javascript - Angular material mat-error cannot show message

Question

I have 2 datetime picker, endDate cannot be less than startDate

endDateAfterOrEqualValidator(formGroup): any {
    var startDateTimestamp: number;
    var endDateTimestamp: number;
    startDateTimestamp = Date.parse(formGroup.controls['startDateForm'].value);
    endDateTimestamp = Date.parse(formGroup.controls['endDateForm'].value);
    return (endDateTimestamp < startDateTimestamp) ? { endDateLessThanStartDate: true } : null;
  }

in html:

<mat-form-field>
    <input matInput  name="endDate" id="endDate" formControlName="endDateForm" [(ngModel)]="endDate" [matDatepicker]="toDatePicker"
    placeholder="To Date">
    <mat-datepicker-toggle matSuffix [for]="toDatePicker"></mat-datepicker-toggle>
    <mat-datepicker disabled="false" #toDatePicker></mat-datepicker>
    <mat-error *ngIf="trainingDetail.hasError('endDateLessThanStartDate')">Not valid<mat-error>
</mat-form-field>

with "mat-error", the message does not display. I try to change by "small"

<small *ngIf="trainingDetail.hasError('endDateLessThanStartDate')">Not valid</small>

the message well. Please advice me how to using mat-error

Answer 1

a mat-error only shows when a FormControl is invalid, but you have the validation on your formgroup. So in that case you need to use a ErrorStateMatcher

In your case it would look like this:

export class MyErrorStateMatcher implements ErrorStateMatcher {
  isErrorState(control: FormControl | null, form: FormGroupDirective | NgForm | null): boolean {
    const invalidCtrl = !!(control && control.invalid);
    const invalidParent = !!(control && control.parent && control.parent.invalid);

    return (invalidCtrl || invalidParent);
  }
}

Also worth mentioning, it's not recommended to have two bindings, i.e formControl and ngModel. Remove the ngModel and utilize the form control instead. If you receive your start date and end date at a later point, you can use patchValue (just set some values to form) or setValue (set all values to form)

mark in component the errorstatematcher:

matcher = new MyErrorStateMatcher();

As for your custom validator, you don't need to parse the dates, just check if end date is smaller than start date:

checkDates(group: FormGroup) {
  if (group.controls.endDate.value < group.controls.startDate.value) {
    return { endDateLessThanStartDate: true }
  }
  return null;
}

and then mark the error state matcher in your template:

<mat-form-field>
  <input matInput [matDatepicker]="picker2" type="text" formControlName="endDate" [errorStateMatcher]="matcher">
  <mat-datepicker-toggle matSuffix [for]="picker2"></mat-datepicker-toggle>
  <mat-datepicker #picker2></mat-datepicker>
  <mat-error *ngIf="myForm.hasError('endDateLessThanStartDate')">End date cannot be earlier than start date</mat-error>
</mat-form-field>

Here's a StackBlitz