Filter predicate function with `instanceOf` issues on type inference

[jmoliugp]

Bug Report

🔎 Search Terms

instanceOf
predicate

🕗 Version & Regression Information

• This error is appearing to me in versions 4.9.4, 4.8.2 & 4.4.4. Nothing makes me think it worked before in prior versions.

⏯ Playground Link

link

💻 Code

const foo = () => {
  const a = 5
  const b = new Error('b')
  const c = new Error('c')
  const d = 'Hello world'
  const ls = [a, b, c, d]

  ls.filter<Error>(curr => curr instanceof Error)
  // Result: (2) [Error: b, Error: c]

  // Throws error:
  // Argument of type '(curr: string | number | Error) => boolean' is not assignable to parameter of type '(value: string | number | Error, index: number, array: (string | number | Error)[]) => value is Error'.
  // Signature '(curr: string | number | Error): boolean' must be a type predicate.ts(2345)
}

🙁 Actual behavior

It is neither recognizing the predicate function nor that the result will be an array of Error values.

🙂 Expected behavior

Interprets that the function is a predicate and filter the errors with the operator instanceOf.

[MartinJohns]

Duplicate of #38390.

Your function is not a type guard. Add a type annotation to make it a type guard and it will work.

[jmoliugp]

Duplicate of #38390.

Your function is not a type guard. Add a type annotation to make it a type guard and it will work.

Can you try it on the playground and paste the code?

Tried it and still having the same problem. I'm not understanding how adding a type annotation would make a difference, the compiler infers the result of evaluating the expression it returns a boolean.

[MartinJohns]

ls.filter<Error>((curr): curr is Error => curr instanceof Error)

I'm not understanding how adding a type annotation would make a difference, the compiler infers the result of evaluating the expression it returns a boolean.

The difference is, by adding a type annotation for the return type you don't let the compiler infer the result type at all, so it won't end up being boolean.

[typescript-bot]

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