Composable array functions that avoid intermediate creation of resultant array.

array_fp_util.js

/*
Ref: https://monet.github.io/monet.js/
*/
const { Some, None } = Monet;

function map(fn) {
  return function (v) {
    return Some(fn(v));
  }
}

function filter(fn) {
  return function (v) {
    const r = fn(v);
    return r ? Some(v) : None();
  }
}

function pipe(...fns) {
  return function (arr) {
    const result = [];
    for (const e of arr) {
      let m = Some(e);
      for (const fn of fns) {
        m = m.flatMap(fn);
        if (m.isNone()) {
          break;
        }
      }
      if (m.isSome()) {
        m.forEach(v => result.push(v));
      }
    }
    return result;
  }
}

const evenSquares = pipe(
  filter(n => n % 2 === 0),
  map(n => n * n),
);
log(evenSquares([1, 2, 3, 4])); // [4, 16]

const oddCubes = pipe(
  filter(n => n % 2 !== 0),
  map(n => n * n * n),
);
log(oddCubes([1, 2, 3, 4])); // [1, 27]