Javascript multidimensional array length problem

Question

JavaScript multidimensional array length always returning 0, how can I solve this problem?

class test {

  static init() {
    test.arr = [];
  }

  static add() {
    let user_id = Math.floor(Math.random() * 10000);
    if (test.arr["u_" + user_id] === undefined) {
      test.arr["u_" + user_id] = [];
    }
    test.arr["u_" + user_id].push({
      "somedata": "here"
    });
  }

  static length() {
    return test.arr.length;
  }

}
test.init();
test.add();
test.add();
console.log(test.arr.length); //Always returning 0

@hien as it is tagged js and includes no typescript specific code, I guess it is JS ? — Jonas Wilms, Mar 24 '19 at 12:06
[Don't abuse arrays when you need an object](https://andrewdupont.net/2006/05/18/javascript-associative-arrays-considered-harmful/), or maybe a `Map`. — Bergi, Mar 24 '19 at 13:23
possible duplicate of [Length of a JavaScript object](https://stackoverflow.com/q/5223/1048572) — Bergi, Mar 24 '19 at 13:25

Jonas Wilms · Answer 1 · 2019-03-24T12:33:07.297

1

An array is a sorted set of numeric key value pairs. "_u" + user_id is not numeric, it's a string, therefore it gets stored as a regular property on the array (it behaves like an object) and not as part of the array itself. If you want to use a key-value storage with a length, use a Map.

 const test = { // no need for a class if you dont have instances
   arr: new Map(), // no need for that unneccessary init
   add() {
    let user_id = Math.floor(Math.random() * 10000);
    if(!this.arr.has("u_" + user_id)) { // I prefer "this" over "test", both work however
      this.arr.set("u_" + user_id, []);
    }
    this.arr.get("u_" + user_id).push({"somedata": "here"});
   },

   length() {
    return this.arr.size; //Note: it is "size" not "length" on a Map
   },
};

Sidenote: arr and test are very bad names.

edited Mar 24 '19 at 12:33

answered Mar 24 '19 at 12:03

Jonas Wilms

132,000
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same returning 0 – Çağrı Yıldırım Mar 24 '19 at 12:31
@Çağrı Yıldırım whoops, must have been sleeping while answering, you have to use `.get` and `.set` on a `Map` – Jonas Wilms Mar 24 '19 at 12:33
1

You even might want to drop the `"u_"` prefix and just use the `user_id` only as the key – Bergi Mar 24 '19 at 13:28
@ÇağrıYıldırım Yes, that's still the same code (without using `.get()` and `.set()` methods) which doesn't work? – Bergi Mar 24 '19 at 21:37

score 1 · Answer 2 · edited Mar 24 '19 at 14:01

1

An array index can be defined as number only. If you want to get the length of an array, there are two ways to achieve this.

You need to define the index as number instead of a string.

Make a separate object, add your object ({"somedata": "here"}) into the object and push it into the array. Check the code below.

let test=[]
let obj = {}
let user_id = Math.floor(Math.random() * 10000);

if(obj["u_" + user_id] === undefined) {
  obj["u_" + user_id] = [];
}

obj["u_" + user_id] = {"somedata": "here"};
test.push(obj)

Hope this will be helpful.

edited Mar 24 '19 at 14:01

Ivar

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answered Mar 24 '19 at 12:46

Prachi Jain

188
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How do I get the length? – Çağrı Yıldırım Mar 24 '19 at 12:50
You just need to console.log(test.length). – Prachi Jain Mar 25 '19 at 03:05

score 0 · Answer 3 · answered Mar 24 '19 at 13:12

Check out the following jsbin I made you. https://jsbin.com/xeqacuf/edit?console

    constructor()
    {
        console.log("new object created");
        this.test = { arr : {} };
    }

It's close to what you are trying to do... Let me know if you need explanation or more help. I'm willing to assist as much as youd like. Notice that I changed the datatype from collection to objectKeyValue which allows you to query the object by key like you wanted.

Javascript multidimensional array length problem

3 Answers3