Custom validators in Angular - Mini Project: Build & Apply

Creating a Custom Validator in Angular

📖 Scenario: You are building a user registration form in Angular. You want to ensure that the username entered by the user does not contain any spaces.

🎯 Goal: Build a custom validator function called noSpaceValidator that checks if the username contains spaces and use it in an Angular reactive form control.

📋 What You'll Learn

Create a custom validator function named noSpaceValidator

The validator should return { 'noSpace': true } if the value contains spaces

Use the noSpaceValidator in a reactive form control named username

Initialize the form group with the username control using Angular's FormBuilder

💡 Why This Matters

🌍 Real World

Custom validators are used in real-world Angular apps to enforce specific input rules beyond built-in validators, such as checking for spaces, special characters, or custom formats.

💼 Career

Knowing how to write and apply custom validators is essential for Angular developers to build robust forms that meet business rules and improve user input quality.

1

Set up the reactive form with a username control

Import FormBuilder and FormGroup from @angular/forms. Create a form group called form with a control named username initialized to an empty string.

Angular

import { FormBuilder, FormGroup } from '@angular/forms';

// Create a form group with username control
// Your code here

Hint

Use this.fb.group to create the form group and initialize username with an empty string.

2

Create the custom validator function noSpaceValidator

Write a function named noSpaceValidator that takes a FormControl as input and returns { noSpace: true } if the control's value contains any spaces, otherwise returns null.

Angular

import { FormBuilder, FormGroup, FormControl } from '@angular/forms';

form: FormGroup;

constructor(private fb: FormBuilder) {
  this.form = this.fb.group({
    username: ['']
  });
}

// Create noSpaceValidator function below
// Your code here

Hint

Check if control.value contains a space using indexOf(' '). Return the error object if true, else null.

3

Add the noSpaceValidator to the username control

Modify the username control in the form group to include the noSpaceValidator as a validator.

Angular

import { FormBuilder, FormGroup, FormControl } from '@angular/forms';

form: FormGroup;

constructor(private fb: FormBuilder) {
  this.form = this.fb.group({
    username: ['', noSpaceValidator]
  });
}

function noSpaceValidator(control: FormControl) {
  return control.value && control.value.indexOf(' ') >= 0 ? { noSpace: true } : null;
}

// Add noSpaceValidator to username control above
// Your code here

Hint

Pass noSpaceValidator as the second element in the array for the username control.

4

Complete the Angular component with the form and validator

Ensure the Angular component class includes the form property, the constructor with FormBuilder injection, the noSpaceValidator function, and the username control uses the validator. This completes the reactive form setup with the custom validator.

Angular

import { FormBuilder, FormGroup, FormControl } from '@angular/forms';

form: FormGroup;

constructor(private fb: FormBuilder) {
  this.form = this.fb.group({
    username: ['', noSpaceValidator]
  });
}

function noSpaceValidator(control: FormControl) {
  return control.value && control.value.indexOf(' ') >= 0 ? { noSpace: true } : null;
}

// Finalize the component setup
// Your code here

Hint

Wrap the form and validator inside an Angular component class with proper imports and decorator.

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