Though we are filtering out for undefined values, typescript is not able to recognize this.

[JosephJibi]

🔎 Search Terms

🕗 Version & Regression Information

• This is a crash
• I was unable to test this on prior versions.

⏯ Playground Link

No response

💻 Code

    const foo = [1, undefined, 2];
    const foo2: number[] = foo.filter((value) => value);

🙁 Actual behavior

Show this error in 'foo2' :
Type '(number | undefined)[]' is not assignable to type 'number[]'.
Type 'number | undefined' is not assignable to type 'number'.
Type 'undefined' is not assignable to type 'number'.

🙂 Expected behavior

Should not show this any error, because we are filtering out all undefined.

Additional information about the issue

[MichalMarsalek]

This is a duplicate of many issues. It works as intended. Typescript only narrows the output of filter if the predicate is a typeguard, which (value) => value is not.

You can define a typeguard that checks that the value is not undefined and then use that:

function isDefined<T>(x: T | undefined): x is T {
  return x !== undefined;
}

const foo = [1, undefined, 2];
const foo2: number[] = foo.filter(isDefined);

[bgenia]

see #16069 #38390

[fatcerberus]

Search Terms: [image]

I didn't know GitHub had image search 😉

[tomkcook]

On a semi-related note, it'd be nice if the return type of a type predicate were transitive. At the moment, this doesn't work:

class Foo {
  foo?: string;

  isComplete(): this is CompleteFoo { return this.foo !== undefined; }
}

interface CompleteFoo extends Foo {
  foo: string;
}

const arrayOfFoo: Foo[] = [];
const completeFoos = arrayOfFoo.filter<CompleteFoo>((foo) => foo.isComplete());

because even thought the type of Foo.prototype.isComplete is declared as this is CompleteFoo, when it's called that type collapses to boolean and the compiler has to be told again that it's a type predicate:

const completeFoos = arrayOfFoo.filter<CompleteFoo>((foo): foo is CompleteFoo => foo.isComplete());

The repetition should be unnecessary.

[fatcerberus]

@tomkcook That should be solved by #57465

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.