Form Data Type

Problem

Form data flows through validation, transformation, and submission logic with inconsistent types. Required fields get forgotten, validation rules drift from actual requirements, and submission payloads silently fail due to type mismatches. Refactoring forms is risky because there’s no compile-time safety.

Solution

Model form state with types that match your domain rather than treating everything as strings. This catches validation errors at compile time and makes form logic easier to reason about.

Example

This example demonstrates defining a typed interface for form data to ensure type safety throughout the form lifecycle.

// Define the shape of form data with proper types
interface UserForm {
  name: string;      // Required string field
  email: string;     // Required string field
  age: number;       // Number field, not string
  isActive: boolean; // Boolean field, not checkbox string value
}

// Use the interface to type form state
const [formData, setFormData] = useState<UserForm>({
  name: '',
  email: '',
  age: 0,
  isActive: true
});

Benefits

• Catches type mismatches at compile time before they cause runtime errors.
• Makes form state self-documenting by encoding requirements in the type system.
• Enables better IDE support with autocomplete and type checking for form fields.
• Prevents common bugs like treating numbers as strings or forgetting required fields.

Tradeoffs

• Requires converting between string inputs and typed values, adding transformation logic.
• Can be verbose for simple forms where type safety overhead isn’t worth the benefit.
• May complicate forms with dynamic fields that don’t fit rigid type structures.
• Needs careful handling of partial/incomplete forms during user input.