Function parameters not previously defined

Question

I have seen some javascript functions with parameters that are not previously defined, but still work. The doubt arose with the function below. The function myFunction() calls the method pessoas.sort(), which in turn takes an auto function invoked as argument. It has the parameter a and the b. Why does it work even though these haven’t been previously defined? Does the JS engine interpret them by itself? How this can be done at other times and with other parameters?

var pessoas = [
{country:"Brazil", name:"Lucas"},
{country:"EUA", name:"Jhon"},
{country:"Japan", name:"Sushi"}]

function myFunction() {
    pessoas.sort(function(a, b){return a.name - b.name});
    show();
}

function show() {
  document.getElementById("demo").innerHTML =
  pessoas[0].country + " " + pessoas[0].name + "<br>" +
  pessoas[1].county + " " + pessoas[1].name + "<br>" +
  pessoas[2].country + " " + pessoas[2].name;
}

<div id="demo"></div>
<button onclick="myFunction()">Mostrar</button>

Answer 1

Having optional parameters is common in several native methods, what is "strange here" is the organization callback itself being optional. This is documented, and if it is not passed this callback says to MDN:

If omitted, the array is ordered according to the Unicode code input of each of the characters, according to the conversion of each element to string.