[new rule] Disallow !?? (Non-null + nullish)

[fregante]

! in TypeScript (non-null assertion operator) cancels the null type and ?? (nullish coalescing operator) actually handles it. The first one is redundant.

This is a suggestion for a new rule:

Fail

return document.querySelector('a')! ?? defaultElement

Pass

return document.querySelector('a') ?? defaultElement

[bradzacher]

Seems like a logical cousin to no-non-null-asserted-optional-chain

[ypresto]

It seems covered by no-unnecessary-conditional?

[fregante]

You're right, no-unnecessary-condition was disabled in my preset due to

• [no-unnecessary-condition] ignore array index return #1798

My example is covered by that rule:

This issue can be closed as far as I'm concerned.

[ypresto]

It would be appreciated if you can paste the code (example) caused false-positive in your project. Just at assigning array element to variable?

[fregante]

There's no false positive. My example matches your example except it uses the same API I used initially.

[bradzacher]

Thanks for pointing that out @ypresto!

For reference this could also be handled via no-restricted-syntax:
https://github.com/typescript-eslint/typescript-eslint/blob/master/docs/getting-started/linting/FAQ.md#how-can-i-ban-specific-language-feature

{
  "no-restricted-syntax": [
    "error",
     {
       "selector": "LogicalExpression[operator='??'] > TSNonNullExpression.left",
       "message": "Why the heck are you non-nulling a nullish-coalesce?"
     },
  ],
}

(dump the config into here to see it working!)

Closing as there are multiple ways to achieve this without a new rule.

---

For reference - no-non-null-asserted-optional-chain makes sense as a rule because it's not achievable with a simple selector alone due to the complexity of the optional chaining AST.

[fregante]

It would be appreciated if you can paste the code (example) caused false-positive in your project

Oh maybe you were referring to this:

• [no-unnecessary-condition] ignore array index return #1798

I didn't personally encounter the issue; I said "my preset" but I meant "the preset I use"