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PopupFirstNameButton.addEventListener('click', FirstNameFunction);
// This gets called
function FirstNameFunction(){
alert("Hello");
}
// This does not
var FirstNameFunction = function (){
alert("Hello");
}
PopupFirstNameButton.addEventListener('click', FirstNameFunction);
// This gets called
function FirstNameFunction(){
alert("Hello");
}
// This does not
var FirstNameFunction = function (){
alert("Hello");
}
5 Answers
Reset to default 7var 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
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.
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.
The first function gets bound at pile 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...
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)
本文标签： javascriptDifference between these two ways of calling a functionStack Overflow
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