Question

I have the following JSON:

{"data":[{"id":1,"client_id":1},{"id":2,"client_id":1}]}

I’m trying to do a for...in but something is not going well.

Look at my code:

for (post in data) {
    console.log(post.client_id); //return undefined
}

But if I do this:

for (i=0 ; i<data.length; i++) {
    console.log(data[i].client_id); //return the correct value
}

Why can’t I iterate with for..in in this case?

Answer 1

First off, you should NEVER iterate arrays with for/in. That structure iterates properties of the object. You will get all the array elements, but you may also get other iterable properties of the object too.

Arrays should always be iterated with a traditional for loop as in:

for (var i = 0; i < arr.length; i++).

Second off, you have to make sure you’re following your own data structure properly. You have this:

var item = {"data":[{"id":1,"client_id":1},{"id":2,"client_id":1}]};

which spread out some more looks like this:

var item = {
    "data":[
        {"id":1,"client_id":1},
        {"id":2,"client_id":1}
     ]
};

So, your JSON is an object. That object has one property in it called data and that one property’s value is an array, which contains objects. You would get the first item in the array with this:

item.data[0]

Or the properties on that object with this:

item.data[0].id
item.data[0].client_id

You would iterate all the items in that array like this:

for (var i = 0; i < item.data.length; i++) {
    // access each object in the array
    item.data[i].id
    item.data[i].client_id
}