一

var names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice'];
var countedNames = names.reduce((obj, name) => { 
  if (name in obj) {
    obj[name]++
  } else {
    obj[name]=1
  }
  return obj
}, {})
//reduce的第二个参数就是obj的初始值
console.log(countedNames)
// { 'Alice': 2, 'Bob': 1, 'Tiff': 1, 'Bruce': 1 }

二 

function getEleNums(data) {
 var map = {}
 for (i = 0; i < data.length; i++) {
     var key = data[i]
     if (map[key]) {
         map[key] += 1
     } else {
         map[key] = 1
     }
 }
 return map
}
var data = ['b','a','c','a','b','b','b','c','c','a','c','a','a','a','b','c']
console.log(getEleNums(data))