Function withCounter

Creates a function with a single parameter, that will call the given fn with the received argument and an internal counter, that indicates how often this function was invoked.

Not all iterables guarantee a consistent order or have the concept of an index. That's why all callbacks for iterables extension functions accept only a single argument. With the withCounter function you can include a counter/index in your callback. This means that you must not reuse the returned function, if you want the counter to match the index! The internal counter cannot be reset.

Example

const sumOfEvenIndices = sum(new Set([1,2,3,4]), withCounter((v, i) => (i % 2 === 0 ? v : 0)));
expect(sumOfEvenIndices).toBe(1 + 3);