How to set quantity to 0 but not negative value in my code? if its shows negative value then put an error message in alert

Everything is working fine in below code. But when I want to put a greater quantity than exiting quantity in a specific products, It returns me negative value. Where to write the code to return the following two reuslts? (1). I want to put value up to 0.
(2). If It goes below 0, alert with “No product in Store”.

const inputValue = id => {
    const input = document.getElementById(id);
    const inputValue = input.value;
    input.value = '';

    return inputValue;
}

const addProduct = () => {
    const productName = inputValue('product-name');
    const productQnt = inputValue('product-quantity');

    // console.log(productName, productQnt);

    // check typeof input value
    // console.log(typeof productName, typeof productQnt)

    // check input type
    const number = Number(productQnt);

    if (!isNaN(productName || !Number.isInteger(number))) {
        alert('nai');
        return;
    }

    // console.log(Number.isInteger(number));
    setProductInLocalStorage(productName, productQnt);
    //    showdata on table at console
    //    getLocalStorageData();
    displayProduct();
}

// get items
const getLocalStorageData = () => {
    const products = localStorage.getItem('All-Products');
    const parseProducts = JSON.parse(products);

    return parseProducts;
}

// set items
const setProductInLocalStorage = (productName, productQnt) => {
    let products = getLocalStorageData();

    if (!products) {
        products = {};
    }

    // add updated quantities 
    if (products[productName]) {
        products[productName] = parseInt(products[productName]) + parseInt(productQnt)
    } else {
        products[productName] = productQnt;
    }

    localStorage.setItem('All-Products', JSON.stringify(products));
};

// display product on UI
const displayProduct = () => {
    const allProducts = getLocalStorageData();
    const section = document.getElementById('all-products');
    section.textContent = '';

    for (const product in allProducts) {
        const name = product;
        const quantity = allProducts[product]
        const div = document.createElement('div');

        div.innerHTML = `
            <div class="shadow-sm p-3 mb-2 bg-body rounded">
            <span class="fs-4">${name}</span>
            Quantity:<small class="fw-bold">
                ${quantity}
            </small>
            </div>
        `;

        section.appendChild(div);
    }
}

// call display products to display on UI
displayProduct();

Answer

Code written below is maybe helpful. And i thought your way of checking productQnt whether is a Integer seems doesn’t work, therefore i show you one valid way meanwhile. Hope it shall be useful to you.

    // check input type
    const number = Number(productQnt);
    if(!productName || !Number.isInteger(number)){
        alert('nai');
        return;
    }
    if (number < 0) {
      alert ("No product in Store");
      return;
    }

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Answer