Understanding the 'Never' Type in TypeScript

The correct answer to the question, "In TypeScript, which type is used when a function never returns a value?" is 'Never'. This might initially sound confusing to some people who are new to TypeScript, but it actually makes a lot of sense once you explore the TypeScript's type system further.

In TypeScript, the never type represents the type of values that never occur. That is, if you have a function that doesn't have a return statement, or if it always throws an error, then it's return type is never. This is especially useful when dealing with error handling or other situations where a function is expected to not return a value under normal circumstances.

Let's take a look at a practical example:

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

In this example, the throwError function never returns a value because it always throws an error. This is a clear use case where the never type is applicable.

It's important to note that the never type is not the same as void. Both types represent the absence of a value, but in a different way. A function with a void return type can return undefined or null, while never indicates a function that doesn't return at all, or always throws an exception.

When designing TypeScript programs, it's always a good practice to be explicit about the function's return type. That means if a function is not expected to return a value, specify it using void or never accordingly. This greatly reduces confusion, improves code readability, and helps to uncover bugs. Make sure to use the never type appropriately and understand its implications within your codebase.

In conclusion, the TypeScript never type is a powerful addition to the toolbox of a TypeScript developer, which can help in maintaining high-quality, robust code bases.