How can I make typescript treat a typed function as a throw-statement?-CodePudding

I am struggling with a typescript inconvenience - I have a custom function that always throws an error, and I am using it to throw errors in case certain variables are null. If I don't use the function, but rather just throw an error, typescript understands that the variable cannot be null after that statement, but if I use a function typed with the 'never' return type, it does not.

See this example on TS Playground that perfectly illustrates the problem.

Is there any way to get around this?

Currently, I have to do one of the following:

Change my types to allow null (that would be very bad for various reasons)
Add a reduntant mock throw statement similar to the one in the code example (annoying but works)

I have tried using the recommended function signature with the return type of never, but typescript still treats it differently from an explicit throw statement. I expected that if a function never returns, typescript understands that the rest of the code won't execute.

CodePudding user response：

You may be looking for an assertion function.

function customThrow(str: string | null, message: string): asserts str is string {
    if (str === null){
        throw new Error(`custom-error-${message}`);
    }
}

const cat: string | null = Math.random() > 0.5 ? 'cat' : null;

customThrow(cat, "cat-missing")

const uppercaseCat = cat.toUpperCase();

The function asserts that whatever is passed into it must be a string, otherwise the function throws an Error. You might also change the naming of the function to something like assertStringNotNull.

Playground

CodePudding user response：

If anyone is curious, the problem stemmed from this issue

And the solution is explicitly typing the function every time we want it to participate in the control flow.

A working playground can be found here

The example from my first playground

const customThrow = (message: string): never => {
    throw new Error(`custom-error-${message}`);
}

Needs to be explicitly typed like this

const customThrow: (message: string) => never = (message) => {
    throw new Error(`custom-error-${message}`);
}