What is the difference between Feature Detection and Feature inference

Question

I wanted to know the main difference between the two forms Feature Detection and Feature inference.

Answer 1

Feature Detection is to verify that the function is available to be used explicitly.

Feature inference consists in checking whether a function is available and, based on this, assuming that another one will also be available.

Feature Detection:

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

Feature inference:

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

Source: Question SO En