Value of varying text fields

I’m building a shopping cart that can have one or many lines of items, each with a single text field displaying the Quantity of that line.

When a quantity is changed, I need to know what it was changed to.

Each text field does have a unique name which I’m successfully loading to a variable. With the variable knowing the name of the field, how do I load the value to another variable?

Essentially, where XXX is a changing field name, I’m trying to resolve:

var newvalue = document.cart.XXX.value

I’m stumbling on how to add the variable bearing the field name into the equation.

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