On: TypeScript 5.0

[quonn]

The missing feature I want most is type narrowing across chained functions like:

arr.filter(a => a.kind === „bar“).map(a => /* a should now be of type Bar just like in a branch */)

[silasdavis → quonn]

It was would be nice for this to be automatic but filter does work like this it you pass a type guard as the predicate:

``` arr.filter((a: A): a is A & { kind: 'bar'} => a.kind === 'bar') ```

If you define:

``` function isKind<K extends A['kind']>(kind: K): (a: A) => a is A & { kind: K } { return (a): a is A & { kind: K } => a.kind === kind; } ```

Then you can write:

``` arr.filter(isKind('bar')) ```

and get a properly narrowed return type array

[quonn → silasdavis]

Nice, thank you

[acdibble → quonn]

This is possible with type predicates, example: https://www.typescriptlang.org/play?#code/C4TwDgpgBAYg9nKBeK...

docs: https://www.typescriptlang.org/docs/handbook/2/narrowing.htm...

[subarctic → quonn]

Wait, are you saying they added that in this release? That's awesome!

[dhritzkiv → subarctic]

This is not part of this release.