Exclude variables from object

So basically I understand you can loop through an array in either of these ways.

var testarray = new Array(1,2,3,4);
for(i in testarray)console.log(testarray[i]);
//outputs 1,2,3,4
for(var i=0;i<testarray.开发者_Python百科length;i++) console.log(testarray[i]);
//outputs 1,2,3,4

My question how can I duplicate/emulate that array. With support for both of the for loop methods? Because when I do the same thing but I create my own function it includes length in the 1st for loop.

function emulatearray(){
     for(var i = 0;i<arguements.length;i++)this[i]=arguments[i];
     this.length = i;
}                       
var testarray = new emulatearray(1,2,3,4);
for(i in testarray)console.log(testarray[i]);
//outputs 1,2,3,4,null
for(var i=0;i<testarray.length;i++) console.log(testarray[i]);
//outputs 1,2,3,4

The for...in statement shouldn't be used to iterate over an array.

Quoting the Mozilla Dev Center:

for...in Statement

Although it may be tempting to use this as a way to iterate over an Array, this is a bad idea. The for...in statement iterates over user-defined properties in addition to the array elements, so if you modify the array's non-integer or non-positive properties (e.g. by adding a "foo" property to it or even by adding a method or property to Array.prototype), the for...in statement will return the name of your user-defined properties in addition to the numeric indexes.

Also, because order of iteration is arbitrary, iterating over an array may not visit elements in numeric order. Thus it is better to use a traditional for loop with a numeric index when iterating over arrays.

This is exactly the reason why you shouldn't use the for (i in array) ... construct. The JavaScript array's length property is internally declared as non-enumerable, so it doesn't appear when you iterate through the object, but any properties that you define are always enumerated.

The upcoming ECMAScript 5 has a way to define your own properties as non-enumerable, but most browsers don't support it as yet.