一

const arr = ["b", "c","b", "c","a", "b", "c"] 
    const obj = arr.reduce((obj,key)=>{
      if (key in obj){
          obj[key]++
      }else{
          obj[key]=1
      }
      return obj
    },{})
    console.log(obj);
//reduce的第二个参数就是obj的初始值
// {b: 3, c: 3, a: 1}

二

//利用遍历
const arr = ["b", "c","b", "c","a", "b", "c"] 
const obj = {}
arr.forEach(item=>{
    if( obj[item] ){
        obj[item]++
    }else{
        obj[item] = 1
    }
})
console.log(obj)
// {b: 3, c: 3, a: 1}