(string | undefined) array to string array

Question

Good afternoon!

I'm working on creating a function in TypeScript that receives a string | undefined array that returns another array containing only string elements. In other words, a function that matches this signature:

functionName(arraySource: (string | undefined)[]): string[]

I've tried filtering the array by its type (via functional programming) with a line like:

array.filter(x => typeof x === "string")

The problem here is the compiler: it says that the previous method returns a (string | undefined)[] variable (which cannot be assigned to a string[]).

Thanks in advance!

Does this answer your question? TypeScript filter out nulls from an array. The answer is applicable to your problem. — k.tten
– k.tten, Commented Sep 9, 2022 at 23:30

kshetline · Accepted Answer · 2022-09-09 23:24:57Z

1

You can simply cast the result like this:

array.filter(x => typeof x === "string") as string[]

I'm not sure that there's a more sophisticated was to convince the compiler to recognized that the logical consequence of the filtering will be to remove undefined values.

answered Sep 9, 2022 at 23:24

kshetline

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1 Comment

diego.guerrero Over a year ago

Thanks! Your answer is perfect and works fine, answer given in question referenced by @kelly with guards seems to work fine too and avoids casting

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