Issue with accessing custom method in subclass in Typescript

Recently delving into TypeScript, I decided to subclass Error and add a new method called getCode() in my MyError class.

export class MyError extends Error {
    public code: number;
    
    constructor(code: number, message?: string) {
        super(message);
        Error.captureStackTrace(this, MyError);
        this.name = 'MyError';
        
        this.code = code;
    }
    
    public getCode() {
        return 405;
    }
}
let errorInstance: MyError = new MyError(404, "test");
console.log(errorInstance.getCode());

In attempting to access the getCode() method of an instance of MyError class, I encountered the following error. It made me question whether custom methods can be added to subclasses, as I couldn't comprehend why the function getCode() was not being found.

console.log(errorInstance.getCode());
TypeError: errorInstance.getCode is not a function
    at Object.<anonymous> (c:\Users\test\vscode-ws\MyError.js:35:15)
    at Module._compile (internal/modules/cjs/loader.js:701:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:712:10)
    at Module.load (internal/modules/cjs/loader.js:600:32)

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