Looping through list items with jQuery

You need to use .each:

var listItems = $("#productList li");
listItems.each(function(idx, li) {
    var product = $(li);

    // and the rest of your code
});

This is the correct way to loop through a jQuery selection.

In modern Javascript you can also use a for .. of loop:

var listItems = $("#productList li");
for (let li of listItems) {
    let product = $(li);
}

Be aware, however, that older browsers will not support this syntax, and you may well be better off with the jQuery syntax above.

You can use each for this:

$('#productList li').each(function(i, li) {
  var $product = $(li);  
  // your code goes here
});

That being said - are you sure you want to be updating the values to be +1 each time? Couldn't you just find the count and then set the values based on that?