Lab 03: Custom Validator

📖 Resources

• Form Validation Documentation
• Custom Validators Guide

🚀 Starter Code

step-3-album-wholesale-v16-post-add-album-form-array

In this lab, you’ll create a custom validator function to enforce a business rule that album prices must end with the digit 9. You’ll implement the validator logic, apply it to a form control, and display custom error messages.

📝 Instructions

Step 1: Create Custom Validator

Create a validator function:

import { AbstractControl, ValidationErrors, ValidatorFn } from '@angular/forms';

export function priceEndingWith9Validator(): ValidatorFn {
  return (control: AbstractControl): ValidationErrors | null => {
    const value = control.value;
    if (!value) return null;

    const endsWithNine = value.toString().endsWith('9');
    return endsWithNine ? null : { priceEndingWith9: true };
  };
}

Step 2: Apply to FormControl

Add validator to FormControl:

albumForm = this.fb.group({
  price: [0, [
    Validators.required,
    priceEndingWith9Validator()  // Custom validator
  ]]
});

Step 3: Display Custom Error

Show custom error message:

<mat-error *ngIf="albumForm.get('price')?.errors?.['priceEndingWith9']">
  Price must end with 9
</mat-error>