The difference between function name and function name () in JS

Beginner JS, encountered a problem. The code reads "

<script type= "Text/javascript" >
var fn=function () {
Alert ("Hello");
SetTimeout (fn,2000);
};
FN ();
</script>

This code is in the page each two seconds pop-up prompt box "Hello". However, to change settimeout (fn,2000) to SetTimeout (FN (), 2000), there is no interval pop-up prompt box. On this question, I do a simple analysis:

FN acts as a parameter to the SetTimeout function, waits for the 2s body with the FN function, and FN () represents an immediate call, and the code runs here without waiting for 2s to call the FN function directly.

The difference between function name and function name () in JS