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49. asserts never

Here is the implementation of the assertsNever function:

function assertsNever(x: never): never {  throw new Error("Unexpected value: " + x);}

This function takes a parameter x of type never, which means that this parameter can never have a value. It is used to annotate the default case of the switch statement, indicating that it should never be reached.

If the assertsNever function is ever called, it means that the switch statement is not exhaustive and there is a missing case. The function simply throws an Error with an error message indicating that an unexpected value was encountered.

With this function added to the code, TypeScript will now complain if there are missing cases in a switch statement, making it easier to catch errors at compile time.