Struggling to understand how the value of certain function parameters are determined

Question

I often do not understand how the value of certain function parameters is determined. Take the following code snippet for example:

function gotItAll(res) {

$('#actionscreen').modal('hide')

if (res.ErrorHtml != "" && res.ErrorHtml.substring(0,31) != "Error copying to sub data table") {
    
    document.getElementById('GridView').innerHTML = res.ErrorHtml;
    InfoMessage("Error", res.ErrorHtml);
} else {

    if (res.BrowseTreeBreadCrumbs != null) {
        //generateBrowseTreeHtml(res);
        sessionStorage.setItem("browseTreeBC", res.BrowseTreeBreadCrumbs);
        if (res.BrowseTreeBreadCrumbs.indexOf("-1") > 0 || res.BrowseTreeBreadCrumbs.indexOf("- -1") > 0 || res.BrowseTreeBreadCrumbs.indexOf("- -999") > 0 || res.BrowseTreeBreadCrumbs.indexOf("-999") > 0 || res.BrowseTreeBreadCrumbs.indexOf("-999") > 0 || res.BrowseTreeBreadCrumbs.indexOf("</b>  </span") > 0) {
            $('#breadCrumbsID').hide();
        }
    }
    
    BaseTable = res.TableName; 
    recs = res.records;

'res' is a parameter that I could not find defined anywhere, yet it returns a value. I was not able to identify where gotItAll() is called and passed a value other than res. In addition, we have res.ErrorHtml, res.BrowseTreeBreadCrumbs.indexOf, res.TableName etc. that do not appear to be defined anywhere, yet they all return different values.

Answer 1

res is passed in from whatever calls gotItAll

The res.whatever are the attributes of the res object

The name res is just for use in that function, and will likely have a different name to what was used as the parameter.

Example

funciton addOneToNumber(numberToAddTo)
{
   return numberToAddTo += 1;
}

function mainFunction()
{
   var newNumber = addOneToNumber(1)
}

Edit:

You can use any object in javascript to be used as a parameter. In this case the function has been used. It also looks like failedCallback is also a function. Which would be called if the call to the server fails

Inside InventoryEditor.ItemEdit.AddNew somewhere, gotItAll is called with res which would of been a result of the server call from whichever type of call is made

Answer 2

The code snippet you posted is just the function declaration. Basically that snipper defines the existence of a function called gotItAll, which takes one parameter as the input. The parameter is called res, so inside the function, the word res refers to the parameter of the function.

Let's use a simple function as example:

function double(x) {
  return x * 2
}

This function as a parameter as well, called x. Inside the function, you refer to it as x.

When you want to use the function, you call the function and you gave a parameter, such as:

double(2) // returns 4
double(4) // returns 8
double(7) // returns 14

As you can see, we can pass to the function double different values. When you call double(4), basically the function considers x = 4, and so everytime inside the function declaration you use x, you will be using the value 4 instead.

You can also pass different variables to the function:

var y = 10
double(y) // returns 20

Same as before, you give to the function the variable y, whose value is 10. So basically the program does x = y and everytime it refers to x inside the function declaration, it will be using the value of y.

Back to your example. I don't know what that function do, but basically res is the placeholder name of the parameter that will be passed to the function. That function can be called from around the code as gotItAll(variableName), and the function will be executed considering res = variableName. In the snipper you pasted, the function is only declared but there are not examples of actual usages of the function, that's why you can't see where res comes from.

EDIT: after your comment, I realized what you are actually referring to. Actually, in Javascript you can pass functions as the parameter of another function. Take for example the double function declared above. Now I want to create a function that doubles the number only if they are even. I can declare this:

function doubleEven(x, doublingFunction) {
  if (x % 2 === 0) {
    return doublingFunction(x)
  } else {
    return x
  }
}

and I can use it as:

doubleEven(2, double) // returns 4
doubleEven(5, double) // returns 5

Basically, I pass double as a parameter to the doubleEven function. So, inside the doubleEven declaration, doublingFunction becomes equal to double and so it is a function.

Of course in this example this is silly, there is no need to pass a function for doubling a number. However this is used a lot in javascript to provide a callback to asynchronous task, such as a network request.