How to Avoid Closure Issues When Using Event Listeners in a Loop?

Avoiding Closure Issues When Using Event Listeners in a Loop

In your attempt to attach event listeners to multiple elements using a for loop, you've encountered the issue where all listeners end up targeting the last object. This is due to the way closures work in JavaScript.

Understanding Closure

When a function refers to a variable declared outside its scope, it retains access to that variable even after the function exits. In your case, the event listener functions defined within the loop reference boxa and boxb, which are defined outside the loop. When the loop completes, these variables point to the last elements in the loop.

Fixing the Issue

To avoid this closure issue, you can use an immediately invoked function expression (IIFE) inside the loop. An IIFE creates a new scope, so the variables declared within it are not accessible outside the function.

Revised Code

Here's the revised code with the IIFEs in place:

// Function to run on click:
function makeItHappen(elem, elem2) {
  var el = document.getElementById(elem);
  el.style.backgroundColor = "red";
  var el2 = document.getElementById(elem2);
  el2.style.backgroundColor = "blue";
}

// 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
}

By using the IIFEs, you ensure that each event listener function has its own scope, preventing the closure issue. Now, each event listener will correctly target the corresponding element within the loop.

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