Flatten Arrays in Vanilla JavaScript with Flat() and FlatMap()

Re:Flatten Arrays in Vanilla JavaScript with Flat() and Flat... by amitport on Wednesday January 05, 2022 @03:43PM (#29814639)

I wonder if it is possible to add a feature to Typescript to help with this:

You could potentially add a syntax for type guards function types, then add a signature to filter that accepts a type guard and returns an array of the guarded types.

Shouldn't be too much of a stretch given that we have type guards.

The syntax is a bit annoying... should be something like filter<A, B>(cb: A => A is B)

[ Parent | on HN ]

Re:Flatten Arrays in Vanilla JavaScript with Flat() and Flat... by abrioy on Wednesday January 05, 2022 @04:27PM (#29815176)

You can use a type guard[1] as an argument to Array.filter, but the function has to be explicitly typed as such.

I don't know why the type isn't narrowed in Array.filter like it is in if statements without this weird workaround.

  const array: (number | string)[] = [];
  
  const mixedArray = array.filter(value => typeof value === 'string');
  // mixedArray: (number | string)[]
  const arrayOfString = array.filter((value): value is string => typeof value === 'string');
  // arrayOfString: string[]

This example in Typescript playground: https://www.typescriptlang.org/play?#code/MYewdgzgLgBAhgJwXA...

[1]: https://www.typescriptlang.org/docs/handbook/advanced-types....

[ Parent | on HN ]

Re:Flatten Arrays in Vanilla JavaScript with Flat() and Flat... by amitport on Wednesday January 05, 2022 @10:07PM (#29818780)

Oh so there is an overload!

filter<U extends T>(pred: (a: T) => a is U): U[];

Additionally, getting TS better at inferring type guards is an open issue (literally): https://github.com/microsoft/TypeScript/issues/38390

[ Parent | on HN ]