[switch-exhaustiveness-check] Require default case if discriminant type is not a union

[raymondwang]

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I have read the FAQ and my problem is not listed.

Repro

{
  "rules": {
    "@typescript-eslint/switch-exhaustiveness-check": "warn"
  }
}

type Day = string; // or number

const day = 'Monday' as Day;
let result = 0;

switch (day) {
  case 'Monday': {
    result = 1;
    break;
  }
}

or

type Day = any; // or unknown, or a JS variable outside of a TS context

const day = 'Monday' as Day;
let result = 0;

switch (day) {
  case 'Monday': {
    result = 1;
    break;
  }
}

Expected Result

I would expect to see an error message compatible with the default-case ESLint rule, indicating that the switch-exhaustiveness-check rule is invalidated because the expression is not a union type.

Actual Result

No error message.

Additional Info

Essentially, I think the best of both worlds is to just have the predictable functionality of the default-case rule operate normally except in the event that a switch statement's expression is a union type. Today, there doesn't seem to be any way for the two rules to operate in tandem. Adding this functionality might be a breaking change, but I'm honestly a bit curious how controversial it would be.

[bradzacher]

As it stands right now - this rule only checks union types.
So this is a large expansion in responsibilities for the rule.

<opinion>
I'll be honest when I say that I disagree that default-case is a "good" rule to have on.
There are cases where you know the constrained value set, but can't constrain the type (eg external library types), meaning you're working with a generic string or number.

So adding a default there just means you're going to add default: throw new Error("shouldn't happen");, which isn't a great practice by default.

Adding a default case also means that if you are able to convert the type to a union type in future, then you'll never catch any missing cases, as the rule would detect that all cases are handled.
</opinion>

I'm not sure if this is a good addition to this lint rule.
Maybe behind a default false flag? But that's, at best, what I'd be suggesting.

[raymondwang]

Thanks for the response @bradzacher. I think that's totally fair, and I don't want to overcomplicate this process or rule by enforcing my opinions with a breaking change. I basically just want to implement some solution for codebases that do want to enforce default-case as well as switch-exhaustiveness-check, which are totally incompatible rules today (despite both being in the interest of encouraging switch statement thoroughness).

Is there a way to recognize whether the default-case rule is being enforced within the rule? Could be an elegant solution, rather than passing in another config, but obviously happy to do the latter if not.

[bradzacher]

Is there a way to recognize whether the default-case rule is being enforced within the rule?

No. Lint rules are all isolated. They have no way to tell if other rules are enabled.

---

Happy to have this behind a default off option!

[SimonSimCity]

Just a work-around I've found which works for me:

If you are the type of guy who wants this feature, it might be worth thinking about an exhaustive matching guard (https://medium.com/technogise/type-safe-and-exhaustive-switch-statements-aka-pattern-matching-in-typescript-e3febd433a7a#be77). This way, you keep default-case active and use the fact that TypeScript has this special type never. When you have an exhaustive switch-case, let the compiler ensure the type inside default is never. Here's an example:

enum Direction {
  Up,
  Down,
  Left,
  Right,
}

(myVar: Direction) => {
  switch (myVar) {
    case Direction.Up:
      break;
    case Direction.Down:
      break;
    case Direction.Left:
      break;
    case Direction.Right:
      break;
    default:
      ((_: never) => { })(myVar)
  }
}

Playground

[ST-DDT]

I'll have a look on this.

[ST-DDT]

PR created: #7880

• feat(eslint-plugin): [switch-exhaustiveness-check] add requireDefaultForNonUnion option #7880