编辑代码

function add(a,b) {
    let res = '',f=0;
    let len = a.length
    for (let i=len-1;i>=0;i--) {
        let r = Number(a[i])+Number(b[i])+f
        f = Math.floor(r/10)
        res=r%10+res
    }
    if(f===1) {
        res = '1'+res
    }
    return res
}
console.log(add('625','456'))