transducers.js

/** Operators */
const map = (mapper) => (reducer) => (acc, val) => reducer(acc, mapper(val));

const filter = (predicate) => (reducer) => (acc, val) =>
  predicate(val) ? reducer(acc, val) : acc;

const some = (predicate) => (_) => (acc, val) =>
  acc !== true ? predicate(val) : true;
/** */

const pipe = (...fns) =>
  fns.reduce((reducer, nextReducer) => (...args) =>
    reducer(nextReducer(...args)),
  );

const arrayOf = (acc, val) => (acc.push(val), acc); // push to Array
const inmutableArrayOf = (acc, val) => [...acc, val]; // push to Array

const transducer = (...fns) => (iterable) => {
  const transform = pipe(...fns)(inmutableArrayOf);
  return Array.from(iterable).reduce(transform, []);
};

/* Example */
const add1 = (x) => x + 1;
const greater2 = (x) => x > 2;
const numbers = [1, 2, 3];

const result = transducer(map(add1), filter(greater2))(numbers);
console.log(result); // [3, 4]

const result2 = transducer(filter(greater2))(numbers);
console.log(result2); // [ 3 ]

const result3 = transducer(
  map(add1),
  some((x) => x > 5),
)(numbers);
console.log(result3); // false