Understanding TypeScript’s any, unknown, and never: A Comprehensive Guide

740 words, 4 minutes read time.

In TypeScript, the types any, unknown, and never can be confusing for developers who are unfamiliar with their use cases. While these types appear similar, each serves a distinct purpose that is crucial for writing safe, maintainable code. Let’s break them down to clear up the confusion and help you make informed decisions when using TypeScript in your projects.

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What is any in TypeScript?

The any type in TypeScript is one of the most flexible types, but also one of the most dangerous when misused. It essentially tells TypeScript to skip type checking for a given variable. You can assign any kind of value to a variable declared with the any type, from strings to numbers, arrays, or even functions, and TypeScript won’t throw any type errors. This can be useful when you’re dealing with code that you know will not have strict typing, such as when porting a JavaScript codebase to TypeScript or using third-party libraries without TypeScript definitions.

However, overusing any leads to runtime errors, as it bypasses TypeScript’s built-in safety mechanisms. A variable of type any can be manipulated in ways that are logically invalid, making your code prone to bugs and harder to maintain. Most of the time, any should be avoided in favor of more precise typing.

When to Use unknown Instead of any

If any is the type that disables type checking, unknown is the safer alternative. Introduced in TypeScript 3.0, the unknown type represents a value that could be of any type, but it requires type checking before any operations can be performed. With unknown, you can no longer perform actions like calling methods or properties on a variable until its type is narrowed down.

For example, if you have a function that accepts a variable whose type is not known ahead of time, you can first check whether the value is a string, number, or object before proceeding with any operations. This not only helps to prevent runtime errors but also encourages writing safer code. If you’re processing external data, such as inputs from a form or API responses, unknown is the way to go.

Here’s an example of how you might use unknown:

let val: unknown = "Hello";
if (typeof val === "string") {
  console.log(val.toUpperCase());  // Safe to call toUpperCase on string
}

The need to narrow the type before performing operations on unknown makes it far superior to any when working with dynamic data, ensuring better type safety and reducing the risk of bugs.

Understanding never: The Empty Set

The never type is unique because it represents values that never occur. It’s most commonly used to indicate that a function will never successfully return a value. For example, a function that throws an error or enters an infinite loop would return never, because these functions never complete normally.

The never type also plays a vital role in TypeScript’s exhaustive checks. For instance, when using a switch statement or handling union types, never can be used in the default case to ensure that all possible values are accounted for. If a new value is added to the union type and not handled in the switch, TypeScript will throw an error, as it will recognize that the value no longer resolves to never.

Example of never in a switch statement:

type UserType = 'admin' | 'guest';

function getUserRole(role: UserType) {
  switch(role) {
    case 'admin':
      return 'Administrator';
    case 'guest':
      return 'Guest';
    default:
      const exhaustiveCheck: never = role;  // Ensures all roles are handled
      throw new Error(`Unhandled role: ${exhaustiveCheck}`);
  }
}

y using never, you can create more robust error handling and exhaustive checking in your code, which can prevent bugs caused by unhandled cases.

Conclusion: When to Use Which Type?

• Use any sparingly: While any is useful in situations where you have no other choice, like porting JavaScript code to TypeScript or working with third-party libraries without type definitions, it should be avoided when possible due to the risk of runtime errors.
• Prefer unknown over any: unknown provides the flexibility of any type while maintaining type safety. It forces you to perform type checks before any operation, which helps in writing safer and more predictable code.
• Use never for exhaustive checks and unreachable code: never is particularly useful in functions that always throw errors or enter infinite loops, and it plays a key role in TypeScript’s exhaustive checking mechanism.

By understanding these three types and applying them properly, you can significantly improve the type safety of your TypeScript projects and avoid common pitfalls.