Define private array inside es6 class on nodejs

Question

I'm just trying to do define a private array as a property of a class but I'm not able to do it.

Here is an example of what I'm looking for:

 class Cart {
      #customerUsername;
      #products[];

     constructor(customerUsername) {
         this.#customerUsername = customerUsername;
         
     }
     
     setCustomerUsername(customerUsername) {
         if(customerUsername) {
             this.customerUsername=customerUsername;
         } else {
             throw Error('Missing customer username');
         }
     }
     
     addProduct(product) {
         if(product) {
             this.products.push(product);
         } else {
             throw Error('Missing product');
         }
     }

 }

But seems like define something like #products[] is not a valid declaration.

Any idea of how can I do this?

Thanks!