3 ms·
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 sign
by amitport 5y ago
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)
:/
- abrioy 5y agoYou 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/MYewdgzgLgBAhgJwXAngLhgCjAVwLYBGApgjAD4zQICWYA5gJQDaAujALwysDcAUL6Eiw81AB5EAJgEEkqDvFkoAdADNqAGyglMANzjqcRDgD4YUFAAciIFTD0Gj7JzADkVWnRcM+Aeh8wRcWlFDGx8YlIKd3pmFgFwaAVkFAB5FQBlKBp6eURk1Q0tBExdfUMGDHtDGGoISiyPEzNLa1sqx2c3BvovX3881DTM7LoMaLpWIA https://www.typescriptlang.org/play?#code/MYewdgzgLgBAhgJwXA... [1]: https://www.typescriptlang.org/docs/handbook/advanced-types.html#type-guards-and-differentiating-types https://www.typescriptlang.org/docs/handbook/advanced-types....
- amitport 5y agoOh 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 https://github.com/microsoft/TypeScript/issues/38390