This will never fail to return an array of strings:

(arr: (string | undefined)[]): string[] => arr.filter(item => item !== undefined);

_{playground link}

But TypeScript does not accept to compile this because it doesn't infer that the filter predicate will return true if and only item is a string. Explicitly typing the predicate doesn't seem to help either:

const predicate = (item: string | undefined): item is string => item !== undefined;
(arr: (string | undefined)[]): string[] => arr.filter(item => predicate(item));

_{playground link}

This works but is much wordier (and if I'm not mistaken, much slower):

(arr: (string | undefined)[]): string[] => {
    const ret: string[] = [];
    for (const item of arr) {
        if (item !== undefined) {
            ret.push(item);
        }
    }
    return ret;
}

_{playground link}

Is there an experimental flag I can enable in my tsconfig.json that'll enable TypeScript to make type inferences based on the .filer predicate?

CodePudding user response：

You are using an anonymous inline function instead of the predicate:

TS Playground link

const predicate = (item: string | undefined): item is string => item !== undefined;

(arr: (string | undefined)[]): string[] => arr.filter(item => predicate(item)); // error

(arr: (string | undefined)[]): string[] => arr.filter(predicate); // ok

// inline predicate
(arr: (string | undefined)[]): string[] => arr.filter((item): item is string => item !== undefined);

CodePudding user response：

You can use something like this this

(arr: (string | undefined)[]): string[] => arr.filter(String) as string[];