candid wind
hey all, considering filing an issue on GH about typescript not properly narrowing in this scenario:
let values = [undefined, 1, 2, 3, undefined]
// -> (undefined | number)[]
values = values.filter(Boolean)
// TS should now know that values cannot be undefined, and the type of values is number[]
Is this behavior intentional? It's frustrated me for the longest time that you have to manually cast the result of the filter call to, say, number[] for it to know it won't be undefined
mossy elk
it's normal
especially considering how the type system and this function works
imagine you have a string[], applying filter will returning you the same type as the source, so string[]
the type stayed the same, only some values got removed
@candid wind
filter doesn't narrow anything
candid wind
hmm
tribal minnowBOT
candid wind
maybe it should though in this specific case
mossy elk
maybe it should though in this specific case
yes, but you have to specify it manually
candid wind
gotcha
mossy elk
const values = [undefined, 1, 2, 3, undefined];
const isNumber = (n: unknown): n is number => typeof n === "number";
const filtered = values.filter(isNumber);
clever harbor
A generic isDefined helper is also useful:
tribal minnowBOT
const isDefined = <T>(x: T | undefined): x is T => x !== undefined
const foos = [undefined, 1, 2, 3, undefined]
// ^? - const foos: (number | undefined)[]
const bars = foos.filter(isDefined)
// ^? - const bars: number[]
ts-reset actually makes .filter(Boolean) work as you expect, but I wouldn't really recommend it.
mossy elk
also, @candid wind , Boolean won't work to keep all numbers, since it will exclude 0 which is falsy