Custom Form Validator in Angular

This challenge focuses on creating a reusable custom validator in Angular to enforce specific data validation rules beyond the built-in validators. Building custom validators is crucial for handling unique business logic and ensuring data integrity within your Angular forms. This exercise will solidify your understanding of Angular's reactive forms and validator concepts.

Problem Description

You need to create a custom validator in Angular that checks if a given input field contains only alphanumeric characters (letters and numbers). This validator should be reusable and applicable to any input field within your Angular forms. The validator should return an error message if the input is invalid and null if the input is valid.

Key Requirements:

• Reusable Validator: The validator should be designed as a class that can be easily imported and used in different forms.
• Alphanumeric Check: The validator must accurately determine if the input string contains only alphanumeric characters.
• Error Message: When the input is invalid, the validator should return an object with an error key containing a user-friendly error message.
• Null for Valid Input: When the input is valid, the validator should return null.
• Reactive Forms: The validator should be implemented using Angular's reactive forms approach (FormGroup, FormControl, Validators).

Expected Behavior:

• When a FormControl is updated, the validator should be triggered.
• If the input contains non-alphanumeric characters, the FormControl's invalid property should be true, and the errors property should contain the error message.
• If the input contains only alphanumeric characters, the FormControl's invalid property should be false, and the errors property should be empty.

Edge Cases to Consider:

• Empty input string: Should be considered valid (no error).
• Input containing spaces: Should be considered invalid.
• Input containing special characters (e.g., punctuation, symbols): Should be considered invalid.
• Null or undefined input: Should be considered valid (no error).

Examples

Example 1:

Input: FormControl value = "HelloWorld123"
Output: null
Explanation: The input contains only alphanumeric characters, so the validator returns null, indicating a valid input.

Example 2:

Input: FormControl value = "Hello World!"
Output: { alphanumeric: "Input must contain only alphanumeric characters." }
Explanation: The input contains a space and an exclamation mark, which are not alphanumeric characters. The validator returns an error object.

Example 3:

Input: FormControl value = ""
Output: null
Explanation: The input is an empty string, which is considered valid by this validator.

Constraints

• The validator must be implemented using TypeScript.
• The validator class should be exportable and reusable.
• The error message should be clear and informative to the user.
• The validator should be efficient and avoid unnecessary computations.
• The validator should be compatible with Angular versions 8 and above.

Notes

• Consider using regular expressions for efficient alphanumeric character checking.
• Think about how to make the error message configurable if needed.
• Remember to import AbstractValidator from @angular/forms to create your custom validator.
• Focus on creating a clean, well-documented, and reusable validator class.