如何在LocalStorage的对象中访问数组的长度

Question

I am having some troubles with the following: I have an array in an object in localStorage and I need to find the number of elements contained in the array.

My code is as following

for (var i = 0; i < localPosts.length; i++){
        local_tags = window.localStorage.getItem(localStorage.key(i)).tags
        console.log(local_tags);
        for (var f = 0; f < local_tags.length; f++) { // <--- error occurs here
            tagf = local_tags[f]
            console.log(tagf);
            for (var j = 0; j < local_tags.length; j++) {
                if(tagf == tags[j]){
                    print_post(i);
                }
            }
        }
    }

Please excuse code messiness

This is an example myh json file(in Json format(this is stringified before it goes into the loops)):

{
"article1": {
    "title": "Hello World!",
    "desc": "This is the first post on this website!",
    "img": "",
    "id": 0,
    "tags": ["1", "3"]
},
"article2":{
    "title": "",
    "desc": "",
    "img": "",
    "id": 1,
    "tags": ["1","2"]
}

}

The problem with this code is whenever I try accsessing the parent function i get an error that goes like this:

Uncaught TypeError: Cannot read property 'length' of undefined

and the log of local_tags returns

undefined

Thanks for the help, I hope I explained it well!

EDIT: made the error more visible in the code

EDIT2: clarified the JSON data is stringified

EDIT3: Added whole Json file

Answer 1

I think the problem is created by tags array

for (var j = 0; j < tags.length; j++) {
   if(tagf == tags[j]){
      print_post(i);
   }
}

it is not defined tags.length

Answer 2

Your code has some errors,at line if(tagf == tags[j]){

Try below code, make some changes as per your requirements

window.localStorage.setItem("test_article1",JSON.stringify({"article1": { "title": "Hello World!", "desc": "This is the first post on this website!", "img": "", "id": 0, "tags": ["1", "3"]}}))
localPosts = ["test_article1"];
for (var i = 0; i < localPosts.length; i++){
        local_tags = JSON.parse(window.localStorage.getItem(localStorage.key(i)))["article1"]["tags"];
        console.log(local_tags);
        for (var f = 0; f < local_tags.length; f++) { // <--- error occurs here
            tagf = local_tags[f]
            console.log(tagf);
            for (var j = 0; j < local_tags.length; j++) {
                    console.log(local_tags[j]);
            }
        }
    }