Answer 1

function fun(num){
    let num1 = num / 10;
    let num2 = num % 10;
    if(num1<1){
        return num;
    }else{
        num1 = Math.floor(num1)
        return `${num2}${fun(num1)}`
    }
}
var a = fun(12345)
console.log(a)
console.log(typeof a)

我觉着这个还是得加一些异常case，比如1000，或者负数

/**
 * @case 1234 => '4321'
 * @case 1000 => '1'
 * @case -1000 => '-1'
 * @case -1234 => '-4321'
 *
 * @param {number} number 传入的数值
 */
function solution(number) {
  if (number < 0) {
    return `-${solution(Math.abs(number))}`;
  }

  if (number < 10) {
    return `${number}`;
  }

  return `${number % 10 || ''}${solution(~~(number / 10))}`;
}

const assert = require('assert').strict;

assert.strictEqual(solution(1234), '4321');
assert.strictEqual(solution(1000), '1');
assert.strictEqual(solution(-1234), '-4321');
assert.strictEqual(solution(-1000), '-1');

Answer 2

type API 个人感觉这么写更好些~

const typeMap = "Symbol Boolean Number String Function Array Date RegExp Object Error"
  .split(" ")
  .reduce((prev, item) => {
    prev[`[object ${item}]`] = item.toLowerCase();
    return prev;
  }, {});

function type(obj) {
  //解决IE6 中的兼容
  if(null == obj) return '' + obj;

  return typeof obj !== "object" 
    ? typeof obj
    : typeMap[Object.prototype.toString.call(obj)] || 'object';
}