Let us assume, that you wrote some code to extend the Array class to a method. In this case we will add the min() method, which will return the lowest numeric value of all elements inside the array:

Array.prototype.min = function min(){
 
  var returnValue = Number.MAX_VALUE;
 
  if(this.length === 0){
    return false;
  }
 
  for(var i = 0; i < this.length; i++){
    if(this[i] < returnValue){
      returnValue = this[i];
    }
  }
 
  return returnValue;
 
};

This methods work fine, but you will notice a strange behavior if you cycle through all array elements using a for loop construct.

var list = [1, 2, 3];
for(var i in list){
  console.log(list[i]);
}

The code will produce this output:

1
2
3
min()

It seems that the for loop also iterates over the recently added method.

The method can only be removed from the Array class, not from an instance of it: 

delete Array.prototype.min;

It can not be removed, but overwritten by assigning a value to it: 

list.min = 0;

So, if you still want to use the for loop to iterate over the entries, you have to check the type of each entry:

for(var i in list){
  if(typeof(list[i]) != 'function'){
    console.log(list[i]);
  }
}