Is it bad practice to have a single javascript file that gets loaded accross all pages even if certain functions are only needed on certain pages? Or should the files be split up according to functionality on a given page and loaded only by pages that need them?
According to YSlow less files is better, but try to keep each file under 25k. Also make sure you minify or otherwise reduce the size of the js (and css) files. If possible turn on GZip for js on the webserver and set a far future expires header.
See here for the Yahoo Developer performance best practices.
If this file is really large, it could impact certain user's perceived performance (i.e. download and render times). IMHO you should split it up into reasonable groups of functions, with each group having similar functions (such that pages only reference the files they need).
depends on the size and complexity of the unused functions.
the javascript-parser anyway only stores the location and the signature of each function. as far as i know, it is only parsed when executed.
if traffic is a problem for you, rather include only those you need...
regards
Since the JS files are cached once they are downloaded and the JS parser shows no noticable performance difference btw a big JS file(not a HUGE one ;)) and a small js file, you should go with the single file approach.
Also it is known that multiple js files reduces the performance.
You're best off with a single JS file, as browsers will cache it after the first request for it (unless they've turned that off, in which case they deserve what they get). Another thing that will vastly, vastly increase your perceived performance on page load is turning on gzip compression in the web server - most JS files compress very well.
I would recommend to use one big file, for each file the browser launches a web request. Most browsers, I'm not quite sure how much it is with the newest versions of the known browsers, only launch a few concurrent web requests. The browser will wait until the files have been downloaded before launching the next web requests.
The one big file will be cached and other pages will load faster.
As #Frozenskys mentions YSlow states that less files is better, one of the major performance enhancements proposed by the Yahoo team is to minimize the amount of http requests.
Of course if you have a HUGE javascript file that literally takes seconds to download, it's better to split it up to prevent that the user has to wait seconds before the page loads.
A single file means a single download; as this article explains, most browsers will only allow a limited number of parallel requests to a single domain. Although your single file will be bigger than multiple small ones, as the other answers have pointed out:
The file will be cached
Techniques like minification and server-side gzip compression will help to reduce the download time.
You can also include the script at the end of the page to improve the perceived load time.
Related
I don't know much about JS parsing or performance.
But I would like to know if there is a performance difference when the browser parses a large file, vs smaller ones with the same total size.
What parses faster:
10kb vs. (2 x 5kb)
Does it matter? Or is it negligible? What if we compare:
1mb vs. (1000 x 1kb)
Note1: This is about browsers parsing JS.
Note2: I'm assuming it's all the same JS code. I mean, either in a single file, or split across multiple smaller files, the parsed code should be the same.
(V8 developer here.)
For parsing speed, the overall amount of script matters, not the organization into files.
That said, there could be secondary effects, which could have more impact on overall perceived page load performance than raw parsing speed. As Ashok's answer points out, the downloading is part of the picture, where historically many small resources were at a disadvantage, which as you point out should be addressed by HTTP2. On the flip side, a few resources might get a speedup thanks to concurrent connections, compared to a single larger chunk.
Another effect worth considering is caching. If you have one part of your code that changes rarely (e.g. a third-party library, which you only update every few months) and another part that changes a lot (e.g. your own code, where you deploy new versions every other day), then it makes sense to split the script files along this line, so that the browser can cache the parts that don't change. That would at least avoid the download of the part that hasn't changed; some browsers might even be able to cache the result of parsing/compiling the code, which would save even more work.
The applicable rules of thumb are:
(1) Do what makes sense for your case (i.e. what's most convenient); or at least start with that and see if it works well enough. Premature optimization (i.e.: making things more complicated in the hope of enabling more speed, without having verified whether that's actually necessary or helpful) is usually a bad idea.
(2) Measure any alternatives yourself, with a test that's as close as possible to your real situation. For example, apply a realistic split to your actual sources (maybe in a handful of chunks? or combine them into one if splitting was what you did before) and test that, rather than generating thousands of files with dummy content. If you can't measure a difference, then there is no difference that matters! And if you can measure a difference, then you have your answer :-)
Speed-wise, a single JS file is always better than multiple JS files as browser would be making fewer network requests. Also, to fetch each JS file, the browser would first open an HTTP request connection, perform handshake and then make the data (JS file) transfer.
Due to this, the multiple JS files makes the content load slower.
But again it depends on what code does the JS file holds. For more detailed info please have a look at One big javascript file or multiple smaller files?
I'm working on improving the page performance of my company's intranet page. We're looking to (dynamically) combine our javascript files as well as cache them for 30+ days. The page launches on login for everyone.
One of my coworkers asked if it's worth the time to combine the JS files if we're already caching them for a month. He's hesitant to do both because the combining tool is server side and doesn't run on our desktop, requiring a somewhat hacky workaround.
I've been doing some research and most of the recommendations for performance I've seen are for external sites. Since we're in a closed system it would seem like we wouldn't get much benefit from combining the files once everyone's cache is primed. What would combining the files buy us that aggressive caching wouldn't?
We're on IE8 if that makes any difference.
The most notable impact with having multiple JavaScript files is the time required to render the page. Each script tag is processed separately and adds time to the overall render process.
A pretty good answer can be found here # multiple versus single script tags
If we are talking a large number of scripts then you may see an improvement in render time; if it is just two or three files then it likely won't bring abount a noticable difference once the files have been cached.
I would suggest testing the page render time in both cases and see how much improvement you see in your case and decide based on that information.
As a useful example, here are some stats from Xpedite (runtime minification tool I created a while back); note the difference in time from load to ready for combined vs uncombined scripts.
Combine all your JavaScript files into a single big file (thus minimizing the number of requests made to the server), and set its name to something like "application_234234.js"; the number represents the last time you have changed the file and will help the browser know it's a new file (thus no cache when you change it).
Add an Expires or a Cache-Control Header (set it really far into the future). Since the file name will change each time you'll modify it, you don't have to worry.
Last and not least, compress and gzip the JavaScript file.
These are some important advices, but learn more about best practices on: http://developer.yahoo.com/performance/rules.html
i simply would like to know from who has well documented about, if i have 10 js/css external files to append at my site does is better to compress them into only 1 file or is good to have 10,20 external source links anyway in a page speed point of view?
i also ask this, cause using both firebug google page speed and yahoo Yslow page speed tools, they conflict in this, google says to separate files, yahoo says compress all in one :| .. normally i would trust in bigG but who knows :|
One of the key performance enhancements you can make is to reduce the number of HTTP requests. Each external resource means one extra request, so grouping them together will have a positive impact on page performance.
If you want to learn more about front-end performance, check out Steve Souders' books. You can find a simplified overview of the topics in the books on the Yahoo! Developer Network (he was at Yahoo! when he wrote the first book).
First of all, compressing files is (nearly) always a good idea. Both using specific JS and/or CSS compressors, and using GZIP compression at the HTTP-level.
Deciding whether to combine files or not is not so easy; you need to juggle different goals:
Minimize the total number of bytes loaded; this includes making sure files can be retrieved from cache
Make sure the files loaded arrive in as few requests as possible.
Combining files optimizes for #2, but can be at the cost of #1. If different pages use different CSS / JS, then every page might get a different combined file (permutation of component files), making caching impossible.
A quick-and-dirty solution is to include all generic JavaScript and all CSS used on all the pages in two single compressed files (one JS, one CSS). If your visitors stay on your site for a longer time they will have the best experience, since all CSS/JS needs to be loaded only once, and that one time is as quick as possible.
If you combine your scripts into one script on one host and this host is slow -- your page will be slow. If you have your script broken up into a few scripts hosted on a CDN with different sub domains for the scripts, your browser will download more of them in parallel. Read that site on boosting download times. It has the conclusion "boosting parallel downloads can realize up to a 40% improvement in web page latency. You can use two or three hostnames to serve objects from the same server to fool browsers into multithreading more objects."
Make sure you have the Google Page Speed, YSlow and Firebug addons installed, then use them. They will help you make your website faster.
I am interested to know about the possibilities of reducing http requests on servers by sending different kind of contents in a single compressed files and later uncompress on client's browser and place the stuff(images,css,js) where it should be.
I read somewhere that firefox is working on plan to give such features in future releases but it has not been done yet plus it would not be a standard version.
Will you guys suggest any solution for this?can Flash be used to uncompress compressed files on client side for later use?
Thanks
We did more or less what you describe in our web an are extremely happy of the response time.
The original files are all separated (HTML, CSS, JS, images) and we develop on them.
Then when moving to production we have a shell script that:
use YUI compressor to compress CSS and JS
all images are read and converted to data:image/png;base64,...
all blank spaces and comments are removed from the HTML
all these resources are put inline in the HTML
The page is ~300kb and usually cached.The server gzip it, the real size travelling the network is then lower.We don't use any additional compression.
And then there is a second call to get the data(JSON for us) and start rendering it client side.
I had to read your question a few times before I got what you were asking. It sounds like you want to basically combine all the elements of your site into a single downloadable file.
I'm fairly confident in saying I don't believe this is possible or desirable.
Firstly, you state that you've heard that Firefox may be supporting this. I haven't heard about that, but even if they do, how will you be able to use the feature while still supporting other browsers?
But even if you can do it, you've tagged this as 'performance-tuning', on the grounds that you'll be saving a few http requests. But in your effort to save http requests to speed things up, you need to be cautious that you don't actually end up slowing things down.
Combining all the files may cut you down to one http request, but your site may then load slower as the whole thing would need to load before any of it would be ready for display (as opposed to a normal page load where your page load may take time but at least some of it may be ready for display quite quickly).
What you can do right now, and which will be useful for reducing http requests, is combine your stylesheets into a single CSS, your scripts into a single JS file, and groups of related images into single image files (google CSS Sprites for more info on this technique).
Even then, you need to be careful about which files you combine - the point of the exersise is to reduce http requests so you need to take advantage caching, or you'll end up making things worse rather than better. Browsers can only cache files that are the same over multiple pages, so you should only combine the files that won't change between page loads. So for example, only combine the Javascript files which are in use across all the pages on your site.
My final comment would be to re-iterate what I've already said: Be cautious about over-optimising to the point that you actually end up slowing things down.
I am trying to improve the performance of a web application. I have metrics that I can use to optimize the time taken to return the main HTML page, but I'm concerned about the external CSS and JavaScript files that are included from these HTML pages. These are served statically, with HTTP Expires headers, but are shared between all the pages of the application.
I'm concerned that the browser has to parse these CSS and JavaScript files for each page that is displayed and so having all the CSS and JavaScript for the site shared into common files will negatively affect performance. Should I be trying to split out these files so I link from each page to only the CSS and JavaScript needed for that page, or would I get little return for my efforts?
Are there any tools that could help me generate metrics for this?
Context: While it's true that HTTP overhead is more significant than parsing JS and CSS, ignoring the impact of parsing on browser performance (even if you have less than a meg of JS) is a good way to get yourself in trouble.
YSlow, Fiddler, and Firebug are not the best tools to monitor parsing speed. Unless they've been updated very recently, they don't separate the amount of time required to fetch JS over HTTP or load from cache versus the amount of time spent parsing the actual JS payload.
Parse speed is slightly difficult to measure, but we've chased this metric a number of times on projects I've worked on and the impact on pageloads were significant even with ~500k of JS. Obviously the older browsers suffer the most...hopefully Chrome, TraceMonkey and the like help resolve this situation.
Suggestion: Depending on the type of traffic you have at your site, it may be well worth your while to split up your JS payload so some large chunks of JS that will never be used on a the most popular pages are never sent down to the client. Of course, this means that when a new client hits a page where this JS is needed, you'll have to send it over the wire.
However, it may well be the case that, say, 50% of your JS is never needed by 80% of your users due to your traffic patterns. If this is so, you should definitely user smaller, packaged JS payloads only on pages where the JS is necessary. Otherwise 80% of your users will suffer unnecessary JS parsing penalties on every single pageload.
Bottom Line: It's difficult to find the proper balance of JS caching and smaller, packaged payloads, but depending on your traffic pattern it's definitely well worth considering a technique other than smashing all of your JS into every single pageload.
I believe YSlow does, but be aware that unless all requests are over a loopback connection you shouldn't worry. The HTTP overhead of split-up files will impact performance far more than parsing, unless your CSS/JS files exceed several megabytes.
To add to kamen's great answer, I would say that on some browsers, the parse time for larger js resources grows non-linearly. That is, a 1 meg JS file will take longer to parse than two 500k files. So if a lot of your traffic is people who are likely to have your JS cached (return visitors), and all your JS files are cache-stable, it may make sense to break them up even if you end up loading all of them on every pageview.