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TypeScript optional function argument throws error TS2345 with `ow.optional.function` #147

kaatt posted onGitHub

interface Options {
  foo?: (baz: number) => string
  bar: (baz: number) => string
  baz?: number
}

function func1(options: Options): any {
  const { foo, bar, baz } = options
  ow(bar, ow.function)
  ow(foo, ow.optional.function) // throws TS2345
  ow(baz, ow.optional.number) // doesn't throw
}

The above code throws this error:

src/test.ts:11:11 - error TS2345: Argument of type 'Predicate<Function>' is not assignable to parameter of type 'BasePredicate<((baz: number) => string) | undefined>'.
  Types of property '[testSymbol]' are incompatible.
    Type '(value: Function, main: Main, label: TimerHandler) => void' is not assignable to type '(value: ((baz: number) => string) | undefined, main: Main, label: TimerHandler) => void'.
      Types of parameters 'value' and 'value' are incompatible.
        Type '((baz: number) => string) | undefined' is not assignable to type 'Function'.
          Type 'undefined' is not assignable to type 'Function'.

11   ow(foo, ow.optional.function)
             ~~~~~~~~~~~~~~~~~~~~

Not sure what's wrong here. Is it a bug with ow's typings?

This workaround works:

if (foo) ow(foo, ow.function)

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posted by IssueHuntBot almost 6 years ago

Hi! I can't seem to re-create the error.

import ow from './dist/source'

interface Options {
    foo?: (baz: number) => string
    bar: (baz: number) => string
    baz?: number
}

function test(options: Options): any {
    const { foo, bar, baz } = options
    ow(bar, ow.function)
    ow(foo, ow.optional.function)
    ow(baz, ow.optional.number)
}

const o1 = {
    bar: (x) => 'asd',
    baz: 2,
};

test(o1);

The above code doesn't throw any errors for me.
Maybe your options object that you pass to test doesn't look like this, so that's maybe where I'm wrong

posted by sbencoding over 5 years ago

@kaatt It happens because you're using strictNullChecks, I will open a PR with a possible fix.

posted by qti3e over 5 years ago

After some more digging, I realized it's somewhat the expected behaviour. But you can use the non-null assertion operator. (!)

- ow(foo, ow.optional.function) // throws TS2345
+ ow(foo!, ow.optional.function)

posted by qti3e over 5 years ago

I've fixed the issue by approaching it in the other direction, which seems to work well - tests are all still passing.

posted by TheGuardianWolf over 4 years ago

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posted by issuehunt-app[bot] over 4 years ago

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Allow potentially undefined functions to be checked #184submitted bytheguardianwolf(102)

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TypeScript optional function argument throws error TS2345 with `ow.optional.function` #147

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