Arrow functions

1hello.md

This is code from an episode of Fun Fun Function: https://youtu.be/6sQDTgOqh-I

2events.js

const dragonEvents = [
  { type: 'attack', value: 12, target: 'player-dorkman' },
  { type: 'yawn', value: 40 },
  { type: 'eat', target: 'horse' },
  { type: 'attack', value: 23, target: 'player-fluffykins' },
  { type: 'attack', value: 12, target: 'player-dorkman' },
]

3vanilla.js

const totalDamageOnDorkman = dragonEvents
  .filter(function (event) {
    return event.type === 'attack'
  })
  .filter(function (event) {
    return event.target === 'player-dorkman'
  })
  .map(function(event) {
    return event.value
  })
  .reduce(function(prev, value) {
    return (prev || 0) + value
  })

4arrow.js

const totalDamageOnDorkman = dragonEvents
  .filter(e => e.type === 'attack')
  .filter(e => e.target === 'player-dorkman')
  .map(e => e.value)
  .reduce((prev, x) => (prev || 0) + x)

Hej, why don't you use the initial value parameter for reduce?

reduce((prev, x) => prev + x, 0)

running this exactly produce error:

  .reduce(function(prev, value) {
   ^

TypeError: Reduce of empty array with no initial value
    at Array.reduce (native)
    at Object.<anonymous> (C:\BackendServicesSandbox\dragons.js:31:4)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:141:18)
    at node.js:933:3

Any clue ?

by the way, my platform is: node.js v.5.7.0 on Windows 8.1

sorry, my bad. The issue was due to a typo.

Why .reduce(summerInitialValue) does not work?

const dragonEvents = [
  {type: 'attack', value: 40, victim: 'dorkman'},
  {type: 'sleep', value: 10},
  {type: 'attack', value: 20, victim: 'aquaman'},
  {type: 'attack', value: 40, victim: 'dorkman'},
]

const v = 'dorkman';
const field = 'victim';
const selectVictim = (e) => e[field] === v;
const selectAttack = e => e.type === 'attack';
const initialValue = 9
const summer = (total, v) => ( total + v) ; 
const summerInitialValue = (initialValue) => (summer, initialValue) 
const myReduce = (reducer, initialValue) => (summer, initialValue)

const totalDamageOnDorkman = dragonEvents
                      .filter(selectAttack)
                      .filter(selectVictim)
                      .map(e => e.value)
                      .reduce(summer,4) //works
// Does not work      .reduce(summerInitialValue)
// Does not work      [myReduce]


console.log(totalDamageOnDorkman)