Cannot set property 'hello' of undefined in Typescript

Question

I am trying to create a class in typescript, but it always throws an error mentioned below.

Below is the log of the execution and it throws an error .

[LOG]: "adding" 
[LOG]: undefined 
[ERR]: Cannot set property 'hello' of undefined 

class CustomDataStructure {
    private _data: any;

    public CustomDataStructure() {
        this._data = {};
    }

    public addItem(value: string) {
        console.log("adding");
        console.log(this._data)
        this._data[value] = new Date().getTime();
    }

    public removeItem(key: string) {
        delete this._data[key];
    }

    public showData() {
        return this._data;
    }
}


let ss = new CustomDataStructure();
ss.addItem("hello");

Answer 1

You need to call a constructor that sets this._data value to something like empty object:

class CustomDataStructure {
 private _data: any;

 constructor() {
     this._data = {};
 }

 public addItem(value: string) {
    console.log("adding");
    console.log(this._data)
    this._data[value] = new Date().getTime();
    console.log(this._data)
 }

 public removeItem(key: string) {
    delete this._data[key];
 }

 public showData() {
    return this._data;
 }
}


let ss = new CustomDataStructure();
ss.addItem("hello");

Answer 2

class CustomDataStructure {
    private _data: any;

    constructor() {
        this._data = {};
    }

    public addItem(value: string) {
        console.log("adding");
        console.log(this._data)
        this._data[value] = new Date().getTime();
    }

    public removeItem(key: string) {
        delete this._data[key];
    }

    public showData() {
        return this._data;
    }
}
let a = new CustomDataStructure();
a.addItem("Hello")
console.log(a.showData())