recursiveReduce.js

// Takes an `array` a `callback` and an `initialValue`
// uses an inner `recursiveFn` to iterate over all array values
// calling the `callback` on each one
const reduce = (orgArray, callback, initialValue) => (
  (function recursiveFn(idx, acc) {
    // base condition to end looping    
    if (idx > orgArray.length - 1) return acc;
    // calculate and pass values to next step
    return recursiveFn(idx + 1, callback(acc, orgArray[idx], idx, orgArray))
    // start the iterations at orgArray[0] 
    // with (initialValue or array[0]) as the acc
  })(0, (initialValue || orgArray[0])) 
);

// Basic sum of an array
// No `initialValue` supplied so starts with Array[0]
reduce([0,1,2,3], (acc, item) => acc + item);
// => 10;

// Add 1 to all items in array
// callback is using es6 to destructure acc into a new array
recuce([0,1,2,3], (acc, item) => {
  acc.push(item + 1);
  
  return acc;
}, []) // initialValue is [] // acc === [] => true
// => [1,2,3,4]