add support throw keyword

[sahandevs]

Search Terms

Error Type, throws, throw, throw keyword

Suggestion

Explanation

add a throw keyword that when it reached in a type it reports a message/error.
it's useful when used with "Template literal types".

usage:

type RouteParams<T extends string> =
  T extends `/${infer Route}/:${infer Param}`
    ? { [Route]: { [Param]: string }}
    : throws `${T} doesn't have a "/:param" part`;

function addRoute<T extends string>(path: T, params: RouteParams<T>) ...

for now, we can only return types like this to inform the user why the input is incorrect:

type Error<T> = T & { error: true };

type RouteParams<T extends string> =
  T extends `/${infer Route}/:${infer Param}`
    ? { [Route]: { [Param]: string }}
    : Error<`${T} doesn't have a "/:param" part`>;

function addRoute<T extends string>(path: T, params: RouteParams<T>) ...

which doesn't show error lines and place of the error message is really indirect and the user may not found it/

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[MartinJohns]

Duplicate of #23689.