腾讯一面题

1. 考闭包的

function fun(n,o) {
  console.log(o)
  return {
    fun:function(m){
      return fun(m,n);
    }
  };
}
var a = fun(0);  a.fun(1);  a.fun(2);  a.fun(3);//undefined,?,?,?
var b = fun(0).fun(1).fun(2).fun(3);//undefined,?,?,?
var c = fun(0).fun(1);  c.fun(2);  c.fun(3);//undefined,?,?,?
//问:三行a,b,c的输出分别是什么？

//答案：
//a: undefined,0,0,0
//b: undefined,0,1,2
//c: undefined,0,1,1

2. 大数相加

let a = '9007199254740991'
let b = '1234567899999999999'

function add(a, b) {
  //取两个数字的最大长度
  let maxLength = Math.max(a.length, b.length)
  //用0去补齐长度
  a = a.padStart(maxLength, 0) //"0009007199254740991"
  b = b.padStart(maxLength, 0) //"1234567899999999999"
  //定义加法过程中需要用到的变量
  let t = 0
  let f = 0 //"进位"
  let sum = ''
  for (let i = maxLength - 1; i >= 0; i--) {
    t = parseInt(a[i]) + parseInt(b[i]) + f
    f = Math.floor(t / 10)
    sum = (t % 10) + sum
  }
  if (f == 1) {
    sum = '1' + sum
  }
  return sum
}