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Ben Nadel at InVision In Real Life (IRL) 2018 (Hollywood, CA) with: Michelle Kong and Johnathan Hunt
Ben Nadel at InVision In Real Life (IRL) 2018 (Hollywood, CA) with: Michelle Kong Johnathan Hunt ( @JHuntSecurity )

Dereferencing Javascript Variables in Dynamic Methods

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There is one problem in Javascript that I cannot seem to get a handle on and it is killling me! I can't seem to get variables to pass by value as I would hope. Take the following example:

for (var intI = 0 ; intI < 10 ; intI++){
var objA = document.createElement( "div" );

// Set the click for the link.
objA.onclick = function(){ alert( intI ); };

// Set the link into the body div.
objDiv.appendChild( objA );
}

Now, in my head, each one of those links, when clicked should alert the appropriate intI value (0, 1, 2, 3, etc.); however, each of them will alert 10 which is the value that broke the FOR loop. It's like they all point to one variable and then get updated for each loop of the FOR iteration.

I can't seem to find a good solution to this. One method that seems to work, but is poop is something along the lines of:

// Define a function INSIDE this function.
function GetI( intX ){
return(
function(){ alert(intX); };
);
}

for (var intI = 0 ; intI < 10 ; intI++){
var objA = document.createElement( "div" );

// Set the click for the link.
objA.onclick = GetI( intX );

// Set the link into the body div.
objDiv.appendChild( objA );
}

This method works as would be expected, though I seem to think that it is doing the exact same thing. It must be something to do with the scoping. Since the intI value is getting passed to a local scope (int GetI()), and then getting passed back, it must be unique (since the local scope of the GetI() method is created unique of each FOR iteration.

This solution seems truly ganky to me. There has to be a better way. And this is just a simple example. I have many places where I want to be doing this with object reference and dynamic event handling. This one simple bumb is really holding me back!

Reader Comments

1 Comments

for (var intI = 0 ; intI < 10 ; intI++){
var objA = document.createElement( "div" );
var thisI = intI;

// Set the click for the link.
objA.onclick = function(){ alert( thisI ); };

// Set the link into the body div.
objDiv.appendChild( objA );
}

15,377 Comments

@TC,

Won't you still run into the same problem since "thisI" is not re-var'd on each iteration (all var commands are actually run at the top)? Seems like you'd run into the same problem.

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Ben Nadel