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?
Yes, it is a pure function. Pure functions are referentially transparent, i.e. one can replace the function call with its result without changing the behaviour of the program.
In your example, it is always valid to replace e.g. calculateTip (100) anywhere in your program with its result of 15 without any change in behaviour, hence the function is pure.