TypeScript - Advanced Type Guards

// Type predicates for runtime checks
function isString(value: unknown): value is string {
  return typeof value === 'string'
}

function isUser(obj: unknown): obj is User {
  return (
    typeof obj === 'object' &&
    obj !== null &&
    'id' in obj &&
    'email' in obj &&
    typeof obj.email === 'string'
  )
}

// Discriminated union guard
type Shape =
  | { kind: 'circle'; radius: number }
  | { kind: 'square'; size: number }
  | { kind: 'rectangle'; width: number; height: number }

function getArea(shape: Shape): number {
  switch (shape.kind) {
    case 'circle':
      return Math.PI * shape.radius ** 2
    case 'square':
      return shape.size ** 2
    case 'rectangle':
      return shape.width * shape.height
  }
}

// Array type guard
function isStringArray(arr: unknown): arr is string[] {
  return Array.isArray(arr) && arr.every(item => typeof item === 'string')
}