我也来试试

[DingSJi]

const fn = function(str){
let arr = str.split("")
let obj = {}
let maxStr = "",
maxNum = 0;
arr.reduce((pre, next) => {
if(pre in obj){
if(pre == next)
obj[pre]++
}else{
obj[pre] = 1
}
return next
},arr[0])
console.log(obj)
for(let key in obj){
if(maxNum < obj[key]){
maxNum = obj[key]
maxStr = key
}
}
return ${maxStr}-${maxNum}
}

console.log(fn('aaaaaabbbba'))