counter.js

// creates a new variable 'counter' and set it's initial value to 0;
let counter = 0;
// creates a new function 'addOne' which takes 0 parameters
function addOne() {
    // adds one to counter. Equivalent to counter = counter + 1;
    counter++;
}
// calling a function. This executes the body of the named function.
addOne();
// prints our result to the console 
console.log(counter);