Functions allowed in weird cases where an indexer signature is expected, where functions don't have one.

[benlesh]

Bug Report

Looking for a workaround or an explanation, really. But it seems like a bug?

🔎 Search Terms

• arraylike
• index
• function

🕗 Version & Regression Information

This is the behavior in every version I've tried.

⏯ Playground Link

Playground link with relevant code

💻 Code

interface ArrayLike<T> {
    readonly length: number;
    readonly [n: number]: T;
}

function testB<T>(arg: ArrayLike<T>) { }

// GOOD: Errors here as expected. "index signature is missing".
testB(() => {});

function testA(arg: ArrayLike<any>) {}

// BAD: No errors here? The index signature is still missing, though.
testA(() => {});

🙁 Actual behavior

testA allows a developer to pass a function where we didn't intend them to be able to pass a function.

In some (much more complicated) cases in RxJS this becomes an unintuitive footgun.

🙂 Expected behavior

I'd expect that testA would error with the same error that testB errors with. Functions don't have indexers in their signatures.

Related to:

• Remove ArrayLike from ObservableInput ReactiveX/rxjs#6011
• Operator functions are acceptable to concat() function in Typescript. ReactiveX/rxjs#4563

[RyanCavanaugh]

This came up on the bird website the other day: https://twitter.com/OliverJAsh/status/1357374631938830337

ArrayLike<unknown> would be a better type at testA

[typescript-bot]

This issue has been marked as 'Question' and has seen no recent activity. It has been automatically closed for house-keeping purposes. If you're still waiting on a response, questions are usually better suited to stackoverflow.