TypeScript: any vs unknown vs never Explained

Understanding these special types is crucial for type-safe TypeScript code.

The any Type

let value: any;
value = 42;
value = "hello";
value.toUpperCase(); // No error at compile time

The unknown Type

let value: unknown;
value = 42;

// Must check type first
if (typeof value === "string") {
  console.log(value.toUpperCase()); // Safe!
}

Real-World Example:

async function fetchUser(id: string) {
  const response = await fetch(`/api/users/${id}`);
  const data: unknown = await response.json();
  
  if (isUser(data)) {
    return data;
  }
  throw new Error("Invalid data");
}

function isUser(obj: unknown): obj is User {
  return typeof obj === "object" && obj !== null && "id" in obj;
}

The never Type

function throwError(message: string): never {
  throw new Error(message);
}

// Exhaustive check example
type Shape = { kind: "circle" } | { kind: "square" };

function getArea(shape: Shape): number {
  switch (shape.kind) {
    case "circle":
      return 0;
    case "square":
      return 0;
    default:
      const _exhaustive: never = shape;
      throw new Error("Unhandled shape");
  }
}

Comparison

| Type | Assign To | Assign From | Use Case | |------|-----------|-------------|----------| | any | Everything | Everything | Migration only | | unknown | any, unknown | Everything | External data | | never | Nothing | Nothing | Impossible states |

Best Practices

1. Never use any in new code 2. Always use unknown for dynamic data 3. Use never for exhaustive type checking 4. Always validate unknown values with type guards

Your code will be safer and more maintainable!