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POJavascript object reference breaks after changing initial object's value
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  1. POJavascript object reference breaks after changing initial object's value
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    copied!<p>I am trying to understand a point of confusion I have with JavaScript objects. Specifically, I am interested in finding what, if anything, causes an object reference to break.</p> <p>To demonstrate the phenomenon, I have included a copy of some output from Chrome's JavaScript console. Note that I am working with arrays here, but we would expect objects to behave similarly given the subtle distinction between arrays and objects in JS. I have added comments for clarity.</p> <pre><code> // Set x to some array literal &gt; x = [1, 2, 3, 4, 5] [1, 2, 3, 4, 5] // Set y to x &gt; y = x [1, 2, 3, 4, 5] &gt; x [1, 2, 3, 4, 5] // as expected &gt; y [1, 2, 3, 4, 5] // as expected </code></pre> <p>As demonstrated above, both <code>x</code> and <code>y</code> output the expected value. Now I shuffle the values of x using a function called <code>shuffle</code> (specified at the bottom of this question).</p> <pre><code> // Shuffle x &gt; x = shuffle(x) [5, 1, 4, 2, 3] &gt; x [5, 1, 4, 2, 3] // x changes as expected &gt; y [5, 1, 4, 2, 3] // y changes as expected </code></pre> <p>Again, everything works as expected above. The variables <code>x</code> and <code>y</code> have maintained reference to the same object. However, when we repeat this operation, the results are strange.</p> <pre><code> // Shuffle x &gt; x = shuffle(x) [3, 1, 5, 4, 2] &gt; x [3, 1, 5, 4, 2] // x changes as expected &gt; y [5, 1, 4, 2, 3] // y didn't change this time </code></pre> <p>Below is the shuffle function, adapted from <a href="https://stackoverflow.com/questions/2450954/how-to-randomize-a-javascript-array">here</a>. Its purpose is to shuffle the contents of an array (parameter <code>r1</code>) and to return the first <code>n</code> items of the mixed array.</p> <pre><code>function shuffle(r1,n) { var i = r1.length, j, tempi, tempj, r2; r2 = r1; while (--i) { j = Math.floor(Math.random() * (i + 1)); tempi = r2[i]; tempj = r2[j]; r2[i] = tempj; r2[j] = tempi; } return r2.slice(0,n); } </code></pre> <p>I have since fixed the problem by rewriting my shuffle function based on <a href="http://bost.ocks.org/mike/shuffle/" rel="nofollow noreferrer">this function</a>. However, I would still like to understand what's going on. For a quick look at the code in action, I have made a <a href="http://jsfiddle.net/chrisfargen/hc4Jf/" rel="nofollow noreferrer">jsFiddle</a>.</p> <p>Any ideas? I appreciate your time.</p>
 

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