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    <p>This is a frequently asked question but I will try to give a better explanation than I have found. When you pass a function as a parameter (as when defining your event handler), javascript does not evaluate the function at that time, but instead stores the function along with a reference to its parent <code>scope</code>.</p> <p>The function does not get evaluated until the event handler is triggered. At that time, the interpreter will check the value of <code>sub</code> in the parent <code>scope</code>. Since this will always happen after your <code>for</code> loop has completed, it will always find the last value of <code>sub</code>, i.e. whatever <code>sub</code> was when your <code>for</code> loop was completed. So all of your event listeners will use the last value of <code>sub</code>.</p> <p>We can get the desired behavior by creating a <a href="https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Closures?redirectlocale=en-US&amp;redirectslug=Core_JavaScript_1.5_Guide%2FClosures" rel="nofollow">closure</a>. Replace this:</p> <pre><code>supcont.addEventListener("click",function() { toggleVisibility(sub); }); </code></pre> <p>with this:</p> <pre><code>(function(localSub) { supcont.addEventListener("click",function() { toggleVisibility(localSub); }); })(sub); </code></pre> <p>The reason this works is because we wrap each event handler declaration with a new parent <code>scope</code> by invoking an IIFE. This forces the event handler to retain a copy of the <code>scope</code> inside the IIFE (called creating a closure over that <code>scope</code>). Now, when the event handler goes looking for <code>localSub</code>, it will find it in the new parent <code>scope</code> and it will have the value we want.</p>
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    1. POaddEventListener in JS not working as expected
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    1. COThanks sir. BTW I just learned what is closure. Leaving link here it just in case : http://stackoverflow.com/questions/111102/how-do-javascript-closures-work :)
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