朋友给的练习题，本以为简单，但不成想需要考虑的条件多多：

function divider(dividend,divisor,decimal=0){
     if(divisor == 0){//除数不能为0
       throw new Error('divisor should not be 0')
     }
     if(decimal < 0 || parseInt(decimal) != decimal){//精确度只能为正整数
       throw new Error('decimal should be a positive int')
     }
     //判断最终结果是负数还是正数
     let isResultPositive = true
     if(dividend * divisor > 0){
       isResultPositive = true
     }else{
       isResultPositive = false
     }
     dividend = Math.abs(dividend)
     divisor = Math.abs(divisor)
     //整数部分的除法
      function intDivider(dividend,divisor){
        let i = 0
        for(i; dividend - (divisor * i) >= divisor; i++){
        }
        let quoten = i
        return quoten
      }
      //10的阶乘，为小数部分的除法做准备
      function power10 (pow){
        let num = 1
        for(let i=0;i<pow;i++){
          num *= 10
        }
        return num
      }
      //如果被整除了，那么加几个精确度0，为能整除的小数部分准备
      function add0WhenNoRemainder(decimal){
        if(decimal == 0){
          return ''
        }
        let float0 = '.0'
        for(let i = 1;i<decimal;i++){
          float0 += '0'
        }
        return float0
      }
      //小数部分的除法
      function remainderDivider(remainder,divisor,decimal){
        if(remainder !=0){//如果不能被整除，则按需保留小数后decimal位
          let enlarge = power10(decimal)//将余数按“保留几位小数的decimal”放大再除
          let quoten = intDivider(remainder * enlarge, divisor)
          return '.' + quoten
        }else{//如果能被整除，则直接按保留小数后decimal位添加0，如.0000
          return add0WhenNoRemainder(decimal)
        }
      }
      //拼接整数部分和小数部分的结果
      let intPart = intDivider(dividend,divisor)//整数部分
      let remainder = dividend - divisor * intPart
      let floatPart = remainderDivider(remainder,divisor,decimal)//小数部分
      // console.log(floatPart)
      let result = intPart + floatPart

      if(!isResultPositive){
        result =  '-' + result
      }
      return result
    }
    let result1 = divider(9,5,2)//1.80
    let result2 = divider(-9,5,3)//-1.800
    let result3 = divider(9,3,5)//3.00000
    let result4 = divider(-9,3,4)//-30000
    let result5 = divider(9,7,1)//1.2
    console.log(result1,result2,result3,result4,result5)