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    <p>Considering that <code>var</code> declarations have function scope (or global if declared in the global scope), your code is equivalent to:</p> <pre><code>var tempS = shapes.slice(), i, S; for (i = 0; i &lt; shapes.length; i++) { S = tempS.pop(); if (S.name == 'pieChart') { document.getElementById(S.id).addEventListener('mousedown', function (e) { alert(S.id); }, false); } } </code></pre> <p>Now, it is clear that <code>S</code> is scoped outside of the <code>for</code> block, and as the event handlers will be triggered after the loop finishes, <code>S</code> will have the value of the last iteration.</p> <p>You need to create a new scope, either through a callback factory (as in this <a href="https://stackoverflow.com/a/750506/1331430">related answer</a>) or an <a href="http://benalman.com/news/2010/11/immediately-invoked-function-expression/" rel="nofollow noreferrer">IIFE</a>, to trap the current <code>S</code> value of the iteration.</p> <pre><code>var tempS = shapes.slice(); for (var i = 0; i &lt; shapes.length; i++) { (function() {//introduces a new scope var S = tempS.pop(); //now S is scoped inside this iteration's IIFE if (S.name == 'pieChart') { document.getElementById(S.id).addEventListener('mousedown', function (e) { alert(S.id); //will reference the IIFE scope's S }, false); } }()); } </code></pre> <p>You can do this in many alternative ways as well, for example, using <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach" rel="nofollow noreferrer">Array#forEach</a>:</p> <pre><code>var tempS = shapes.slice(); shapes.forEach(function(S) { //S is now scoped in the forEach callback scope if (S.name == 'pieChart') { document.getElementById(S.id).addEventListener('mousedown', function (e) { alert(S.id); }, false); } }); </code></pre> <p>Note that this will change the iteration order, but I believe it won't matter for this use case.</p> <hr> <p>In ES6 it will be possible to use <code>let</code> to create block-scoped variables:</p> <pre><code>for (var i = 0; i &lt; shapes.length; i++) { let S = tempS.pop(); //scoped inside the `for` block if (S.name == 'pieChart') { document.getElementById(S.id).addEventListener('mousedown', function (e) { alert(S.id); }, false); } } </code></pre> <p>I'm leaving this approach here for future readers, as <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let" rel="nofollow noreferrer"><code>let</code></a> is not ready for production yet. Most recent browsers do have experimental implementations though (see <a href="http://kangax.github.io/es5-compat-table/es6/" rel="nofollow noreferrer">ES6 compat table</a>).</p>
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