Preparation code

 
<script>
  Benchmark.prototype.setup = function() {
    function concatList(list, otherList) {
      var pad = list.length, length = otherList.length;
      while (length--) list[pad + length] = otherList[length];
      return list;
    }
    
    function nativeConcat(list, otherList) {
      return list.concat(otherList);
    }
    
    var array2 = [11, 12, 13, 14, 15, 16, 17, 18, 19, 20];
  };
</script>

Test runner

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var array1 = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'];
var r = nativeConcat(array1, array2)[15];

var array1 = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'];
var r = concatList(array1, array2)[15];

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