Set default select list value Angular2

问题

I want to set the default option for the select in angular 2 as "Select an option". Here is the code that I have so far:

HTML

<div class="form-group">
                        <label class="col-md-4 control-label" for="CustomFix">Select an option:</label>
                        <div class="col-md-4">
                            <select id="example" name="example" class="form-control" [(ngModel)]="exampleArray" (ngModelChange)="changes()">
                                <option disabled selected>Select a Custom Fix</option>
                                <option *ngFor="let example of exampleArray" [ngValue]="example">{{example.attribute }}</option>
                            </select>
                        </div>
                    </div>

Typescript

changes(){
console.log(this.example.option);
}

I have the line in my html:

<option disabled selected>Select a Custom Fix</option>

How can I enable this as the default option? It is separate from my array used with ngModel

回答1:

If you use [ngValue] then you need to assign the identical object instance to exampleArray. Another object instance with the same properties and values won't do.

If you use [value]="..." instead of [ngValue], then only strings can be used and for string comparison to different string instances containing the same characters are considered equal, but that's not the case for object instances where exampleArray needs to reference the exact same object reference used with [ngValue].

Actually

[(ngModel)]="exampleArray"

in your example is invalid, because the model should not be the array that is used to generate the <option> elements, it should be a property that holds the selected value.

   <div class="form-group">
        <label class="col-md-4 control-label" for="CustomFix">Select an option:</label>
        <div class="col-md-4">
            <select id="example" name="example" class="form-control" [(ngModel)]="selectedItem" (ngModelChange)="changes()">
                <option disabled selected>Select a Custom Fix</option>
                <option *ngFor="let example of exampleArray" [ngValue]="example">{{example.attribute }}</option>
            </select>
        </div>
    </div>

constructor() {
  this.selectedItem = exampleArray[1]; // will make the 2nd element of `exampleArray` the selected item
}

回答2:

If you want to have this option selected at the start - that's usually when the ngModel's value is undefined, you just have to tell Angular that this option is responsible for undefined value, so it should be:

<option disabled [ngValue]="undefined">Select a Custom Fix</option>

You also need do correct your [(ngModel)] binding - right now you're trying to bind selected value to.. the array itself. You should rather bind to some other property, e.g.:

<select id="example" name="example" class="form-control" [(ngModel)]="example">

(You can see the working solution here: http://plnkr.co/edit/Zu29ztqaaDym1GYDAhtJ?p=preview)

回答3:

You should give that option a value, bind the select element to an ID variable, and set that variable on component load.

// controller
exampleArraySelectedValue = -1;

<div class="form-group">
  <label class="col-md-4 control-label" for="CustomFix">Select an option:</label>
  <div class="col-md-4">
    <select id="example" name="example" class="form-control" [(ngModel)]="exampleArraySelectedValue" (ngModelChange)="changes()">
      <option value="-1">Select a Custom Fix</option>
      <option *ngFor="let example of exampleArray" [ngValue]="example">{{example.attribute }}</option>
    </select>
  </div>
</div>

来源：https://stackoverflow.com/questions/40640085/set-default-select-list-value-angular2

标签

html5

select

angular

angular2-ngmodel