transduction-block07.js

const data = [
  1,
  2,
  3,
  4,
  5,
];

/**
 * 
 * This utility function is often available on various 
 * functional libraries (i.e ramdajs)
 * `pipe` is the opposite of `compose`
 */
const pipe = (...fns) => initialValue => fns.reduce((result, fn) => fn(result), initialValue);

const addBy1 = (x) => x + 1;
const multiplyBy2 = (x) => x * 2;

const getItemsBelow10 = (x) => x < 10;
const sum = (accumulator, currentValue) => accumulator += currentValue;

const mapReduce = mapperFn => combinerFn => (accumulator, currentValue) => {
  return combinerFn(accumulator, mapperFn(currentValue));
};

const filterReduce = predicateFn => combinerFn => (accumulator, currentValue) => {
  if (predicateFn(currentValue)) {
    return combinerFn(accumulator, currentValue);
  }
  return accumulator;
};

const combinerConcat = (accumulator, currentValue) => {
  accumulator.push(currentValue);
  return accumulator;
};

const transducer = pipe(
  mapReduce(addBy1),
  mapReduce(multiplyBy2),
  filterReduce(getItemsBelow10)
);

const res = data
  .reduce(transducer(combinerConcat), [])
  .reduce(sum, 0)

// 35