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angular value vs ngvalue

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In this video we will discuss how to validate a select element if it has a custom option like one of the following.
  • Please Select
  • Select Department
  • Etc...

Consider the following example :

<div class="form-group"
     [class.has-error]="department.touched && department.invalid">
  <label for="department" class="control-label">Department</label>
  <select required #department="ngModel" name="department"
          [(ngModel)]="employee.department" id="department"
          class="form-control">
    <option [ngValue]="null">Select Department</option>
    <option *ngFor="let dept of departments" [value]="dept.id">
      {{dept.name}}
    </option>
  </select>
  <span class="help-block"
        *ngIf="department.touched && department.errors?.required">
    Department is required
  </span>
</div>


Code explanation : 
  • <option [ngValue]="null">Select Department</option>. Notice we are using ngValue instead of value. If you use value, null is treated as a string and not as a null. Hence the required validation does not work. Along with using ngValue, also make sure you set the department property on the employee model object to null.
  • <option *ngFor="let dept of departments" [value]="dept.id">{{dept.name}}</option>. Here we are using value instead of ngValue, because we just want the selected department id as a string. If you want the department object itself instead of just the department id string, then use ngValue.
  • <option *ngFor="let dept of departments" [ngValue]="dept">{{dept.name}}</option>. In this example we are using ngValue and binding it to the dept object. If we select a department now, we get the selected department object.
    "department": { "id": 3, "name": "IT" }
  • Use the disabled attribute, if you do not want the user to be able to select the "Select Department" option.
    <option disabled [ngValue]="null">Select Department</option>
Please note :  The built-in required validator will only work with the SELECT element, if the default option value is null.
<option disabled [ngValue]="null">Select Department</option>

In a real world application, most of the time we load the SELECT list options from a database table. In some case we may also load the default option (like SELECT DEPARTMENT etc) also from the database. In this case the default option value may not be NULL. Depending on your use case it could be -1, or SELECT or something else. So in scenarios like this, built-in required validator does not work with the SELECT element. To make it work, we have to implement our own custom required validator. We will discuss how to do this in our next video.

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