Custom validators in Angular

Introduction

Custom validators help you check if form inputs follow your own rules. They make sure users enter data correctly beyond built-in checks.

You want to check if a password has special characters.

You need to verify a username is unique.

You want to confirm two password fields match.

You want to validate a date is in the future.

You want to block certain words in a text input.

Syntax

Angular

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

export function customValidator(): ValidatorFn {
  return (control: AbstractControl): ValidationErrors | null => {
    const value = control.value;
    // Your validation logic here
    const isValid = /* condition */;
    return isValid ? null : { customError: true };
  };
}

A custom validator is a function that returns another function.

The inner function checks the control's value and returns null if valid or an error object if invalid.

Examples

This validator checks if the input contains spaces and returns an error if it does.

Angular

export function noSpaceValidator(): ValidatorFn {
  return (control: AbstractControl) => {
    const hasSpace = (control.value || '').includes(' ');
    return hasSpace ? { noSpace: true } : null;
  };
}

This validator checks if the input length is at least a minimum number.

Angular

export function minLengthValidator(min: number): ValidatorFn {
  return (control: AbstractControl) => {
    const length = (control.value || '').length;
    return length >= min ? null : { minLength: { requiredLength: min, actualLength: length } };
  };
}

Sample Program

This Angular component uses a custom validator to block the word 'forbidden' in the input. If the user types it, an error message shows below the input.

Angular

import { Component } from '@angular/core';
import { FormControl, ValidatorFn } from '@angular/forms';

// Custom validator to block the word 'forbidden'
export function forbiddenWordValidator(): ValidatorFn {
  return (control) => {
    const forbidden = (control.value || '').toLowerCase().includes('forbidden');
    return forbidden ? { forbiddenWord: true } : null;
  };
}

@Component({
  selector: 'app-root',
  standalone: true,
  template: `
    <label for="input">Enter text:</label>
    <input id="input" [formControl]="textControl" aria-describedby="error" />
    <div *ngIf="textControl.errors?.forbiddenWord" id="error" role="alert" style="color: red;">
      The word 'forbidden' is not allowed.
    </div>
  `
})
export class AppComponent {
  textControl = new FormControl('', [forbiddenWordValidator()]);
}

Important Notes

Always return null when the input is valid.

Use descriptive error keys in the returned object to identify the error.

Combine custom validators with built-in ones by passing them in an array.

Summary

Custom validators let you add your own rules to form inputs.

They return null if valid or an error object if invalid.

Use them to improve user input quality and app reliability.

Practice

1. What does a custom validator function in Angular return when the input is valid?

easy

A. an empty string

B. true

C. false

D. null

Solution

1. Step 1: Understand Angular validator return values

   Angular custom validators return null when the input passes validation, meaning no errors.

2. Step 2: Identify the valid return for no errors

   Returning null signals the form control is valid, while any object means invalid.

3. Final Answer:

   null -> Option D

4. Quick Check:

   Valid input returns null [OK]

Hint: Valid inputs always return null from validators [OK]

Common Mistakes:

• Returning true or false instead of null or error object
• Returning empty string instead of null
• Confusing valid with invalid return values

2. Which of the following is the correct signature for a synchronous custom validator function in Angular?

easy

A. function myValidator(event: Event): string

B. function myValidator(control: AbstractControl): ValidationErrors | null

C. function myValidator(control: FormGroup): void

D. function myValidator(value: string): boolean

Solution

1. Step 1: Recall Angular validator function signature

   A synchronous custom validator takes an AbstractControl and returns ValidationErrors or null.

2. Step 2: Match the correct function signature

   function myValidator(control: AbstractControl): ValidationErrors | null matches the required signature exactly, others do not fit Angular patterns.

3. Final Answer:

   function myValidator(control: AbstractControl): ValidationErrors | null -> Option B

4. Quick Check:

   Validator signature = function myValidator(control: AbstractControl): ValidationErrors | null [OK]

Hint: Validator functions take AbstractControl and return errors or null [OK]

Common Mistakes:

• Using wrong parameter types like string or Event
• Returning boolean instead of error object or null
• Missing return type or returning void

3. Given this custom validator code, what will be the validation result for input value 'abc123'?

function alphaOnly(control: AbstractControl) {
  const valid = /^[a-zA-Z]+$/.test(control.value);
  return valid ? null : { alphaOnly: true };
}

medium

A. { alphaOnly: true }

B. true

C. null

D. false

Solution

1. Step 1: Analyze the regex test on input 'abc123'

   The regex /^[a-zA-Z]+$/ matches only letters. 'abc123' contains digits, so test returns false.

2. Step 2: Determine validator return value

   Since test is false, the validator returns the error object { alphaOnly: true } indicating invalid input.

3. Final Answer:

   { alphaOnly: true } -> Option A

4. Quick Check:

   Input with digits fails alphaOnly validator [OK]

Hint: Regex test false means return error object, not null [OK]

Common Mistakes:

• Assuming digits pass the regex
• Confusing true/false return values
• Expecting null for invalid input

4. Identify the error in this custom validator code snippet:

function minLengthFive(control: AbstractControl) {
  if (control.value.length < 5) {
    return { minLengthFive: true };
  }
}

medium

A. Incorrect error object key name

B. Missing return null for valid input

C. Using control.value.length without checking null

D. Validator should return boolean instead of object

Solution

1. Step 1: Check for runtime errors

   The code accesses control.value.length without checking if control.value is null or undefined, causing TypeError when value is null (common for new FormControls).

2. Step 2: Angular validator best practice

   Always check if (control.value == null) return null; before accessing properties like length. Explicit return null; for valid cases is also recommended.

3. Final Answer:

   Using control.value.length without checking null -> Option C

4. Quick Check:

   control.value can be null causing crash [OK]

Hint: Always check control.value != null before .length [OK]

Common Mistakes:

• Not checking control.value for null before .length
• Returning undefined instead of null for valid input
• Confusing error object keys with validator names

5. You want to create a custom validator that checks if a password contains at least one uppercase letter, one lowercase letter, and one digit. Which of these validator implementations correctly applies this logic?

hard

A. function passwordValidator(control: AbstractControl) {
const val = control.value || '';
const hasUpper = /[A-Z]/.test(val);
const hasLower = /[a-z]/.test(val);
const hasDigit = /\d/.test(val);
return hasUpper && hasLower && hasDigit ? null : { passwordStrength: true };
}

B. function passwordValidator(control: AbstractControl) {
if (!control.value.match(/[A-Z]/) || !control.value.match(/[a-z]/) || !control.value.match(/[0-9]/)) {
return null;
}
return { passwordStrength: true };
}

C. function passwordValidator(control: AbstractControl) {
const val = control.value;
if (val.length < 8) return { passwordStrength: true };
return null;
}

D. function passwordValidator(control: AbstractControl) {
const val = control.value;
if (val.includes(' ')) return { passwordStrength: true };
return null;
}

Solution

1. Step 1: Understand password requirements

   The password must have at least one uppercase, one lowercase, and one digit character.

2. Step 2: Analyze each option's logic

   function passwordValidator(control: AbstractControl) { const val = control.value || ''; const hasUpper = /[A-Z]/.test(val); const hasLower = /[a-z]/.test(val); const hasDigit = /\d/.test(val); return hasUpper && hasLower && hasDigit ? null : { passwordStrength: true }; } correctly tests all three conditions and returns null if all pass, error object otherwise. function passwordValidator(control: AbstractControl) { if (!control.value.match(/[A-Z]/) || !control.value.match(/[a-z]/) || !control.value.match(/[0-9]/)) { return null; } return { passwordStrength: true }; } reverses logic, returning null on failure. The remaining options check unrelated conditions.

3. Final Answer:

   function passwordValidator(control: AbstractControl) { const val = control.value || ''; const hasUpper = /[A-Z]/.test(val); const hasLower = /[a-z]/.test(val); const hasDigit = /\d/.test(val); return hasUpper && hasLower && hasDigit ? null : { passwordStrength: true }; } -> Option A

4. Quick Check:

   All conditions true returns null, else error [OK]

Hint: Return null only if all conditions are met, else error [OK]

Common Mistakes:

• Returning null on invalid input
• Checking only length or spaces, not required chars
• Mixing up logic conditions in if statements