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I just learned that #TypeScript can actually type narrowOn twitter.com
Array.prototype.filtervia anisclause:const values = ['a', 'b', null, 'd']; const result = values.filter((v): v is string => !!v); // Inferred asstring[]!typescriptlang.org/play?#code/MYewdgzgLgBAbgQwDYFcCmEYF4YG0DkC+ANDPgEYkxgpJKn4Am+AugNwBQoksAThrVg5EqDADoAZgEskUNLwAUCuAEoAXPBhTM0XlLABzbAD4YAQjOq2QA
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Mood 0
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What's extra cool is that this is *not* special-cased in the compiler. Apparently you can usePermalink On twitter.com
isin a type declaration to declare a new generic as part of the expression, here is the source forArray.prototype.filter: github.com/microsoft/TypeScript/blob/404a7d602df9c19d98d49e6a6bf2295e423be676/lib/lib.es5.d.ts#L1176
Mood +1 🙂
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We can even do this generically to removePermalink On twitter.com
nullfrom the input type, rather than restating a non-nullable version:const list = [1, 2, null, 3]; list.filter((v): v is NonNullable<typeof list[number]> => v !== null);typescriptlang.org/play?#code/PTAEBkHsDsHNQC4E8AOBTANKAJjA5AqAO4CG0hCkoATmuiYQJYIB0AUAMYwDOhANo14AuUAEFq1EkgA8vaozigAPqABGkSHzRlloaAFcAtqrTVdBvnwB8oALygA2gTS88WAExYE1fZj37LAF0AbjY2EFAAJTRDSAA3F1AAAws+JNAAM2pIQ0QACzRQBRR9ClRCsmwi6AzTblAuCTQOBD4kdi5oXhoXAMJ7AV4WDMY+BFMACgm4kj5fAEoRGbnCwVAAORh1gL4SVS1pZHRIDNBBhAcDY1NAm1sbZd9QAEJbe1T54KA
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Mood 0
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Now if only we could somehow make:Permalink On twitter.com
(value: T) => !!valueinfer to(value: T): value is NonNullable<T> => !!valuewe'd be golden. Is there any reason the compiler can't do that? It already type narrows, we'd just need to plumb it through to the return type.
Mood 0
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I guess the tricky part here is probably remembering that it came from a function parameter and tying the return type to the narrowed form of that parameter. It also assumes thatPermalink On twitter.com
valueis not modified, which may not always be true, but would probably break inference anyways.
Mood +2 🙂