transducer from scratch in JS, just to learn and understand how it works.

transducer.js

// Function Composition
const identity = x => x
const compose = (f, g) => x => f(g(x))
const composer = fnArr => fnArr.reduce(compose, identity)

// Map implemented in terms of reduce
const mmap = fn => reducer => (acc, val) => reducer(acc, fn(val))

// Filter implemented in terms of reduce
const mfilter = fn => reducer => (acc, val) => fn(val) ? reducer(acc, val) : acc

// Transducer Helper Function
const transduce = (fns, reducer, init) => data => data.reduce(composer(fns)(reducer), init)

// Reducers
const concat = (a, b) => a.concat([b])
const sum = (a, b) => a + b

const fns = [
  mmap(x => x + 1),
  mmap(x => x * 3),
  mfilter(x => x < 20),
  mfilter(x => x % 2 == 0)
]

// Data
const data = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

const t = transduce(fns, concat, [])
const t2 = transduce(fns, sum, 0)
  
console.log(t(data))
console.log(t2(data))