var a = '142',b = '99'
function add(a,b){
var len1 = a.length,len2 = b.length
var maxLen = Math.max(len1,len2)
var s1 = a.padStart(maxLen,0)
var s2 = b.padStart(maxLen,0)
var res = '',addOne = 0
for(let i=maxLen-1;i>=0;i--){
var temp = Number(s1[i])+Number(s2[i])+addOne
addOne = temp>=10?1:0
temp = temp>=10?temp%10:temp
console.log(res,addOne,temp)
res=temp+res
}
return addOne===1?1+res:res
}
console.log(add(a,b))