Type not automatically changing after .filter

[git-no]

Bug Report

Type not automatically changing after .filter

🔎 Search Terms

typescript error filter undefinied

Found
issue 20707
issue 20812

🕗 Version & Regression Information

Typescript 4.5.3 and below
ESNext and below

Please keep and fill in the line that best applies:

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________

⏯ Playground Link

Playground Link

💻 Code

const mixedArray = ['string',undefined,'string'] // result of await Promise.all(apps.map(app => fetchData(app)))

// none of them following new arrays are just type string[], they are still (string | undefined)[]
const shouldBeStringArray = mixedArray.filter(v => v != undefined)
const shortVersionShouldBeStringArray = mixedArray.filter(v => !!v)
const typeCheckShouldBeStringArray = mixedArray.filter(v => {return typeof v == "string"})

🙁 Actual behavior

const mixedArray = ['string',undefined,'string'] // result of await Promise.all(apps.map(app => fetchData(app)))
const shouldBeStringArray = mixedArray.filter(v => v != undefined)
// shouldBeStringArray is type (string | undefined)[]

🙂 Expected behavior

const mixedArray = ['string',undefined,'string'] // result of await Promise.all(apps.map(app => fetchData(app)))
const shouldBeStringArray = mixedArray.filter(v => v != undefined)
// shouldBeStringArray type string[]

[MartinJohns]

Duplicate of #20218. You're not passing a type guard function to filter.

[Jamesernator]

In these situations I feel like it would be helpful for TypeScript to infer these functions as typeguards automatically. In particular if we have one of the following callbacks to infer as a typeguard:

v => v != undefined; // or null
v => typeof v === "string"; // and others
v => v instanceof SomeClass;
v => v === "someLiteral"; // and other types
// ideally disriminants as well but this might be harder to statically analyze
v => v.kind === "someKind"

[MartinJohns]

@Jamesernator See #38390.

[git-no]

Duplicate of #20218. You're not passing a type guard function to filter.

Im doubt that it would work. I do miss your specific solution.

This type guard does not work correct

type A = string | undefined
const array: A[] = ['hello', undefined, 'salve'];
const a1 = array.filter((e): e is string => e == undefined);

because a1 is type string[] but got no string value, got only undefined.
Conclusion, Typescript has the next error, it does not check the types assigned to a1. There is no warning.

[git-no]

Potential Solution

This solution works

// Create this helper function
function isNotNullOrUndefined<T extends Object>(input: null | undefined | T): input is T {
    return input != null;
}

// This actually determines that result is of type (string)[]
let correctStringArray = mixedArray.filter(isNotNullOrUndefined);

Source / found in 16069

[MartinJohns]

This type guard does not work correct

Type guards are not checked for correctness. You can say the very same about the helper function from your last comment.

[conartist6]

iter-tools contains a bunch of type guards (referred to there as predicates), and is the only library I know of that has:

1. users
2. not guards (which are critical because the types are written differently)
3. the guards you need (as of v7.2.0, which I just released)

I should probably split this out into a separate @iter-tools/predicates package, but I have not done this yet.

The method it sounds like you want is notNil.