Why Custom validators in Angular? - Purpose & Use Cases

The Big Idea

Discover how custom validators can save you from messy, repetitive form checks!

The Scenario

Imagine you have a form where users must enter a username that is not only filled but also unique and follows special rules like no spaces or special characters.

You try to check all these rules manually every time the user types something.

The Problem

Manually checking each rule every time is slow and messy.

You have to write repeated code for each input, and it's easy to forget or make mistakes.

Also, the form won't update properly if you don't handle errors right, confusing users.

The Solution

Custom validators let you write reusable, clear rules that Angular runs automatically on your form inputs.

This keeps your code clean and your form responsive, showing errors exactly when needed.

Before vs After

✗ Before

if(username.includes(' ')) { showError('No spaces allowed'); }

✓ After

function noSpacesValidator(control) { return control.value && control.value.includes(' ') ? { 'noSpaces': true } : null; }

What It Enables

You can create smart, reusable checks that keep your forms user-friendly and error-free.

Real Life Example

Think of a signup form that blocks usernames with spaces or special characters instantly, guiding users to fix mistakes before submitting.

Key Takeaways

Manual validation is slow and error-prone.

Custom validators automate and simplify input checks.

They improve user experience by showing clear, timely errors.

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