<select> element property "value" not detected by Jasmine in test cases

Question

I am making an Amazon project and I encountered a problem when i try to test addToCart function with Jasmine. The only issue that prevents it from working is that it can't acces the "value" in addToCart function.

const quantity = Number(selectQuantityElement.value);

Jasmine gives this error message: TypeError: Cannot read properties of null (reading 'value')

Is there a way maybe to simulate that i selected some value in selector so it knows it's a number? Or to acces "value" from the test case?

When I change a value of quantity to some number it works: const quantity = 2;

Here is the test case:

import {addToCart, cart, loadFromStorage} from '../../data/cart.js';

describe('test suite: addToCart', () => {

  const productId = 'e43638ce-6aa0-4b85-b27f-e1d07eb678c6';

  it('adds an existing product to the cart', () => {
    document.querySelector('.js-test-container').innerHTML = `  
    <div class="js-added-to-cart-text-${productId}"></div>
    <div class="js-quantity-selector"></div>
    `

    spyOn(localStorage, 'setItem');

    spyOn(localStorage, 'getItem').and.callFake(() => {
      return JSON.stringify([{
        productId: 'e43638ce-6aa0-4b85-b27f-e1d07eb678c6',
        quantity: 1,
        deliveryOptionId: '1'
      }]);
    });
    loadFromStorage();

    addToCart('e43638ce-6aa0-4b85-b27f-e1d07eb678c6');
    
    expect(cart.length).toEqual(1);
    expect(localStorage.setItem).toHaveBeenCalledTimes(1);
    expect(cart[0].productId).toEqual('e43638ce-6aa0-4b85-b27f-e1d07eb678c6');
    expect(cart[0].quantity).toEqual(2);
  });

And here is the addToCart function:

export function addToCart(productId) {

  displayAddedMessage(productId);

  const selectQuantityElement = document.querySelector(`.js-quantity-selector-${productId}`)
  const quantity = Number(selectQuantityElement.value); <-- This value

  let matchingItem;

  cart.forEach((cartItem) => {
    if (productId === cartItem.productId) {
      matchingItem = cartItem;
    }
  });

  if (matchingItem) {
    matchingItem.quantity += quantity;
  } else {
    cart.push({
      productId: productId,
      quantity: quantity,
      deliveryOptionId: '1'
  })
  }

  saveToStorage();
}

And here is the HTML for the selector:

<div class="product-quantity-container js-product-quantity-container">
        <select class="js-quantity-selector-${product.id} js-quantity-selector">
          <option selected value="1">1</option>
          <option value="2">2</option>
          <option value="3">3</option>
          <option value="4">4</option>
          <option value="5">5</option>
          <option value="6">6</option>
          <option value="7">7</option>
          <option value="8">8</option>
          <option value="9">9</option>
          <option value="10">10</option>
        </select>
      </div>