`Array.filter` does not properly narrow down type

[bpasero]

Bug Report

🔎 Search Terms

array filter narrow down

🕗 Version & Regression Information

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

⏯ Playground Link

Playground link with relevant code

💻 Code

interface IFoo {
  foo: boolean;
}

interface IBar {
  bar: boolean;
}

const items: (IFoo | IBar)[] = [];

function isFoo(obj: unknown): obj is IFoo {
  if (!obj) {
    return false;
  }

  const candidate = obj as IFoo;
  return typeof candidate.foo === `boolean`;
}

const fooItems = items.filter(item => isFoo(item));
for (const fooItem of fooItems) {
  console.log(fooItem.foo); // error: Property foo does not exist on type IFoo | IBar
}

🙁 Actual behavior

The fooItems is of type IFoo | IBar

🙂 Expected behavior

I would have expected fooItems to be IFoo[].

[MartinJohns]

filter() only narrows when you pass a type guard function. You don't do this.

[bpasero]

How is function isFoo(obj: unknown): obj is IFoo { not a type guard function?

[MartinJohns]

The inline function item => isFoo(item) is not. It lacks a type annotation, so the type is inferred to be boolean.

[bpasero]

Cool thanks for the help! Very hard to figure this out though for me, even though having used TypeScript since 2012.

[fatcerberus]

See #38390