JS transducers

transducers.js

/** Implementing transformations (like map and filter) with reducers. **/

// https://www.webpackbin.com/bins/-Kvmn0gWkIMmtnFnkcu_
// https://medium.com/@roman01la/understanding-transducers-in-javascript-3500d3bd9624

// take all evens, multiply by 2, log that.
const arr = ['1', '2', '3', '4', '5', '6']

console.log('Chaining map and filter.')

 const arred = arr.filter((elem, index) => {
  return parseInt(elem) % 2 === 0
 }).map(elem => {
   return elem * 2;
 })
arred.forEach(console.log, console) // 4 8 12 

console.log('Implement map with reduce.')

const mapReducerOld = (result, input) => {
  result.push(input * 2)
  return result
}
const mapped = arr.reduce(mapReducerOld, [])

mapped.forEach(console.log, console)
// 2 4 6 8 10 12

console.log('Same stuff, but passing in mapping function from outside.')

const mapReducer = (f) => (result, input) => {
  result.push(f(input))
  return result
}

// code above is the ES6 equivalent to:
const mapReducerNoES6 = function(f) {
  return function(result, input) {
     result.push(f(input))
    return result
  }
}

const mappedIt = arr.reduce(mapReducer((f) => f * 2), [])

mappedIt.forEach(console.log, console)
// 2 4 6 8 10 12

console.log('With this new syntax, we can filter too!')

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

const filteredIt = arr.reduce(filterReducer((x) => x <= 4), [])

filteredIt.forEach(console.log, console)
// "1" "2" "3" "4"