Why do Event Listeners in JavaScript Loops Always Target the Last Element?

Understanding Event Listener Behavior in Loops

In JavaScript, event listeners allow elements to respond to user actions. Using loops to add event listeners to multiple objects can raise concerns about listener functionality.

The Issue with Traditional Approach

Consider the following situation: you're trying to add click event listeners to multiple elements within a "container" class. However, when you use a traditional for loop, you notice that all listeners end up targeting the last element in the loop.

The Reason: Closures

This behavior occurs due to closures in JavaScript. When you assign variables within the loop, they reference the same memory location. So, when you attempt to access these variables in the event listener functions, they always refer to the last iteration's values.

The Solution: Immediate Invocation

To solve this issue, use immediate invocation within the loop. By doing so, you create a new execution context for each iteration, ensuring that the variables within that context remain local and unbound to the loop variables.

Fixed Code:

The corrected code below makes use of immediate invocation to achieve the desired behavior:

// Autoloading function to add the listeners:
var elem = document.getElementsByClassName("triggerClass");

for (var i = 0; i < elem.length; i += 2) {
    (function () {
        var k = i + 1;
        var boxa = elem[i].parentNode.id;
        var boxb = elem[k].parentNode.id;
        elem[i].addEventListener("click", function() { makeItHappen(boxa,boxb); }, false);
        elem[k].addEventListener("click", function() { makeItHappen(boxb,boxa); }, false);
    }()); // immediate invocation
}

In this corrected code, the immediate invocation creates a new execution context for each iteration, allowing the boxa and boxb variables to maintain their proper values within the event listener functions.

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