Both unknown and any are TypeScript types that can represent any value. However, the ways they allow you to interact with those values differ, which leads to distinct use cases and scenarios where each should be used.

unknown

• Safety: unknown is a safer alternative to any. If a value has the type unknown, you can't perform any operations on it, assign it to variables of most other types, or access any properties on it without some sort of type assertion or type guarding.

• Use Cases: Use unknown when:

  • You're dealing with values from external sources (e.g., user inputs, API responses) and you want to ensure you perform adequate type checking before using them.
  • You want to write more type-safe, predictable, and self-documenting code.

• Example:

  function processInput(input: unknown) {
      if (typeof input === "string") {
          console.log(input.toUpperCase());  // Type checking ensures this is safe
      }
  }

any

• Flexibility at the cost of safety: any is the most flexible type in TypeScript. It tells the compiler to turn off all type checking for a variable declared of this type. A variable of type any can be assigned to a variable of any other type and vice versa without any issues.

• Use Cases: While overuse of any can lead to less type-safe and less maintainable code, there are scenarios where it's handy:

  • When you're migrating a JavaScript codebase to TypeScript and gradually adding types. Using any can be a starting point.
  • When you're using third-party libraries or code that isn't typed, and creating type definitions is impractical.
  • In certain metaprogramming scenarios where type dynamism is required.

• Example:

  let data: any = fetchDataFromApi();
  console.log(data.someProperty);  // No type checking, this might be risky

Recommendations:

1. Default to stricter types: Always aim to use the strictest type possible. This means using specific types (e.g., string, number, or your custom types) over unknown, and using unknown over any.
2. Use unknown for greater safety: If you're unsure about the shape or type of a value (like from an external source), start with unknown. This will force you to perform necessary type checks or type assertions, ensuring safety.
3. Limit the use of any: Only use any when you have a strong reason, and it's impractical to use any other type. If you do use any, try to limit its scope and document why it's necessary, so future maintainers are aware of potential type risks.
4. Leverage compiler options: Consider enabling the noImplicitAny compiler option in your tsconfig.json. This ensures that you explicitly mark variables or return types as any when you truly intend to, rather than accidentally omitting types.