Callback references as arguments in Javascript

Question

var clientData = {
    id: 1969,
    fullName: "Not Set",
    setUserName: function (firstName, lastName)  {
      clientData.fullName = firstName + " " + lastName;
      console.log(clientData.fullName);
    }
};

function getUserInput(firstName, lastName, callback)  {  
  //why can't I say clientData.callback (firstName, lastName); instead
  callback (firstName, lastName);
}

getUserInput ("Peter", "Pan", clientData.setUserName);

As is commented in the code, why can't I say clientData.callback (firstName, lastName); if what the callback argument is referencing is the function that should be accessible from clientData with dot access? If clientData.setUserName is just a reference to a function object, I shouldn't be accidentally saying clientData.clientData.setUsername with my commented code, right?

I'm a beginner, and because it's not working I'm sure I'm thinking of something wrong.

You can say clientData.setUserName(), you cannot say clientData.callback() - there is no such method. — Bergi
– Bergi, Commented Apr 14, 2015 at 4:19

Felix Kling · Accepted Answer · 2015-04-14 04:15:38Z

2

clientData.callback would literally try to access property callback on clientData. Think about clientData.callback being equivalent to clientData['callback'].

What comes after the . is not treated as a variable. Even if it were, the value of callback is a function. You can't use a function as property name. Property names are strings.

answered Apr 14, 2015 at 4:15

Felix Kling

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Comments

concrete_d · Accepted Answer · 2015-04-14 04:17:47Z

2

When you create a function, think of the arguments as variables used in the function. When you do:

function getUserInput(firstName, lastName, callback)

and you call it with:

getUserInput ("Peter", "Pan", clientData.setUserName);

What implicitly happens at the top of the function is:

var firstName = "Peter";
var lastName = "Pan";
var callback = clientData.setUserName

Whatever you pass in as callback is referenced inside the function as callback. So if you pass in clientData.setUserName, it doesn't matter that it belongs to an object called clientData - it is still referred to as callback inside.

answered Apr 14, 2015 at 4:17

concrete_d

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Comments

akh · Accepted Answer · 2015-04-14 04:21:15Z

0

Try using:

function getUserInput(firstName, lastName)  {  
  clientData.setUserName (firstName, lastName);
}

answered Apr 14, 2015 at 4:21

akh

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