javascript - Two types of for loop - how come only one of them is working? -

this question has answer here:

• why using “for…in” array iteration bad idea? 22 answers

i have following code in javascript:

var controller = { initapp: function(){     var getcartdata = $.ajax({ url: '/cart' }),         getcategorydata = $.ajax({ url: '/categories' }),         getsupplierdata = $.ajax({ url: '/suppliers'}),         getprodcuts = $.ajax({url: '/products'});      // main controller logic starts when data loaded in     $.when(getcartdata, getcategorydata, getsupplierdata, getprodcuts     ).done( function( cart, categories, suppliers, products ) {         var model = new model(cart, categories, suppliers, json.parse(products[0]));          console.log(model.products);         console.log(typeof model.products);          for(var = 0; i<model.products.length; i++){             console.log(model.products[i]);         }          for(product in model.products){             console.log(product);         }      }); } 

};

i iterate through model.products. @ end of code use 2 types of loops. how come 1 of them (the first one) working expected? second 1 prints numbers in length of array.

produkt key too:

a={a:1,b:2} for(key in a){  alert(key);// a.   b.   } 

so need do:

for(product in model.products){   console.log(model[product]);  } 

but should use normal loop. why? because prototype handled in keys:

array.prototype.test=2; 

run booth loops code executed before. in echo 2, dont want...