javascript - Transfer value of object inside array to another array -

i have object this:

var animals_data = { category : [     {         class : "reptiles",         animals : [             {                 image1 : "some image",                 image2 : "some image 2",                 name : "snake",                 description : "some description"             },             {                 image1 : "another image",                 image2 : "another image 2",                 name : "crocodilia",                 description : "another description"             },         ]     },     {         class : "mammals",         animals : [             {                 image1 : "more image",                 image2 : "more image 2",                 name : "cat",                 description : "more description"             },             {                 image1 : "image",                 image2 : "image 2",                 name : "dog",                 description : "long description"             }         ]     },     {         class : "birds",         animals : [             {                 image1 : "bird image",                 image2 : "bird image 2",                 name : "american flamingo",                 description : "bird description"             },             {                 image1 : "another bird image",                 image2 : "another bird image 2",                 name : "atlantic puffin",                 description : "another bird description"             },         ]     }  ]}; 

i want values of firstindex , put them in different array. however, having hard time doing this. code did is:

for (var = 0; < animals_data.category.length; i++) { var currentiteration = animals_data.category[i];     var currentclass = currentiteration.class;  animals_array.push(currentclass); }; 

when run code error says: cannot read property 'firstindex' of undefined. thank help.

edit: included actual object working with. have omitted parts basically, that's gist. values want put in new array ones in "class" property.

solved it. thank xotic750 pointing out mistake. question is:

when used:

for (var = 0; < obj.firstkey.length; i++); { var currentiteration = obj.firstkey[i]; var currentindex = currentiteration.firstindex;  new_array.push(currentindex); }; 

the array doubled (i.e 2 of each added).

however when replaced with

new_array.push(obj.firstkey[i].firstindex; 

it runs correctly. may know why there such difference? thank you.