An article I was reading gives this as an example of an impure function (in JavaScript):
const tipPercentage = 0.15;
const calculateTip = cost => cost * tipPercentage;
That struck me as a bit of an odd example, since tipPercentage is a constant with an immutable value. Common examples of pure functions allow dependence on immutable constants when those constants are functions.
const mul = (x, y) => x * y
const calculateTip = (cost, tipPercentage) => mul(cost, tipPercentage);
In the above example, correct me if I'm wrong, calculateTip would usually be categorised as a pure function.
So, my question is: In functional programming, is a function still considered pure if it relies on an externally defined constant with an immutable value, when that value is not a function?