统计某个字符在字符串中出现的次数

var arr = ['a','b','c','d','a','a','b','c','d','a'];
var countarr = arr.reduce((allarr, arrs) => {
  if(arrs in allarr) {
    allarr[arrs]++;
  } else {
    allarr[arrs]=1;
  }
  return allarr;
},{})