step1.js

// action generator returns action object!!
const incrementCount = ( payload = {} ) => {
// if payload doesn't exist it defaults to {}
// calling property on undefined object will throw an error
// payload is an object passed to incrementCount
 return {
   // returns new action object!
   type: 'INCREMENT',
   incrementBy: typeof payload.incrementBy === "number" ? payload.incrementBy : 1
 }
}