Open index.html. Two functions required are available there in dev-console. Or, if there is some special testing app that tests js file, import index.js.
Also there is VendingMachine class available to test it with any custom supply, and its instances limitedMachine and unlimitedMachine with predefined supplies given in task. These instances are used in functions, required in task.
I assume that parameter to be passed is a number or a string, that contains number, and this number is of float type with not more then two decimals.