How can you indicate that a function in TypeScript is expected to throw an error?

An error occurs when trying to compile the following code: "a function whose declared type is neither void nor any must return a value or consist of a single throw statement."

Is there a way to indicate to the compiler that _notImplemented throws an exception?

function _notImplemented() {
   throw new Error('not implemented');
}

class Foo {
    bar() : boolean { _notImplemented(); }

The only workaround I can think of is to use generics, but it feels like a hack. Is there a more elegant solution?

function _notImplemented<T>() : T {
   throw new Error('not implemented');
}

class Foo {
    bar() : boolean { return _notImplemented(); }

Answer №1

If you want _notImplemented to return the type never, you can specify it in your code. The type never is specifically used for functions that never return, which could be due to an infinite loop or always throwing an error.

function _notImplemented() : never {
     throw new Error("Function not implemented")
}

Answer №2

Instead of using a throw, consider utilizing an Either. An Either is a data type that typically contains either an error or a result. TypeScript can easily work with it since it behaves like any other type.

For instance:

function getSixthChar(str: string): Either<Error, string> {
    if (str.length >= 6) {
        return Either.right<Error, string>(str[5]);
    }
    else {
        return Either.left<Error, string>(new Error("String length is too short"));
    }
}

A function calling the getSixthChar function can choose to handle the result in various ways such as unwrapping, returning a maybe, throwing an error, etc.

To use the Either type, you will need a library that supports it - consider monad libraries like TsMonad or monet.js.

Answer №3

As far as my knowledge goes, there is currently no clean solution to address this issue.

The TypeScript team is actively investigating this issue on their GitHub repository (https://github.com/Microsoft/TypeScript/issues/1042) and we may see a resolution in the near future.

Answer №4

It is suggested that we consider enter image description here treating the action of `throw` as being equal to `return` in this particular scenario.

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