Which loads first css or javascript

I include CSS files before Javascript for a different reason. I am not guessing. In retrospect, I guess the important things are just to avoid dynamic loading of CSS and putting the JS inside a domReady event if you need to do dynamic sizing — hugomg.

Scripts should should not change any display. That's the CSS job. Also js should only act after or while the DOMContentLoaded is fired with some little but very specific situations.

But I understand what you mean. Stupid IE! Show 1 more comment. Know your users and what they value. Know your users and what browsing environment they use. Know what each file does, and what its pre-requisites are. Making everything work will take precedence over both speed and pretty.

Use tools that show you the network time line when developing. Test in each of the environments that your users use. It may be needed to dynamically server side, when creating the page alter the order of loading based on the users environment. When in doubt, alter the order and measure again. It is possible that intermixing styles and scripts in the load order will be optimal; not all of one then all of the other.

Experiment not just what order to load the files but where. In Body? After Body? Consider async and defer options when appropriate to reduce the net delay the user will experience before being able to interact with the page.

Test to determine if they help or hurt. There will always be trade offs to consider when evaluating the optimal load order. Pretty vs. Responsive being just one. Ted Cohen Ted Cohen 9 9 silver badges 16 16 bronze badges. The linked article no longer claims "Stylesheet loads block script execution". Is that no longer true? Greg - It's still true. Scripts need to be able to query DOM. They might not block script loading , if they're smart, but they will block script.

Updated I was not sure about the tests in OP. Salman A Salman A k 78 78 gold badges silver badges bronze badges. That's also why document. The reason is that the script may want to get coordinates and other style-dependent properties of elements, like in the example above. Naturally, it has to wait for styles to load. Doesn't make much sense to me, order of executed JS doesn't tell anything about it's purpose. However consider this One page on your site is 50k which is not unreasonable.

Its a small optimization but thats the reason for it. You are also, apparently, unaware of the fact that they now use a CDN. It should be a small script just for the purpose for reading session and then setting the style either css variables or other means based on the user session.

This prevents a flash of rendering default style before the script switches to set style. The answer: it probably doesn't matter The best answer here was from , so I decided to test for myself.

Steve Souders has already given a definitive answer but I wonder whether there's an issue with both Sam's original test and Josh's repeat of it. Andy Davies Andy Davies 5, 2 2 gold badges 24 24 silver badges 21 21 bronze badges.

If you look at this waterfall you can see where Chrome speculatively opens up a second connection before it's needed webpagetest. I was thinking normal latency for TCP purposes rather than low - what sort of environment was the test done in? Re: "Steve Souders has already given a definitive answer but The actual correct semantic should actually have been for Steve to say "Put CSS before Synchronous JavaScript" Otherwise people get it wrong by generalizing as it being a rule for all scripts Yes but most people just include scripts synchronously so Steve's advice is good to the uninitiated.

Consider for the same case, Instead of having single css, take 2 or 3 css files and try it out these ways, 1 css.. George Katsanos George Katsanos Performance is always an issue.

I just almost ignore that browsers that do not follow the spec exist. And the elements inside HEAD are called as they appear…. CSS should be loaded first then JS for the simple reason that I would rather my visitors see a styled non interactive page first, then get the interactive part after the css has loaded.

By the way: Most JS should be loaded at the bottom of the page unless of course it needs to be in the head , it speeds up page load times considerably.

Typically, in the head, I include all the text-based tags first title, meta, etc , then the favicon, then CSS, then JS. Essentially, the order boils down to what is necessary first to get the page to display or what loads faster first to get the page to display. Anything in the head must be downloaded first before proceeding further down into the HTML, so there would be a delay in displaying the HTML while everything in the head is being downloaded. On top of that, CSS, JS, and image files that need to be downloaded can be placed on different subdomains, which then allows for simultaneous downloading.

But—that has to be balanced between the amount of time it takes to actually do a DNS lookup versus the download time or filesize of the file being downloaded. If you use http-equiv meta tags, especially with the Content-Type attribute, be sure to add the before anything else including the title in the document. This increases rendering speed, because any content rendered before that has to be re-rendered by the browser. I would place the title immediately after. Such incompatibility problems persisted well into the early s, as old browsers were still being used and still needed supporting.

This is one of the main reasons why libraries like jQuery came into existence — to abstract away differences in browser implementations e. Things have improved significantly since then; modern browsers do a good job of supporting "classic JavaScript features", and the requirement to use such code has diminished as the requirement to support older browsers has lessened although bear in mind that they have not gone away altogether. If you are not already familiar with the basics of Troubleshooting JavaScript , you should study that article before moving on.

There are a number of common JavaScript problems that you will want to be mindful of, such as:. Note: The easiest solution is to declare the iteration variable with let instead of var —the value of i associated with the function is then unique to each iteration.

Unfortunately this does not work correctly with IE11, which is why we haven't used this approach in the "good" for loop. As with HTML and CSS , you can ensure better quality, less error-prone JavaScript code using a linter, which points out errors and can also flag up warnings about bad practices, etc. The JSHint homepage provides an online linter, which allows you to enter your JavaScript code on the left and provides an output on the right, including metrics, warnings, and errors.

It is not very convenient to have to copy and paste your code over to a web page to check its validity several times. What you really want is a linter that will fit into your standard workflow with the minimum of hassle.

Many code editors have linter plugins, for example Github's Atom code editor has a JSHint plugin available. Other popular editors have similar linting packages available. It is worth mentioning command line uses — you can install these tools as command line utilities available via the CLI — command line interface using npm Node Package Manager — you'll have to install NodeJS first.

For example, the following command installs JSHint:. JSHint loader for Webpack. Browser developer tools have many useful features for helping to debug JavaScript. For a start, the JavaScript console will report errors in your code. Make a local copy of our broken-ajax. If you look at the console, you'll see the error message "Uncaught TypeError: can't access property "length", heroes is undefined", and the referenced line number is If we look at the source code, the relevant code section is this:.

So the code falls over as soon as we try to access a property of jsonObj which as you might expect, is supposed to be a JSON object. This is supposed to be fetched from an external. You may already know what is wrong with this code, but let's explore it some more to show how you could investigate this.

It has a number of features available, but the main one you'll use often is console. Refresh the page in the browser, and you will get an output in the console of "Response value:", plus the same error message we saw before.

The console. A very common problem with async requests like this is when you try to do something with the response object before it has actually been returned from the network. Let's fix this problem by running the code once the load event has been fired — remove the console.

To summarize, anytime something is not working and a value does not appear to be what it is meant to be at some point in your code, you can use console.

Unfortunately, we still have the same error — the problem has not gone away. Let's investigate this now, using a more sophisticated feature of browser developer tools: the JavaScript debugger as it is called in Firefox.

The main feature of such tools is the ability to add breakpoints to code — these are points where the execution of the code stops, and at that point you can examine the environment in its current state and see what is going on.

Let's get to work. The error is now being thrown at line Click on line number 51 in the center panel to add a breakpoint to it you'll see a blue arrow appear over the top of it. At this point, the right-hand side will update to show some very useful information. We'd like you to try fixing this problem yourself.

If you get stuck, consult our fixed-ajax. Note: The debugger tab has many other useful features that we've not discussed here, for example conditional breakpoints and watch expressions. For a lot more information, see the Debugger page. As your apps get more complex and you start to use more JavaScript, you may start to run into performance problems, especially when viewing apps on slower devices.

Performance is a big topic, and we don't have time to cover it in detail here. Some quick tips are as follows:. In this section, we'll look at some of the more common cross-browser JavaScript problems. We'll break this down into:. In the previous article we described some of the ways in which HTML and CSS errors and unrecognized features can be handled due to the nature of the languages.

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