Can anonymous functions only be used once in a given piece of code?

The anonymous function is called, it just gets called later. The code that calls the function might not be visible to you, it might be buried in the source code for a library that you're using. But you can also right code yourself that takes in an anonymous function and then calls it later.

function doSomethingAfterOneSecond(callback) {
  setTimeout(function(){
    callback(); // This is when it gets called
  }, 1000);
}

doSomethingAfterOneSecond(function() {
  console.log("Hello Future-World");
})

In this case, the anonymous function is assigned to the callback parameter. Inside my function doSomethingAfterOneSeconds, I'm only calling it once, but I could have used it bunch of places and called it many times.

Also, in the example in the video at 6:30, this is an anonymous function that gets assigned to a variable. So it starts anonymous but it ends up having a name after all:

var alertRandom = function() {
  var randomNumber = Math.floor(Math.random() * 6 ) + 1;
  alert(randomNumber);
}

alertRandom();