[Javascript] Flattening nested arrays: a little exercise in functional refactoring

const array = [1, [2, 3], [[4, [5, [6], 7], [8, 9]]]];

const concatReducer = (acc, curr) => acc.concat(curr);
const map = f => ary => ary.map(f);
const reduce = (f, init) => ary => ary.reduce(f, init);
const compose = (f, g) => x => f(g(x));

const flatten1Element = x => (Array.isArray(x) ? flatten(x) : x);
const flattenEachElement = map(flatten1Element);
const flattenOneLevel = reduce(concatReducer, []);

const flatten = compose(
  flattenOneLevel,
  flattenEachElement
);

console.log(flatten(array));
//[1,2,3,4,5,6,7,8,9]