// Given an array of numbers, if the index is even, add.
// If the index is odd, subtract.
const addSubtractReducer = (total, current, index) =>
(index % 2) === 0 ?
total + current :
total - current;
const addSubtract = (x) => {
const nums = [];
// Recursive function that accumulates numbers for the operation.
const f = (y) => {
nums.push(y);
return f;
};
// When the recursive function is type cast to a number,
// reduce the accumulated numbers.
f.valueOf = () => nums.reduce(addSubtractReducer, x);
// Return the recursive function, having added the first digit.
return f;
};
const addTo1 = addSubtract(1);
let x = addTo1(2);
console.log(x.valueOf());
