in this video example where does the parameter "error" is defined?

Its defined in the function declaration.

In Javascript, and many other languages, you define a function and also the expected arguments/parameters passed.

Saying "function someFunction(thing)" is declaring "thing" as an expected value right up front. :)

Like in the example below...

//this function is declaring three variables that it demands in order to run
function makeBurger(topping1, topping2, topping3) {
    alert("Here is your burger with " + topping 1 + ", " + topping2 + ", and " + topping 3);

}

//calling the following function would fail with an error because it only passes one value to the function
makeBurger("cheese");

//the following passes three values, even if only one matters
makeBurger("cheese", "", "");

In the example you provided, it looks like a data was sent, and after it was sent, a reply returns and if it contains an error, itll pass an error string through. This means it would fail if it DIDNT pass through a string because it is always expecting an error string, even if the error string is "unknown error occurred."

It would be roughly the same as saying...

//passing a string through the function
xhr.onerror("error - your code never went through :( ");

thanks for your answer. it is my fault I didn't provide the whole code cause I thought that if I sent it via "get help"button in the video page the link will automatically be presented..

function getJSON(url){
  return new Promise(function(resolve, reject){
    var xhr = new XMLHttpRequest();
    xhr.open('GET', url);
    xhr.onreadystatechange = handleResponse;
    xhr.onerror = function(error){reject(error);};
    xhr.send();

    function handleResponse() {
      if(xhr.readyState === 4 && xhr.status === 200) {
        var employees = JSON.parse(xhr.responseText);
        resolve(employees)
      } else {
        reject(this.statusText);
      }
    };    
  });
};

I can't see where "error" is being passed.