itrreduce.js

function itrReduce(src, reductionFn, destination) {
  let result = {value: undefined, done: false}
  let done = {done: true}
  let latestDestinationValue = destination
  
  return {
    next: function() {
      let nextIncomingOpt = src.next()
      if (nextIncomingOpt.done) {
        return done
      } else {
        latestDestinationValue = reductionFn(latestDestinationValue, nextIncomingOpt.value)
        result.value = latestDestinationValue
        return result
      }
    },
    [Symbol.iterator]: function() {
      return this
    }
  }
}

I think gardening brain needs an example :)