js reduce()


```typearray.reduce((state, input, index, array) => {	action()	return state}, initial)```




### reduce - function with array input → output some value
> [Array.prototype.reduce()](https://developer.mozilla.org/ko/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce)


> [Filter object properties by key in ES6](https://stackoverflow.com/questions/38750705/filter-object-properties-by-key-in-es6)




