functional-javascript-workshop/exercises/basic_recursion/solution/solution.js

The basics of functional programming in JavaScript. No libraries required.

function reduce(arr, fn, initial) {
  return (function reduceOne(index, value) {
    if (index > arr.length - 1) return value // end condition
    return reduceOne(index + 1, fn(value, arr[index], index, arr)) // calculate & pass values to next step
  })(0, initial) // IIFE. kick off recursion with initial values
}

module.exports = reduce