subarray.js

var makeSubArray = (function(){

 var MAX_SIGNED_INT_VALUE = Math.pow(2, 32) - 1,
 hasOwnProperty = Object.prototype.hasOwnProperty;

 function ToUint32(value) {
 return value >>> 0;
 }

 function compareMaxIndex(object, index) {
 if (index < -1 || (index + 1 !== MAX_SIGNED_INT_VALUE && hasOwnProperty.call(object, index + 1)))
 return 1;
 else if (index >= MAX_SIGNED_INT_VALUE || (index !== -1 && !hasOwnProperty.call(object, index)))
 return -1;
 else
 return 0;
 }

 function getMaxIndexProperty(object, old_index) {
 var index = old_index,
 direction = interval = compareMaxIndex(object, old_index);
 if (direction == 0)
 return index;
 
 // find low and high by searching from old_index by powers of 2
 do {
 old_index = index;
 interval *= 2;
 index += interval;
 } while (compareMaxIndex(object, index) == direction);

 var high = direction == 1 ? index : old_index;
 var low = direction == 1 ? old_index : index;

 if (high >= MAX_SIGNED_INT_VALUE)
 high = MAX_SIGNED_INT_VALUE - 1;
 if (low < -1)
 low = -1;
 
 // binary search (implementation from underscore's sortedIndex())
 while (low < high) {
 var mid = Math.floor((low + high) / 2);
 // var mid = (low + high) >> 1; // returns negative results for numbers near MAX_SIGNED_INT_VALUE
 compareMaxIndex(object, mid) == 1 ? low = mid + 1 : high = mid;
 }
 return low;
 }

 return function(methods) {
 var length = 0;
 methods = methods || { };

 methods.length = {
 get: function() {
 return getMaxIndexProperty(this, length - 1) + 1;
 },
 set: function(value) {
 var constrainedValue = ToUint32(value);
 if (constrainedValue !== +value) {
 throw new RangeError();
 }
 
 // optimizate for the common case where a built-in
 // prototype is setting the length to what it really is after modifications
 if (compareMaxIndex(this, value) == 0)
 length = value;
 
 var old_len = this.length;
 for (var i = constrainedValue, len = old_len; i < len; i++) {
 delete this[i];
 }
 // the hasOwnProperty() check allows sparse arrays to be made contiguous
 // via setting the length after-the-fact
 for (var i = old_len, len = constrainedValue; i < len; i++) {
 if (!hasOwnProperty.call(this, i))
 this[i] = undefined;
 }
 length = constrainedValue;
 }
 };
 methods.toString = {
 value: Array.prototype.join
 };
 return Object.create(Array.prototype, methods);
 };
})();

function SubArray() {
 var arr = makeSubArray();
 if (arguments.length === 1) {
 arr.length = arguments[0];
 }
 else {
 arr.push.apply(arr, arguments);
 }
 return arr;
}