javascript for each loop

I ran into an issue today with the javascript for each loop. The for each loop would run one extra time and return the value "each" for the variable "field" in example below.

Example 1
for(var field in array){
//some code
}

To solve this I switched up the for each loop to do it this way instead.

Example 2
array.each(function(field){
//some code
});

Not sure what the difference is between Example 1 and Example 2.