I don’t really understand what you mean, maybe this can help you

function Check(){
        var prod = document.getElementById('product1').value
        if(prod <= 0 || prod == ""){
            alert("Failed");
            } else {
            alert("Success")    
            }
        }

Product 1 
  <input type="number" id="product1" value="" size="5">
  <button type="button" onClick="Check()">Click</button>

For example, suppose
We have a product that has an XL size and it has 2 colors, red and blue
We have 2 pieces of blue color in stock
But there are 0 pieces of red in stock

We want to show a message to the user if the user chooses the red size XL, that the stock of this variable in the warehouse is 0.

I want to write for WooCommerce product features with .. alert and I need help to complete the code

If the user selects a variable from the available product and one of the variables selected by the user is null, I want to show a warning to the user.

`

<?php
// PHP program to pop an alert
// message box on the screen
  
// Display the alert box 
echo '<script>alert("Welcome to Geeks for Geeks")</script>';
  
?>

`

`
this is my code