Generic Types in Typescript Cannot be used to index object

Question

I am trying to create to type my class with generics and it alays throws me an error

class CustomDataStructure<T> {
    public _data: object;

     constructor() {
        this._data = {};
        }
    public getAndDeleteRandomKey(): void {
        const allExistingKeys = Object.keys(this._data);
        const randomKey = allExistingKeys[Math.floor(Math.random()*(allExistingKeys.length))];
        this.removeItem(randomKey);
    }
   
    public addItem(value: T): void {
        console.log("adding");
        console.log(this._data)
        this._data[value] = new Date().getTime();
    }

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

let ss = new CustomDataStructure<string>();
ss.addItem("hello");
ss.addItem("hello2");

This throws me 2 different errors

parameter of type 'T'.'T' could be instantiated with an arbitrary type which could be unrelated to 'string'.
Type 'T' cannot be used to index type 'object'

am i missing anything here

TypeScriptPlayground

Why are you using a generic type here? What other possible types would you use to key your data object? — Evan Trimboli
– Evan Trimboli, Commented Nov 10, 2020 at 20:47
They can, but ultimately under the hood they get cast to string. — Evan Trimboli
– Evan Trimboli, Commented Nov 10, 2020 at 20:52

Stefan B. · Accepted Answer · 2020-11-10 20:54:42Z

1

The reason for this is that the generic T could be also a complex object, e.g. a class instance and you only can use primitive objects as key

const object = {}
object["myKey"] = true // works 
object[new Date()] = true // not works, because is a complex object

consider using a map, see

Playground

class CustomDataStructure<T> {
    private _data: Map<T, any> = new Map();

     constructor() {}
    public getAndDeleteRandomKey(): void {
        const allExistingKeys = Array.from(this._data.keys());
        const randomKey = allExistingKeys[Math.floor(Math.random()*(allExistingKeys.length))];
        this._data.delete(randomKey);
    }
   
    public addItem(value: T): void {
        this._data.set(value, new Date().getTime())
    }


}

let ss = new CustomDataStructure<string>();
ss.addItem("hello");
ss.addItem("hello2");

answered Nov 10, 2020 at 20:54

Stefan B.

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