This is a small one-liner function for marking the unreachable code which can be checked to be unreachable at type-level.
The main example use-case is an exhaustive switch-case statement, like this:
type Union = 'First' | 'Second';
declare function getValue(): Union;
import { unreachable } from 'unreachable-ts';
const value: Union = getValue();
switch (value) {
case 'First':
// do something
break;
case 'Second':
// do something else
break;
default:
unreachable(value, new Error("Value must be either First or Second"));
}
If the getValue
function returns any value not corresponding to the declared type, it will fall into the default
branch and throw a runtime error.
On the other hand, if the Union
type is changed but the switch/case
is not, TypeScript will throw a compilation error:
type Union = 'First' | 'Second' | 'Third';
declare function getValue(): Union;
import { unreachable } from 'unreachable-ts';
const value: Union = getValue();
switch (value) {
case 'First':
// do something
break;
case 'Second':
// do something else
break;
default:
// Error: TS2345: Argument of type 'Error' is not assignable to parameter of type 'never'.
unreachable(value, new Error("Value must be either First or Second"));
}
Well, it's mainly for the sake of removing repetition. It's better to have one import in several places than to duplicate that code, don't you think so? And anyway, if you feel like you don't want to pull a whole project for a small one-liner, you are free to just copy the contents to you - it's in public domain, anyway.
Isn't there an unreachable
package already?
Well, there is... but you know, every programmer has to reinvent the wheel sometimes, right?
Just because of license (the Unlicense), probably. Nothing more. It's just too simple to be done wrong.
Just don't forget that this is public domain, and, if I ever get any pull request - this request's contents will be considered public domain too.
It seems that TypeScript has won anyway. And, to be honest, I never worked with Flow, so... how can I compete on this field?