8000 Easier union type narrowing in Angular control statements ('in' / `typeof` operator) · Issue #56875 · angular/angular · GitHub
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Easier union type narrowing in Angular control statements ('in' / typeof operator) #56875

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johnameyer opened this issue Jul 7, 2024 · 2 comments
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Easier union type narrowing in Angular control statements ('in' / typeof operator) #56875

johnameyer opened this issue Jul 7, 2024 · 2 comments

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@johnameyer
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johnameyer commented Jul 7, 2024 •
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Which @angular/* package(s) are relevant/related to the feature request?

compiler, core

Description

If I have a union of types i.e. { one: number } | { two: number } | { three: number }, it can be a bit onerous to narrow the type in my actual template description.

Attempting to write the template exactly as I might in a pure typescript file,

@if('one' in obj) {
    {{ obj.one }}
}

results in

Parser Error: Unexpected token 'in' at column 10 in ['one' in obj] in ...

The documentation does mention these operators are not supported, but chalks it up to side effects when these operators are pure.

You can't use JavaScript expressions that have or promote side effects, including:

  • Assignments (=, +=, -=, ...)
  • Operators such as new, typeof, or instanceof
  • Chaining expressions with ; or ,
  • The increment and decrement operators ++ and --
  • Some of the ES2015+ operators

Naturally, I could write out a type predicate function hasOne(obj: object): obj is { one: number} on my class, but this is a little verbose and can lead to a component having many such functions.

Proposed solution

If / else control expressions support in / typeof / instanceof operators like the other standard algebraic comparison operations.

Alternatives considered

Helper function like $any() that apply the proper type narrowing.

Do not throw errors for accessing property if one member of union contains that property.
@JoostK JoostK mentioned this issue Jul 7, 2024
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JoostK commented Jul 7, 2024

I think this makes sense. There's a similar issue #38560 that has been rolled into #43485, I'm adding this one to that list as well.

@JoostK JoostK closed this as not planned Won't fix, can't repro, duplicate, stale Jul 7, 2024
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