Rhino does not handle Arrays the way that other coding languages do.

I figured it out, unlike most arrays that return an array after you call push- Rhino returns the new length. So if you call push, don't set it's value.

var copyArray = []; //I assume this line tells it to be an array

for (var i = 0; i < postResult.data..length; i++) 
         {
             copyArray[i] = postResult.data[i]; 
//arrays do not behave like .Net arrays, you can change length without initializing, so you just set each index to the index of the array you're copying
         }

copyArray.push(variableToAdd); 
//Do NOT set this as the value of your array, the push method returns an int (length of array) NOT an array!
//you probably dont need to call push at all to add to the array as shown in the for loop above if you already know your length or want a specific length

Then just set your value to the variable, like this

  var entityData = {
    "variableName": copyArray
};
//No need to call Array(copyArray) or any of the prototype. slice stuff. If you call Array() you'll just nest the array a second time. 

Use this page for further info on functions you can do with arrays.