Difference between feature detection, feature inference, and using the UA string

What is the difference between feature detection, feature inference, and using the UA string is one of the favorite questions among interviewers, especially around JavaScript. They are the different methods of understanding and verifying if a web technology feature exists in the user environment. 

To Understand the difference between feature detection, feature inference, and using the UA string with examples, we will go through them one by one. 

What is Feature Detection?

The feature is a specific and distinctive character of a program or object. In feature detection, we aim to check if a particular feature exists or not. For example, when you open any application that uses maps, you may feel ALL the maps in applications appear similar Because they probably use the same API to apply the map feature.  So here, we check if the browser supports geolocation or not.

What is Feature Inference?

Inference means making a conclusion by cause and effect. For example, if you saw someone running after they saw a dog running behind them, you can infer fear. So, In Feature Inference is verified if a feature exists assuming the next web technology feature exists. Basically, it is the same as feature detection, but we first assume one thing. For example, in prior examples we have seen geolocation API in feature detection, there we assume that the user already has a modern browser.

What is UA STRING?

In computer language, a user agent or UA is any software replaced by any user, which “retrieves, renders and facilitates end-user interaction with Web content” or a special kind of software agent. UA String is a Characteristic string that identifies the specific application, system, or version. The user-agent string is inferring or reading the little string that each browser responds to the user requests to figure out which browser are these application targeting.

Different applications or web pages to different browsers may lead to a bad user experience because the internet is accessible to a wide range of populations and we should try building and iterating the features for every browser rather than targeting every single browser. But everything cannot be perfect and we must consider edge cases where browser detection is a need.

	feature detection	feature inference	UA String
1	Feature detection means testing whether a particular browser supports a specific feature or not.	Feature Inference is we have Predetermined a feature exists and assumed the next web technology feature we are implementing. In simple words, if A exists B should exist too.	A browser’s User-Agent string (UA) is known to identify which browser is being used, what version, and on which operating system.
2	Feature detection is somewhat different from browser sniffing.	Feature Inference is similar to feature detection.	Feature detection is similar to browser sniffing.
3	For example, when you open any application that uses maps, you may feel ALL the maps in applications appear similar Because they probably use the same API to apply the map feature.	For example, if the browser supports geolocation APIs, the user is using a modern browser and local storage is available.	Example: a particular feature may work well in Chrome browser but does not have the same functionality in safari.
4	We can consider using feature detection libraries for complex feature detection. Modernizr is a great library to handle feature detection	Similar kinds of libraries are used in feature inference too.	They do not have libraries but strings to identify different browsers.
5	This is the way of checking if a web technology feature exists in the user environment.	if a web technology feature exists in the user environment, by the method of assuming.	All three including the UA string are ways of verifying if a web technology feature exists in the user environment.
6	Code Example For Feature Detection: if (“geolocation” in mapNavigator) {     //get user’s current location }  else {     // Give the user a picture of      // static maps or a default setting }	Code Structure Example For Feature Inference: if(document.getElementsByParentId) {      // assume parentId exists document.getElementById(id);      //check the feature }	In UA String Detection, we get a code string from the user’s browser.
7	This method is still useful and in practical use.	This method is similar to feature detection but not good, as assuming can be bad at times.	Since the Internet is so wide-ranged, building programs for a specific browser can be waste of time and effort. Hence, this method is not so practical these days.

Conclusion:

• Feature detection is trying to verify if a feature exists. For example, if the user’s browser supports the geolocation APIs.
• Feature inference is if A exists we can assume B will exist too.
• UA String is the string that helps you identify which browser is used by the user.