What is calling the function in this code?

I just finished JavaScript Basics and in it we learned to call a function such as : function example(a,b) { doSomething; }

example();

Now I'm working through the Loops, Arrays and Objects in workspaces and I come across the following example:

function randomNumber(upper) { return Math.floor( Math.random() * upper ) + 1; } var counter = 0; while ( counter < 10) { var randNum = randomNumber (6); document.write(randNum + ' '); counter+= 1; }

The code runs perfectly but I'm not sure of what "called" the function. I would have expected to see a line such as: randomNumber(75); to call it. Is the variable randNum somehow calling it? Any help would be appreciated. Thanks.