有人可以解释什么是运营商＆＃34;：？＆＃34;？

Question

有人可以解释什么是运营商吗？

 @observable.ref navigationState: ?NavigationState = null;

Answer 1

它是安全的导航操作员＆＃34;，即x？.y。

for refrence go here

或采用此示例

  <md-select placeholder="Select Month" [(ngModel)]="monthSelected" 
  #userSelect>

  <md-option *ngFor="let userSelection of CalanderContent" 
  [value]="userSelection">
  {{ userSelection.month }}
  </md-option>
  </md-select>
  {{userSelect.selected.value.days}} <- Compile time error beacuse we didnt selected
                                          any options at time of initializing.
  {{userSelect.selected?.value?.days}} <- Correct because it waits if its selected 
                                          then , displays the value