javascript - jQuery Event Handler created in loop

Question

So I have a group of events like this:

$('#slider-1').click(function(event){   
        switchBanners(1, true);
});
$('#slider-2').click(function(event){   
        switchBanners(2, true);
});
$('#slider-3').click(function(event){   
        switchBanners(3, true);
});
$('#slider-4').click(function(event){   
        switchBanners(4, true);
});
$('#slider-5').click(function(event){   
        switchBanners(5, true);
});

And I wanted to run them through a loop I am already running something like this:

for(i = 1; i <= totalBanners; i++){
    $('#slider-' + i).click(function(event){    
            switchBanners(i, true);
    });
}   

In theory that should work, but it doesnt seem to once I load the document... It doesnt respond to any specific div id like it should when clicked... it progresses through each div regardless of which one I click. There are more event listeners I want to dynamically create on the fly but I need these first...

What am I missing?

Answer 1

This is a very common issue people encounter.

JavaScript doesn't have block scope, just function scope. So each function you create in the loop is being created in the same variable environment, and as such they're all referencing the same i variable.

To scope a variable in a new variable environment, you need to invoke a function that has a variable (or function parameter) that references the value you want to retain.

In the code below, we reference it with the function parameter j.

   // Invoke generate_handler() during the loop. It will return a function that
   //                                          has access to its vars/params. 
function generate_handler( j ) {
    return function(event) { 
        switchBanners(j, true);
    };
}
for(var i = 1; i <= totalBanners; i++){
   $('#slider-' + i).click( generate_handler( i ) );
}

Here we invoked the generate_handler() function, passed in i, and had generate_handler() return a function that references the local variable (named j in the function, though you could name it i as well).

The variable environment of the returned function will exist as long as the function exists, so it will continue to have reference to any variables that existed in the environment when/where it was created.

---

UPDATE: Added var before i to be sure it is declared properly.