Understanding the 'never' Type in TypeScript

In TypeScript, the 'never' type is a special type that represents values that should never occur. This might seem a little unusual at first, but a deeper dive into the concept should make its utility more apparent.

The never type – as defined by TypeScript – is a subtype of, and assignable to, every type. However, no type is assignable to never (except never itself). In other words, no values can ever be of type never.

Practical Applications of the 'never' Type

One typical use case for the never type is in functions that do not return a value. Let’s take a look at an example:

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

In this example, the function error does not return a value (it actually throws an error). Therefore, the return type of the function is never.

Another common use is in infinite loops:

function infiniteLoop(): never {
    while (true) {}
}

Here, the infiniteLoop function doesn't return a value as it's going into an infinite loop. Thus, the return type of this function is also 'never'.

Best Practices and Further Insights

While the never type is used less frequently than other TypeScript types, it's still crucial to understand it. When combined with the TypeScript type checker, the never type can help identify areas of your code that may contain logic errors, or aren't behaving as expected.

As a best practice, use the never type in situations where you want to ensure a specific scenario does not happen. For example, when dealing with discriminated unions (a pattern which helps you create well-defined and distinct state objects), using a never type in a default case in a switch statement ensures that all potential cases are handled correctly.

Whether you're debugging a complex function or adding extra safety to your TypeScript projects, the never type serves as a helpful tool to improve readability and robustness of your code.