CODEWITHSUNDEEP
javascript
theJava
theJava

Reputation: 15044

Difference between these two ways of calling a function

PopupFirstNameButton.addEventListener('click', FirstNameFunction);

// This gets called
function FirstNameFunction(){
  alert("Hello");   
}

// This does not
var FirstNameFunction = function (){
    alert("Hello");   
}

Upvotes: 6

Views: 575

Answers (5)

user422039
user422039

Reputation: 1458

You are missing 3rd argument to addEventListener again!

This happens because FirstNameFunction used in line 1 is undefined yet with anonymous function syntax at line 9. With function syntax FirstNameFunction symbol is in scope already.

Upvotes: 2

Joshua Carmody
Joshua Carmody

Reputation: 13740

In the first example, you're creating a named function. The function's name is FirstNameFunction.

In the second example, you're creating an anonymous function (a function that has no name). However, you're also defining a variable named FirstNameFunction that holds a reference to the anonymous function. In this case FirstNameFunction is not the function itself, but is just a variable that references it.

The reason these differences are important when assigning the event handler as you did on the first line, is because global-scope named functions can be referenced from anywhere in the code, as long as their declaration has been parsed and interpreted before you try to use them. On the other hand, variables can only be used while they're in scope. That means after they're defined, and before they fall out of scope. Therefore, you should be able to use the second declaration method with your event handler assignment, as long as you declare the variable pointing to the anonymous function before you call the event handler and you do it in the same scope.

This works:

var FirstNameFunction = function (){
    alert("Hello");   
}    
PopupFirstNameButton.addEventListener('click', FirstNameFunction, false);

This doesn't:

PopupFirstNameButton.addEventListener('click', FirstNameFunction, false); // FirstNameFunction is undefined!!
var FirstNameFunction = function (){
    alert("Hello");   
}

Neither does this:

function declareFunction()
{
    var FirstNameFunction = function (){
        alert("Hello");   
    }    
}  // FirstNameFunction falls out of scope here and is no longer declared

declareFunction(); // The anonymous function exists while this is running but the reference is lost when the function returns

PopupFirstNameButton.addEventListener('click', FirstNameFunction, false); // This doesn't work.

Upvotes: 6

dominicbri7
dominicbri7

Reputation: 2599

Maybe because they both have the same name? I tried :

<HEAD>
        <SCRIPT TYPE="text/JavaScript">
            // This does not get called?
            var FirstNameFunction = function (){
                alert("Hello");
            }
        </SCRIPT>
    </HEAD>
    <BODY>
        <button id="abutton" value="!"/>

        <SCRIPT TYPE="text/JavaScript">
            getById('abutton').addEventListener('click', FirstNameFunction);
            </SCRIPT>
    </BODY>

And it worked (in Chrome at least)

Upvotes: 1

Boldewyn
Boldewyn

Reputation: 82814

The first function gets bound at compile time. The function foo() syntax allows for look-ahead declaration of functions.

The second is a simple variable declaration. And you can't use variables, before they are declared...

Upvotes: 1

wong2
wong2

Reputation: 35760

var FirstNameFunction = function (){
    alert("Hello");   
}

this is an assignment statement , so only after this is executed, FirstNameFunction gets assigned a value. So when PopupFirstNameButton.addEventListener('click', FirstNameFunction); is executing, FirstNameFunction is undefined

Upvotes: 7

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