Help understanding callback functions

What is being referenced if we don't include the parameter as apart of the callback

There is a difference between passing in a callback, and actually calling it.

instead of

processUserInput(greeting);

you could just as easily write this, if this is more understandable for you:

processUserInput(function greeting(name) {
    document.write('Hello ' + name);
});

And inside processUserInput we call our callback by saying callback(name)

Meaning we're actually calling greeting(name) (the function we passed in)

I got your tag notice.

You asked, "How is it possible for a function that is declared with a parameter, be treated as a callback which does not have its parameter?" As Thomas pointed out, when the function is passed as a callback, it is not being invoked ("called"). Only the name of the function is being passed.

But then on line 7 the function (which is now named "callback") does get invoked. And here the argument it needs is being passed to it.

Whenever you call a function, you must have parentheses after the name even if no argument is being passed. This is how JavaScript knows if you are calling the function or just referencing it. Here's a simple example of invoking vs. referencing:

function hey() { return "Hey there!"; }

var reference = hey;         // "reference" now contains a reference to the function
var message = hey();         // "message" now contains "Hey there!"
var message2 = reference();  // "message2" now also contains "Hey there!"

I hope that clears things up. Happy coding!

So treating greeting as a callback it would look like this right? ...

5) function processUserInput(greeting) {
6)    var name = prompt('Please enter your name.');
7)    greeting(name);
  }

Which would look like...

5) function processUserInput(greeting) {
6)    var name = prompt('Please enter your name.');
7)    alert("Hello " + name)(name);
  }

This is what I don't get. I refer to the greeting function in the processUserInput argument. Referring to the function body of greeting I would insert

alert("Hello " + name)

With the function greeting being invoked on it's own. It would generate an alert dialog box with "Hello name"; name being whatever argument that is passed.

In the context of processUserInput, the function body of greeting is stored for the callback. For which a response is retrieved from a user and stored in the variable name. Then it is decided that particular variable will be used as an argument for the callback. Am I understanding this right?