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Types defining Symbol.hasInstance should be eligible to use with the instanceof operator #39064

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lazytype opened this issue Jun 14, 2020 · 5 comments · Fixed by #55052
Closed

Types defining Symbol.hasInstance should be eligible to use with the instanceof operator #39064

lazytype opened this issue Jun 14, 2020 · 5 comments · Fixed by #55052
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In Discussion Not yet reached consensus Suggestion An idea for TypeScript

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@lazytype
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TypeScript Version: 3.9.4

As specified here, https://tc39.es/ecma262/#sec-instanceofoperator, it should be sufficient for an object to define a function for the Symbol.hasInstance well-known symbol in order to be used as the right-hand side of the instanceof operator.

It seems like this was intended to be implemented based on these comments: #12728 (comment)
but for whatever reason that didn't happen it in the PR

Search Terms: Symbol.hasInstance

Code

({}) instanceof {[Symbol.hasInstance]: () => true}
// Running this in the browser evaluates to `true`

Expected behavior:
No type-errors

Actual behavior:

({}) instanceof {[Symbol.hasInstance]: () => true}
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
!!! error TS2359: The right-hand side of an 'instanceof' expression must be of type 'any' or of a type assignable to the 'Function' interface type.

Playground Link: Playground Link

Related Issues:

@lazytype
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I have a tested and working branch that implements this proposal here: https://github.com/lazytype/TypeScript/commit/0c226a775f63f4dd97f6a65422707d4231787188

Holding off on creating a PR until this issue gets tagged with the "Backlog" milestone, per the contributor guidelines.

@RyanCavanaugh RyanCavanaugh added Awaiting More Feedback This means we'd like to hear from more people who would be helped by this feature Suggestion An idea for TypeScript In Discussion Not yet reached consensus and removed Awaiting More Feedback This means we'd like to hear from more people who would be helped by this feature labels Jun 16, 2020
@ExE-Boss
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See also: #17360

@caburj caburj mentioned this issue Jan 31, 2022
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@rotu
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rotu commented Jan 29, 2023

The typescript compiler also spuriously complains when the lhs of instanceof is not an object:

class MyNumber {
  static [Symbol.hasInstance](x:any) {
    return (typeof x === 'number');
  }
}

console.log(1 instanceof MyNumber);

It gives this error:

The left-hand side of an 'instanceof' expression must be of type 'any', an object type or a type parameter.

@shrinktofit
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Why was this issue marked as "Suggestion"? I think it should be labeled as bug. @RyanCavanaugh

@rbuckton rbuckton mentioned this issue Jul 17, 2023
Merged
@MichaelMitchell-at
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I have a tested and working branch that implements this proposal here: lazytype@0c226a7

Holding off on creating a PR until this issue gets tagged with the "Backlog" milestone, per the contributor guidelines.

❤️ Thanks for finally fixing the bug @rbuckton. I can finally delete this branch.

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Support custom 'Symbol.hasInstance' methods when narrowing 'instanceof' microsoft/TypeScript
6 participants
@rotu @lazytype @ExE-Boss @RyanCavanaugh @shrinktofit @MichaelMitchell-at