A user agent is a computer program representing a person, for example, a browser in a Web context.
Besides a browser, a user agent could be a bot scraping webpages, a download manager, or another app accessing the Web. Along with each request they make to the server, browsers include a self-identifying User-Agent HTTP header called a user agent (UA) string. This string often identifies the browser, its version number, and its host operating system.
Spam bots, download managers, and some browsers often send a fake UA string to announce themselves as a different client. This is known as user agent spoofing.
A typical user agent string looks like this: "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:35.0) Gecko/20100101 Firefox/35.0".
Mapping UA string tokens to a more human-readable browser name for use in code is a common pattern on the web today. When mapping the new Edge token to a browser name, Microsoft recommends using a different name than the one developer used for the legacy version of Microsoft Edge to avoid accidentally applying any legacy workarounds that are not applicable to Chromium-based browsers.
When Microsoft is notified about these types of issues, website owners are contacted and informed about the updated UA. In these cases, Microsoft uses a list of UA overrides in our Beta and Stable channels to maximize compatibility for users who access these sites.
There are ways to develop your website to progressively enhance itself based on the availability of features rather than by targeting specific browsers. Using the user agent to detect the browser looks simple, but doing it well is, in fact, a very hard problem.
Also, experts, or people with another point of view, can give you ideas for working around the bug. Your site needs to use a specific Web feature that some browsers don't yet support, and you want to send those users to an older Website with fewer features but that you know will work.
This is the worst reason to use user agent detection because odds are eventually all the other browsers will catch up. The difficulty of successfully using user agent detection is worth a few disruptions to the purity of your HTML.
Also, rethink your design: can you use progressive enhancement or fluid layouts to help remove the need to do this? In those rare cases where behavior differs between browsers, instead of checking the user agent string, you should instead implement a test to detect how the browser implements the API and determine how to use it from that.
Support could have been added to other browsers at any time, but this code would have continued choosing the inferior path. As the above code demonstrates, there is always a way to test browser support without user agent sniffing.
For example, in the above code snippets, using look behind in short-regexp notation (e.g. /reg/IGM) will cause a parser error in unsupported browsers. Progressive enhancement This design technique involves developing your Website in 'layers', using a bottom-up approach, starting with a simpler layer and improving the capabilities of the site in successive layers, each using more features.
Graceful degradation This is a top-down approach in which you build the best possible site using all the features you want, then tweak it to make it work on older browsers. People use user agent sniffing to detect if the users' device is touch-friendly and has a small screen, so they can optimize their website accordingly.
While user agent sniffing can sometimes detect these, not all devices are the same: some mobile devices have big screened sizes, some desktops have a small touchscreen, some people use smart TV's which are an entirely different ballgame altogether, and some people can dynamically change the width and height of their screen by flipping their tablet on its side! Rather, add in touch conveniences such as bigger, more easily clickable buttons (you can do this using CSS by increasing the font size).
Here is an example of code that increases the padding of #exampleButton to 1em on mobile devices. This effect can be easily achieved using CSS flex boxes, sometimes with floats as a partial fallback. Also try to move less relevant/important information down to the bottom and group the page's content together meaningfully.
Although it is off-topic, perhaps the following detailed example might give you insights and ideas that persuade you to forgo user agent sniffing. The boxes can be separated into multiple columns via two equally fair method.
The second method uses a Column layout and resents all the dogs to the left and all the cats to the right. Only in this particular scenario, it is appropriate to provide no fallback for the flex boxes/multi columns, resulting in a single column of very wide boxes on old browsers.
If more people visit the webpage to see the cats, then it might be a good idea to put all the cats higher in the source code than the dogs so that more people can find what they are looking for faster on smaller screens where the content collapses down to one column. Next, always make your code dynamic. The user can flip their mobile device on its side, changing the width and height of the page.
Or, there might be some weird flip-phone-like device thing in the future where flipping it out extends the screen. The simplest way to do this is to separate all the code that moves content around based on screen size to a single function that is called when the page is loaded and at each resize event thereafter.
Also note that there is a huge difference between the media queries (max-width: 25em), not all and (min-width: 25em), and (max-width: 24.99em) : (max-width: 25em) excludes (max-width: 25em), whereas not all and (min-width: 25em) includes (max-width: 25em). One such case is using user agent sniffing as a fallback when detecting if the device has a touch screen.
Internet Explorer (on Windows) and WebKit (on iOS) are two perfect examples. However, Internet Explorer was such a special little wasp exception prior to version 9 that it was very easy to detect the browser based upon the browser-specific features available.
WebKit is a bit worse because Apple forces all the browsers on IOS to use WebKit internally, thus the user has no way to get a better more updated browser on older devices. For example, WebKit 6 has a bug whereby when the device orientation changes, the browser might not fire MediaQueryList listeners when it should.
Most browsers set the name and version in the format BrowserName/VersionNumber, with the notable exception of Internet Explorer. So to detect Safari you have to check for the Safari string and the absence of the Chrome string, Chromium often reports itself as Chrome too or Sea monkey sometimes reports itself as Firefox.
The browser version is often, but not always, put in the value part of the BrowserName/VersionNumber token in the Sergeant String. There are five major rendering engines: Trident, Gecko, Presto, Blink, and WebKit.
It is therefore important to pay attention not to trigger false-positives when detecting the rendering engine. Must containGeckoGecko/xyzWebKitAppleWebKit/pay attention, WebKit browsers add a 'like Gecko' string that may trigger false positive for Gecko if the detection is not careful. PrestoOpera/XYZ Note: Presto is no longer used in Opera browser builds >= version 15 (see 'Blink')Trident Trident/Internet Explorer put this token in the comment part of the Sergeant StringEdgeHTMLEdge/the non-Chromium Edge puts its engine version after the Edge/ token, not the application version.
Note: Edge HTML is no longer used in Edge browser builds >= version 79 (see 'Blink'). BlinkChrome/XYZ Most rendering engines put the version number in the RenderingEngine/VersionNumber token, with the notable exception of Gecko. The Operating System is given in most Sergeant strings (although not web-focused platforms like Firefox OS), but the format varies a lot.
They indicate the OS, but also often its version and information on the relying on hardware (32 or 64 bits, or Intel/PPC for Mac). Like in all cases, these strings may change in the future, one should use them only in conjunction with the detection of already released browsers.
A technological survey must be in place to adapt the script when new browser versions are coming out. The most common reason to perform user agent sniffing is to determine which type of device the browser runs on.
Never assume that a browser or a rendering engine only runs on one type of device. Never use the OS token to define if a browser is on mobile, tablet or desktop.
Internet Explorer Immobile/XYZ token in the comment. Mozilla/5.0 (compatible; MSI 9.0; Windows Phone OS 7.5; Trident/5.0; Immobile/9.0)Edge on Windows 10 Mobile/XYZ & Edge/ tokens outside the comment. Mozilla/5.0 (Windows Phone 10.0; Android 6.0.1; Xbox; Xbox One) Apple WebKit/537.36 (HTML, like Gecko) Chrome/58.0.3029.110 Mobile Safari/537.36 Edge/16.16299In summary, we recommend looking for the string “Mob” anywhere in the Sergeant to detect a mobile device. If the device is large enough that it's not marked with “Mob”, you should serve your desktop site (which, as a best practice, should support touch input anyway, as more desktop machines are appearing with touchscreens).
I have included a zip file with all the example source code at the start of this tutorial, so you don’t have to copy-paste everything… Or if you just want to dive straight in. Download & Notes Mobile Detection Useful Bits & Links The End Click here to download the source code, I have released it under the MIT license, so feel free to build on top of it or use it in your own project.
There's no “standard” way of writing an user agent string, so different web browsers use different formats (some are wildly different), and many web browsers cram loads of information into their user agents. Some mobile web browsers will let you change what the browser identifies itself as (i.e. “Mobile Mode” or “Desktop Mode”) in order to access certain websites that only allow desktop computers.
Newsreaders, screen readers and other web-related applications and systems act as agents as well. Some websites check the browser version in the user agent to ensure the user has a current browser and directs them to various vendors’ websites to download a newer version.
Many enterprise applications check browser type and versions due mostly to heavy customization of the UI and to ensure an optimal experience. The user is typically denied access without the application’s “certified” browser or is notified the experience will be less than optimal.