transduction-block03.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 => (accumulator, currentValue) => {
  accumulator.push(mapperFn(currentValue));
  return accumulator;
};

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

const res = data
  .reduce((accumulator, currentValue) => {
    accumulator.push(addBy1(currentValue));
    return accumulator;
  }, [])
  .reduce((accumulator, currentValue) => {
    accumulator.push(multiplyBy2(currentValue));
    return accumulator;
  }, [])
  .reduce((accumulator, currentValue) => {
    if (getItemsBelow10(currentValue)) {
      accumulator.push(currentValue);
    }
    return accumulator;
  }, [])
  .reduce(sum, 0)

// 18