typescript array not assignable after map and find

The following code

const s: string[] = ["x", "y", "z"].map(i => ["x"].find(t => t === i));

This causes an error "Type ‘(string | undefined)[]’ is not assignable to type ‘string[]’." as find gives string | undefined. So I was hoping adding a filter can fix that, like:

const s: string[] = ["x", "y", "z"].map(i => ["x"].find(t => t === i)).filter(i => i);

But this didn’t work, why?

TS version 4.2.3

>Solution :

If you know for sure that the find call here will never return undefined, you can use an assertion with ‘!’:

const s: string[] = ["x", "y", "z"].map(i => ["x"].find(t => t === i)!);
//                                                                   ^

Otherwise you will have to write a guard:

function isDefinitelyAString(val: unknown): val is string {
    return typeof val === "string";
}

const s: string[] = ["x", "y", "z"].map(i => ["x"].find(t => t === i)).filter(isDefinitelyAString);

You can also inline the guard like @jonrsharpe suggests:

const s: string[] = ["x", "y", "z"].map(i => ["x"].find(t => t === i)).filter((i): i is string => typeof i === "string");