JavaScript多维数组长度问题

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 

Answer 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.

Answer 2

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.

Answer 3

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.