replacing (not sure about the last pat), but it has a cost. A simpler way to clear the DOM would be to use the innerHTML property and set it to empty, jquery does this but it is at a later point in the html() api. open up the api and look at the method.
using innerHTML
$("<selector>")[0].innerHTML=""
Also deletion of pages sounds weird to me as a user, what if I want to go back to the initial comments and please dont think about making it an infinite scroller too.. I have tried and given up after the number of bugs raised but we had a genuine use case for it and I had to stick a button up there, but this wasnt when the user scrolled away from the first page, this is when the user landed on a 3rd page but now needs to see the results above it.
Hope that answers your question and btw infinte scrolling is your friend use it, dont over engineer a case which will probably only be tested by your QA team. Its better to spend your effort somewhere else.
Yes it will, if i may suggest an idea after let's say 5 pages just delete the first page and append the new one instead of deleted all of the previous pages. good luck :)

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