title: Is concatenating an array to itself faster than looping through the array to create more indexes?
body: <p>I was just taking a look through the source for the examples on the three.js github page, and I came across this ImprovedNoise class, which is basically a Perlin noise script:</p>
  
  <p><a href="https://github.com/mrdoob/three.js/blob/master/examples/js/ImprovedNoise.js" rel="nofollow">https://github.com/mrdoob/three.js/blob/master/examples/js/ImprovedNoise.js</a></p>
  
  <p>At the very top of the ImprovedNoise function is this:</p>
  
  <pre><code>var p = [151,160,137,91,90,15,131,13,201,95,96,53,194,233,7,225,140,36,103,30,69,142,8,99,37,240,21,10,
           23,190,6,148,247,120,234,75,0,26,197,62,94,252,219,203,117,35,11,32,57,177,33,88,237,149,56,87,
           174,20,125,136,171,168,68,175,74,165,71,134,139,48,27,166,77,146,158,231,83,111,229,122,60,211,
           133,230,220,105,92,41,55,46,245,40,244,102,143,54,65,25,63,161,1,216,80,73,209,76,132,187,208,
           89,18,169,200,196,135,130,116,188,159,86,164,100,109,198,173,186,3,64,52,217,226,250,124,123,5,
           202,38,147,118,126,255,82,85,212,207,206,59,227,47,16,58,17,182,189,28,42,223,183,170,213,119,
           248,152,2,44,154,163,70,221,153,101,155,167,43,172,9,129,22,39,253,19,98,108,110,79,113,224,232,
           178,185,112,104,218,246,97,228,251,34,242,193,238,210,144,12,191,179,162,241,81,51,145,235,249,
           14,239,107,49,192,214,31,181,199,106,157,184,84,204,176,115,121,50,45,127,4,150,254,138,236,205,
           93,222,114,67,29,24,72,243,141,128,195,78,66,215,61,156,180];
  
  for (var i=0; i &lt; 256 ; i++) {
  
      p[256+i] = p[i];
  
  }
  </code></pre>
  
  <p>You'll notice that p is populated with a randomly-sorted array of the numbers 0 to 255. Once the <code>p</code> array is established, the script does a <code>for</code> loop over every position in the array and effectively latches a copy of itself from positions 256 to 511. The order is the same, but the indexes are shifted by 256.</p>
  
  <p>So my question is this: is it faster in JavaScript to loop over an array like this or to simply do..</p>
  
  <pre><code>p = p.concat(p);
  </code></pre>