React ReducerのもとになったReduce

> [0,1,2,3,4].reduce(function(previousValue, currentValue, index, array){
  return previousValue + currentValue;
}, 10);
20

[Arrow Function]
> ["a","b", "c", "de", "f", "g"].reduce((previousValue, currentValue, index, array)=> {
  return previousValue + currentValue;
}, "X");
"Xabcdefg"