The onlyOneError pipe is designed to prioritize and display a single validation error from a list of possible errors. It simplifies error handling by returning the first error based on a predefined priority order.

By the way this is continuation of my previous post for the "Creating custom field validator in Angular template driven form"

https://www.codingscenes.com/posts/84/Creating-custom-field-validator-in-Angular-template-driven-form

Pipe Implementation:

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({
  name: 'onlyOneError',
  standalone: true,
})
export class OnlyOneErrorPipe implements PipeTransform {
  /**
   * Transforms an object of errors into a single prioritized error.
   * @param allErrorsObj - The object containing all validation errors.
   * @param errorsPriority - An array defining the priority of errors.
   * @returns The first error based on priority or null if no errors exist.
   */
  transform(allErrorsObj: Record<string, any> | null, errorsPriority: string[]): Record<string, any> | null {
    if (!allErrorsObj) return null;


    for (const error of errorsPriority) {
      if (allErrorsObj[error]) {
        return { [error]: allErrorsObj[error] }; // Return the first matching error.
      }
    }


    return null; // Return null if no prioritized errors are found.
  }
}

Let's talk about the parameters and output in the details here

1. Input Parameters:

   • allErrorsObj: An object containing all validation errors (e.g., { required: true, minLength: { requiredLength: 8 } }).
   • errorsPriority: An array of error keys in the order of priority (e.g., ['required', 'minLength', 'noNumeric']).

2. Output:

   • Returns the first error from allErrorsObj that matches the priority list.
   • If no errors match, it returns null.

<ng-container
  *ngIf="password.errors
      | onlyOneError
        : [
            'noNumeric',
            'noUpperCase',
            'noLowerCase',
            'minLength',
            'noSpecial',
            'passwordStrength'
          ] as ​error
  "
>
  <div ​class="alert alert-danger">
    <ng-container *ngIf="error.minLength">
      Password must have at least {{​password.errors?.['minLength'].requiredLength​}} characters.
    </ng-container>
    <ng-container ​*ngIf="error.noNumeric">
      Password must include at least one numeric character.
    </ng-container>
    <ng-container ​*ngIf="error.noUpperCase">
      Password must include at least one uppercase character.
    </ng-container>
    <ng-container ​*ngIf="error.noLowerCase">
      Password must include at least one lowercase character.
    </ng-container>
    <ng-container ​*ngIf="error.noSpecial">
      Password must include at least one special character.
    </ng-container>
    <ng-container *ngIf="error.passwordStrength">
      Password is not strong enough.
    </ng-container>
  </div>
</ng-container>

Little explaination on this part

1. Pipe Usage:

   • The onlyOneError pipe is applied to password.errors with a priority list of error keys.
   • The as error syntax assigns the returned error to the error variable.

2. Dynamic Error Messages:

   • Based on the error object, the appropriate error message is displayed using <ng-container>.

3. Priority Handling:

   • The pipe ensures that only the highest-priority error is shown, even if multiple errors exist.

Ofcourse! there are few benefits of using this pipe

Benefits

• Simplifies error handling by focusing on one error at a time.
• Makes the template cleaner and easier to maintain.
• Ensures consistent error prioritization across the application.