index/interfaces/ValidationResult.md

Standard validation result

Provides consistent interface for all validators with proper TypeScript strict mode compatibility.

Design Rationale

This interface uses explicit T | undefined instead of optional properties to ensure compatibility with TypeScript's exactOptionalPropertyTypes setting.

TypeScript Configuration Context

// tsconfig.json
{
  "compilerOptions": {
    "exactOptionalPropertyTypes": true
  }
}

With exactOptionalPropertyTypes: true, TypeScript strictly enforces that optional properties (property?: Type) cannot be assigned undefined values. Only missing properties are allowed.

Interface Design Choice

// ❌ INCORRECT - Breaks with exactOptionalPropertyTypes
interface ValidationResult<T> {
  success: boolean;
  data?: T;          // Optional property
  errors?: string[]; // Optional property
}

// ✅ CORRECT - Compatible with exactOptionalPropertyTypes
interface ValidationResult<T> {
  success: boolean;
  data: T | undefined;       // Explicit undefined
  errors: string[] | undefined;  // Explicit undefined
}

Usage Patterns

// Success case
const successResult: ValidationResult<string> = {
  success: true,
  data: 'validated-value',
  errors: undefined
};

// Failure case
const failureResult: ValidationResult<string> = {
  success: false,
  data: undefined,
  errors: ['Validation error message']
};

// Type-safe consumption
function handleResult(result: ValidationResult<string>) {
  if (result.success) {
    // TypeScript knows result.data is string | undefined
    console.log(result.data);
  } else {
    // TypeScript knows result.errors is string[] | undefined
    console.error(result.errors);
  }
}

Benefits

1. Type Safety: Explicit undefined makes intent clear
2. Strict Mode: Compatible with exactOptionalPropertyTypes
3. Consistency: All validators return same shape
4. Backward Compatibility: Maintains existing API surface

Type Parameters

Type Parameter	Description
T	The type of the validated data

Properties

Property	Type	Description
data	T | undefined	Validated and transformed data (if successful)
errors	string[] | undefined	Validation error messages (if failed)
success	boolean	Whether validation passed