有人可以解釋什麼是運營商「：？」？

Question

有人可以解釋什麼是運營商？

@observable.ref navigationState: ?NavigationState = null; 

Answer 1

它是安全的導航操作」，做參考go here

或藉此例如

<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