You have already learned how HTML5 provides built-in validation features, such as required attributes, input type checks, pattern matching, and length constraints. However, there are many situations where these built-in tools are not enough to enforce all the rules your application needs. For instance, HTML5 cannot compare values between fields—such as making sure a user has entered the same password twice—or ensure that a username is not already taken in your database.

Another common limitation is when your form requires validation based on complex business logic. For example, you might need to check that a date of birth makes a user at least 18 years old, or that a coupon code matches a specific set of rules only valid for certain users. HTML5 patterns cannot handle these kinds of interdependent or dynamic requirements.

Custom validation with JavaScript lets you implement logic that goes beyond what HTML5 offers. You can compare multiple fields, perform calculations, check values against data from your server, or create unique error messages. This flexibility makes custom validation an essential skill for any developer working on real-world forms.

Some validation needs that require custom JavaScript logic include:

• Ensuring that two password fields match;
• Verifying that an email address is not already registered;
• Checking that an entered age is above a minimum or below a maximum based on other form inputs;
• Validating a phone number format that cannot be described with a simple pattern;
• Enforcing business-specific rules, such as preventing certain usernames or requiring conditional fields if a checkbox is selected.