What's the difference between feature detection, feature inference, and using the UA string

Question

I was asked this question in a job interview recently, specifically around Javascript. I was wondering the proper response.

What exactly is the difference between feature detection, feature inference, and using the User agent string?

Answer 1

Feature detection checks a feature for existence, e.g.:

if (window.XMLHttpRequest) {
    new XMLHttpRequest();
}

Feature inference checks for a feature just like feature detection, but uses another function because it assumes it will also exist, e.g.:

if (document.getElementsByTagName) {
    element = document.getElementById(id);
}

Checking the UA string is an old practice and should not be used anymore. You keep changing the UA checks and never benefit from newly implemented features, e.g.:

if (navigator.userAgent.indexOf("MSIE 7") > -1){
    //do something
}