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    <p>Consider a simple function that adds the first N integers. (e.g. <code>sum(5) = 1 + 2 + 3 + 4 + 5 = 15</code>).</p> <p>Here is a simple JavaScript implementation that uses recursion:</p> <pre class="lang-js prettyprint-override"><code>function recsum(x) { if (x===1) { return x; } else { return x + recsum(x-1); } } </code></pre> <p>If you called <code>recsum(5)</code>, this is what the JavaScript interpreter would evaluate:</p> <pre class="lang-js prettyprint-override"><code>recsum(5) 5 + recsum(4) 5 + (4 + recsum(3)) 5 + (4 + (3 + recsum(2))) 5 + (4 + (3 + (2 + recsum(1)))) 5 + (4 + (3 + (2 + 1))) 15 </code></pre> <p>Note how every recursive call has to complete before the JavaScript interpreter begins to actually do the work of calculating the sum.</p> <p>Here's a tail-recursive version of the same function:</p> <pre class="lang-js prettyprint-override"><code>function tailrecsum(x, running_total=0) { if (x===0) { return running_total; } else { return tailrecsum(x-1, running_total+x); } } </code></pre> <p>Here's the sequence of events that would occur if you called <code>tailrecsum(5)</code>, (which would effectively be <code>tailrecsum(5, 0)</code>, because of the default second argument).</p> <pre class="lang-js prettyprint-override"><code>tailrecsum(5, 0) tailrecsum(4, 5) tailrecsum(3, 9) tailrecsum(2, 12) tailrecsum(1, 14) tailrecsum(0, 15) 15 </code></pre> <p>In the tail-recursive case, with each evaluation of the recursive call, the <code>running_total</code> is updated.</p> <p><em>Note: The original answer used examples from Python. These have been changed to JavaScript, since modern JavaScript interpreters support <a href="https://stackoverflow.com/questions/310974/what-is-tail-call-optimization">tail call optimization</a> but Python interpreters don't.</em></p>
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    1. POWhat is tail recursion?
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    1. USMatthias Braun
    1. USLorin Hochstein
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    1. COPython is kind of an odd choice here, since it does not have AFAIK tail-recursion elimination.
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    2. COChris Conway is correct. Tail calls are not optimized in Python, unfortunately. Guido claims having the stack available for debugging is better than TCO.
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    3. COYou'll find Guido's opinion [here](http://neopythonic.blogspot.com/2009/04/tail-recursion-elimination.html).
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