modification of examples from: https://medium.com/@roman01la/understanding-transducers-in-javascript-3500d3bd9624

js-transduce_experiment.js

/*
  arrays
*/
const _set = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
const _set2 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
/*
  predicates
*/
const isEven = x => x % 2 === 0;
const addOne = x => x + 1;
const squared = x => x * x;
/*
   reducing functions
*/
const mapReducer = f => (result, input) => {
  result.push(f(input));
  return result;
};

const filterReducer = predicate => (result, input) => {
  if (predicate(input)) {
    result.push(input);
  }
  return result;
};

console.log("add one", _set.reduce(mapReducer(addOne), []));
console.log("squared", _set.reduce(mapReducer(squared), []));
console.log("is even", _set2.reduce(filterReducer(isEven), []));
/*
   composable reducers
*/
const mapping = f => reducing => (result, input) => reducing(result, f(input));

const filtering = predicate => reducing => (result, input) =>
  predicate(input) ? reducing(result, input) : result;

const accumulator = (xs, x) => {
  xs.push(x);
  return xs;
};

const transduce = (transformationReducers, reducing, initial, input) =>
  input.reduce(transformationReducers(reducing), initial);

// simple compose helper - ramda `compose` lets you compose n* number of reducers, instead of just the three this one does
const compose = (f, g, h) => x => f(g(h(x)));

const addToEvenAndSquare = compose(
  filtering(isEven),
  mapping(addOne),
  mapping(squared)
);

console.log(
  "add one, is even",
  _set
    .reduce(mapping(addOne)(accumulator), [])
    .reduce(filtering(isEven)(accumulator), [])
);

console.log(
  "add one, is even (reducer composition as reducer)",
  _set.reduce(mapping(addOne)(filtering(isEven)(accumulator)), [])
);

console.log(
  "addToEvenAndSquare using transduce helper function",
  transduce(addToEvenAndSquare, accumulator, [], _set)
);