Array.prototype.reducePolyfill()

reduce.js

import expect from 'expect'

let items = [{price: 19.9}, {price: 59}, {price: 120}, {price: 29.9}]

// the reducer
const add = function add(sum, item) {
  return sum + item.price
}

expect(items.reduce(add, 0)).toEqual(228.8)

Array.prototype.reducePolyfill = (function() {
  let value
  return function reducer(fn, startValue) {
    if(startValue != undefined) {
      value = startValue
    }
    let curr = this.shift()
    if(this.length > 0) {
      reducer.call(this, fn)
    }
    value = fn(value, curr)
    return value
  }
})()

expect(items.reducePolyfill(add, 0)).toEqual(228.8)