Javascript 不确定组合运算符在函数中执行的操作

我一直在编程下面的函数，直到这一行我都理解了

   cost += nightSurcharge;

我在

if

语句中使用了一些条件，用于在晚上8点到早上6点之间将夜间附加费添加到成本中

我需要了解的是，

+=

是否只是说，如果满足条件，将夜间附加费添加到成本中

// add a parameter called hourOfDay to the function
    var taxiFare = function (milesTraveled, hourOfDay) {
      var baseFare = 2.50;
      var costPerMile = 2.00;
      var nightSurcharge = 0.50; // 8pm to 6am, every night

      var cost = baseFare + (costPerMile * milesTraveled);

      // add the nightSurcharge to the cost starting at 
      // 8pm (20) or if it is before 6am (6)
       if (hourOfDay >= 20 || hourOfDay < 6) {
          cost += nightSurcharge;
      } 

      return cost;

    };

//将名为hourOfDay的参数添加到函数中
var出租车费=功能（英里级，小时/天）{
var基准票价=2.50；
var-costPerMile=2.00；
var NightOverlage=0.50；//每晚从晚上8点到早上6点
风险值成本=基本票价+（成本许可*里程等级）；
//从开始将夜间附加费添加到成本中
//晚上8点（20）或上午6点（6）之前
如果（小时数>=20 | |小时数<6）{
费用+=夜间附加费；
} 
退货成本；
};

我需要了解的是，

+=

是否只是说，如果满足条件，将夜间附加费添加到成本中

// add a parameter called hourOfDay to the function
    var taxiFare = function (milesTraveled, hourOfDay) {
      var baseFare = 2.50;
      var costPerMile = 2.00;
      var nightSurcharge = 0.50; // 8pm to 6am, every night

      var cost = baseFare + (costPerMile * milesTraveled);

      // add the nightSurcharge to the cost starting at 
      // 8pm (20) or if it is before 6am (6)
       if (hourOfDay >= 20 || hourOfDay < 6) {
          cost += nightSurcharge;
      } 

      return cost;

    };

是的，此代码等效于：

if (hourOfDay >= 20) {
    cost = cost + nightSurcharge;
}
else if (hourOfDay < 6) {
    cost = cost + nightSurcharge;
}

if（hourOfDay>=20）{
成本=成本+夜间附加费；
}
否则如果（小时/天<6）{
成本=成本+夜间附加费；
}

我需要了解的是，

+=

是否只是说，如果满足条件，将夜间附加费添加到成本中

// add a parameter called hourOfDay to the function
    var taxiFare = function (milesTraveled, hourOfDay) {
      var baseFare = 2.50;
      var costPerMile = 2.00;
      var nightSurcharge = 0.50; // 8pm to 6am, every night

      var cost = baseFare + (costPerMile * milesTraveled);

      // add the nightSurcharge to the cost starting at 
      // 8pm (20) or if it is before 6am (6)
       if (hourOfDay >= 20 || hourOfDay < 6) {
          cost += nightSurcharge;
      } 

      return cost;

    };

是的，此代码等效于：

if (hourOfDay >= 20) {
    cost = cost + nightSurcharge;
}
else if (hourOfDay < 6) {
    cost = cost + nightSurcharge;
}

if（hourOfDay>=20）{
成本=成本+夜间附加费；
}
否则如果（小时/天<6）{
成本=成本+夜间附加费；
}

成本+=夜间附加费相当于
成本=成本+附加费
成本+=夜间附加费相当于
成本=成本+附加费