如何在angular中设置空值

这与我关于角度空值的帖子有关

我试图理解为什么我的应用程序/代码在当前情况下会表现出不同的行为

  calculateAge(dateOne: Date, dateTwo: Date): number {
    let firstDate = new Date(dateOne);
    let secondDate = new Date(dateTwo);
    let differenceInDates = Math.abs(firstDate.getTime() - secondDate.getTime());
    let age = Math.floor(differenceInDates / (1000 * 3600 * 24));
    console.log(age);
    return age;
  }

  getFlowers() {
    this.flowerSubscription = this.flowerService.getFlowersByFlowerId(this.flowerId, this.someName).subscribe(
      res => {
        this.dataToSend = res.DataToSend
        this.flowerSubscription = this.flowerService.getFlowers(this.dataToSend, this.someName).subscribe(
          res => {
            this.flower = res; // this.Flower is interface IFlower
            console.log(this.flower); <--- this is null
            if (this.flower !== undefined) {
              this.flowerNumber = (this.flower.FNumber || '--').toString();
              this.flowerInDate = this.flower.FlowerInDate;
              this.flowerOutDate = this.flower.FlowerInDate;
              this.flowerAge = this.calculateAge(this.flower.flowerWarehouseDate, this.flower.flowerStoreDate);
              this.flowerStatus = (this.flower.FlowerStatus || '--');
              this.customerAmount = (this.flower.CustomerAmount || '--').toString();
              this.warehouseAmount = (this.flower.WarehouseAmount || '--').toString();
            }
        }
    }
}

calculateAge（日期一：日期，日期二：日期）：数字{
设firstDate=新日期（dateOne）；
设第二个日期=新日期（日期二）；
让differenceInDates=Math.abs（firstDate.getTime（）-secondDate.getTime（））；
让年龄=数学楼层（差异数/（1000*3600*24））；
控制台日志（年龄）；
回归年龄；
}
getFlowers（）{
this.flowerSubscription=this.flowerService.getFlowersByFlowerId（this.flowerId，this.someName）。订阅(
res=>{
this.dataToSend=res.dataToSend
this.flowerSubscription=this.flowerService.getFlowers（this.dataToSend，this.someName）。订阅(
res=>{
this.flower=res；//this.flower是花椰菜
console.log（这个花）；Maddy

getFlowers（）{
this.flowerSubscription=this.flowerService.getFlowersByFlowerId（this.flowerId，this.someName）。订阅(
res=>{
this.dataToSend=res.dataToSend
this.flowerSubscription=this.flowerService.getFlowers（this.dataToSend，this.someName）。订阅(
res=>{
this.flower=res；//this.flower是花椰菜
console.log（this.flower）；我通过检查
this.dataToSend
是否为null找到了解决方案。如果为null，我将所有值设置为
--
，否则我将使用实际变量设置值。
@Eliseo，谢谢你的帮助。
if（variable）相当于if（variable！=未定义和变量！=null和变量！=0和变量！=”），if（！variable）相当于if（variable==未定义和变量==null | | variable==0 | | | variable=“”）@Eliseo，不应该
|
处理空值吗？你的意思是
console.log（this.flower；如果
this.flower
为null，那么
this.flower.FNumber
将抛出一个错误。你有错误吗？@user184994，我只得到null异常。否则没有错误。Eliseo，也许我不完全理解，如果你能帮我解释一下。在我的If语句中，“| |”应该验证null。对吗？还有，如果只有null异常呢有一件事在if条件下是空的。我认为如果是这样的话，这个会失败。@Maddy，this.flower可以是空的，也可以不是空的。如果this.flower不为空，this.flower.FlowerStatus可以是空的。我认为this.FlowerStatus=this.FlowerStatus | | |-'一定是好的。另一个说Hyuc Kang this.FlowerStatus=this.FlowerStatus=this.FlowerStatus？th的语句是空的is.flower.FlowerStatus:“--”也可以（是三元运算符）。但最后两条语句给出的错误是this.flower为null
  <table id="FlowerDetails" class="f-fs">
    <tr>
      <td class="f-title">Flower Nmber</td>
      <td class="f-text">{{flowerNumber}}</td>
    </tr>
    <tr>
      <td class="f-title">flowerInDate</td>
      <td class="f-text">{{(flowerInDate != null) ? (claimStartDate | date: 'shortDate') : '--'}}</td>
    </tr>
    <tr>
      <td class="f-title">flowerOutDate</td>
      <td class="f-text">{{(flowerOutDate != null) ? (flowerOutDate | date: 'shortDate') : '--'}}</td>
    </tr>
    <tr>
      <td class="f-title">Age</td>
      <td class="f-text">{{flowerAge}}</td>
    </tr>
    <tr>
      <td class="f-title">Flower Status</td>
      <td class="f-text">{{flowerStatus}}</td>
    </tr>
  </table>

getFlowers() {
    this.flowerSubscription = this.flowerService.getFlowersByFlowerId(this.flowerId, this.someName).subscribe(
      res => {
        this.dataToSend = res.DataToSend
        this.flowerSubscription = this.flowerService.getFlowers(this.dataToSend, this.someName).subscribe(
          res => {
            this.flower = res; // this.Flower is interface IFlower
            console.log(this.flower); <--- this is null
            if (this.flower){ //if this.flower <> undefined and <>null
              this.flowerNumber = (this.flower.FNumber || '--').toString();
              this.flowerInDate = this.flower.FlowerInDate;
              this.flowerOutDate = this.flower.FlowerInDate;
              this.flowerAge = this.calculateAge(this.flower.flowerWarehouseDate, this.flower.flowerStoreDate);
              this.flowerStatus = (this.flower.FlowerStatus || '--');
              this.customerAmount = (this.flower.CustomerAmount || '--').toString();
              this.warehouseAmount = (this.flower.WarehouseAmount || '--').toString();
            }
            else
            {
              this.flowerNumber = '--';
              this.flowerInDate = '--';
              this.flowerOutDate = '--';
              this.flowerAge = 0;
              this.flowerStatus = '--';
              this.customerAmount = '--';
              this.warehouseAmount = '--';
            }
        }
    }